NEED HELP 05 Trailblazer Fuel Gauge Problem. Bad PCM?

[Zmoney]

Hey I'm new. I've been searching around but can't find an answer on this.

I have an 05 14X,xxx miles on it.

I think my PCM is bad and I dont know how to test it.


Its been giving me problems for almost two years now and I can't figure it out. I haven't been driving it much at all since I started noticing it wasn't acting right, but now I want to get her ship shape again.

Not throwing any codes so my mechanic (independent guy) wasn't too interested in digging.

I had an intermittent rough idle, some delayed shifting into second sometimes, and what I thought was atrocious mileage.

I replaced my coil packs, new delco plugs, air filter and it seems to run better in general, but there's still one big issue.

I came to notice (again after not driving more than a couple of miles at a time it for quite some time until recently) that when I filled up, the fuel gauge would not read entirely full. Then within 20 miles it would rapidly drop to 3/4 and then seem to drop at a normal rate with driving.

I thought it must be the sending unit.

I dropped the tank and replaced the whole pump with a delphi. The tank had about 5 gallons left in it at this time when the gauge had been reading empty.

I thought I was right and it was definitely the sending unit.

Tank went back on, went and filled it up, gauge read completely full.

I then drove about 40 miles then filled up again to get a mileage calculation, which looked great to me (at that point it took 1.4x gallons to fill it back up, allowing the pump to tell me it was done) The gauge seemed to behave normally during that drive (it never rapidly fell toward that 3/4 mark and said I'd gone from a tick above full to a tick under)

After refill the gauge went back up to above full.


Drove 30 miles and was at 3/4 of a tank again.

Kept driving and I'm now reading out 115 miles on half a tank - as if the gauge is just not factoring in that first quarter tank.

So I think its not a second bad sending unit having the exact same issue. Its either the gauge itself (which doesn't seem right)

Thoughts or ideas on how to proceed?

 

[NJTB]

Weird one, could it be a stepper motor? Almost seems like the float in the tank is getting hung up, improbable.

 

[TJBaker57]

I would not make any conclusions without a whole lot more observations. Like on the order of 3 or 4 tankfuls and running down to maybe 1/4 tank before filling. Reset the tripmeter at fill up and watch that also, relating that to tank gauge.

 

[Zmoney]

So the idea of it being an obstruction in the tank is a no-go in my opinion. First I just dropped and cleaned the tank and it was perfectly normal inside, second if it was hung up it should read a higher fuel level than actual, shouldn't it? In this case it says its 1/4 lower than actual within 20-30 miles driven after fill up.

As for the stepper being the problem, I suppose that could be. But is the low fuel light dependent on a signal from the pcm or the position of the gauge needle? Because when the gauge reads about empty (but there is still approx. 5.5 gallons in it) the low fuel light goes on too.

I happen to have a spare instrument cluster. I wonder now if just for testing would it create a problem to plug it in and see if the gauge reads the same? And would that even work without programming?

 

[Zmoney]

I also wonder if it could be bad wiring into or from the pcm and how I might even test that if so...

 

[TJBaker57]

Do you have an obd2 adapter?? If so you may be able to read what the PCM is seeing for fuel tank level and also what the PCM is instructing the gauge to read out.

The PCM gets a varying voltage back from the sender. The PCM has programming for the tank size in your truck. The PCM converts that voltage to a corresponding percentage value and sends this to the cluster for display.

So if you see on the serial data line (with obd2 adapter) that the PCM is sending say 50% to the cluster fuel gauge and the gauge reads something else then it may be assumed the gauge is at fault.

I think the low fuel light is dependent on gauge position, not any separate signal from the PCM. I believe this as a result of some benchtop testing I have done with a spare PCM & cluster. Watch here as I short out the wires from the PCM that would go to the sender. The low fuel light doesn't come on until the needle is near zero while the shorted wires remain unchanged.

2002 TrailBlazer Cluster Gas Gauge

The early models had the fuel level sender circuit open or high resistance when full. At some year they reversed this I have read and the sender sent a close...

youtube.com

 

[Zmoney]

Do you have an obd2 adapter?? If so you may be able to read what the PCM is seeing for fuel tank level and also what the PCM is instructing the gauge to read out.

The PCM gets a varying voltage back from the sender. The PCM has programming for the tank size in your truck. The PCM converts that voltage to a corresponding percentage value and sends this to the cluster for display.

So if you see on the serial data line (with obd2 adapter) that the PCM is sending say 50% to the cluster fuel gauge and the gauge reads something else then it may be assumed the gauge is at fault.

I think the low fuel light is dependent on gauge position, not any separate signal from the PCM. I believe this as a result of some benchtop testing I have done with a spare PCM & cluster. Watch here as I short out the wires from the PCM that would go to the sender. The low fuel light doesn't come on until the needle is near zero while the shorted wires remain unchanged.

2002 TrailBlazer Cluster Gas Gauge

The early models had the fuel level sender circuit open or high resistance when full. At some year they reversed this I have read and the sender sent a close...

youtube.com

I had a rather basic scantool. On top of that I have no idea what Im looking at.

These were taken at idle with 1/4 tank showing ?(if anything here is even applicable)

If that calc load line is the fuel level signal, it was at 49% when the vehicle was first started, then gradually fell down as I sat here in the driveway. As Im witing this it fell further and has settled at 25.9%

 

[TJBaker57]

No. The Calc Load is calculated engine load. Not anything to do with fuel level. Fuel level is almost certainly NOT going to be available on a simple code reader/ basic scantool.

 

[budwich]

from a quick read of your manual for the 5210, it should have a list of "custom pids" that should show fuel... maybe. There is a "existing pid" (which I think is what you posted) and "custom pids" which you can select from a list. Just my quick read though.

oops never mind... it looks like "custom" only refers to what is on the screen, not a selection from a list of "available pids"... :-( You need to spend $25 to get torque and and elm interface to do everything your reader does plus more.

 

[Zmoney]

from a quick read of your manual for the 5210, it should have a list of "custom pids" that should show fuel... maybe. There is a "existing pid" (which I think is what you posted) and "custom pids" which you can select from a list. Just my quick read though.

oops never mind... it looks like "custom" only refers to what is on the screen, not a selection from a list of "available pids"... :-( You need to spend $25 to get torque and and elm interface to do everything your reader does plus more.

Lmfao


Anybody have a recommendation for something with the necessary livedata thats not going to be a crazy expensive snapon tool?

 

[Zmoney]

from a quick read of your manual for the 5210, it should have a list of "custom pids" that should show fuel... maybe. There is a "existing pid" (which I think is what you posted) and "custom pids" which you can select from a list. Just my quick read though.

oops never mind... it looks like "custom" only refers to what is on the screen, not a selection from a list of "available pids"... :-( You need to spend $25 to get torque and and elm interface to do everything your reader does plus more.

Oh! You are suggesting something. Im not familiar with one of those. Torque is an app?

 

[Zmoney]

from a quick read of your manual for the 5210, it should have a list of "custom pids" that should show fuel... maybe. There is a "existing pid" (which I think is what you posted) and "custom pids" which you can select from a list. Just my quick read though.

oops never mind... it looks like "custom" only refers to what is on the screen, not a selection from a list of "available pids"... :-( You need to spend $25 to get torque and and elm interface to do everything your reader does plus more.

Okay so.

One of these?

https://www.amazon.com/dp/B07CP5ZJVQ/?tag=gmtnation-20

 

[Zmoney]

No shit. Last I checked (probably 10 years ago) things like this were all garbage.

I ordered this one just now

Amazon.com: Bafx Products - For Android Only - Wireless Bluetooth Obd2 Scanner Diagnostic Code Reader & Scan Tool - Scan, Reset & Clear Car Check Engine Light : Automotive

Buy Bafx Products - For Android Only - Wireless Bluetooth Obd2 Scanner Diagnostic Code Reader & Scan Tool - Scan, Reset & Clear Car Check Engine Light: Code Readers & Scan Tools - Amazon.com ✓ FREE DELIVERY possible on eligible purchases

www.amazon.com

Guess I'll report back when it arrives.

Thanks all who've been firing away with ideas so far. Its great to see a forum with a little juice left in it. Seems like Trailvoy is dead these days...

 

[TJBaker57]

Drove 30 miles and was at 3/4 of a tank again.

Kept driving and I'm now reading out 115 miles on half a tank - as if the gauge is just not factoring in that first quarter tank.

So I think its not a second bad sending unit having the exact same issue. Its either the gauge itself (which doesn't seem right)

Thoughts or ideas on how to proceed?

So having looked at this again.... and checking my own gauge... I came to realize I just about never pay much attention to the gauge beyond "full,,, around half,,, getting low"!! I just don't. I know these gauges are not designed for a high level of precision. They just aren't.

I have had mine since 2010 and I record every fillup in a spreadsheet (it allows far more possibilities than any app ever will). So I have a good handle on how far a tank will go and I watch the trip miles.

All that aside... if one is interested in a PID for fuel tank level here is what I use for a 2002 with 4.2 LL8.

 

[Zmoney]

So having looked at this again.... and checking my own gauge... I came to realize I just about never pay much attention to the gauge beyond "full,,, around half,,, getting low"!! I just don't. I know these gauges are not designed for a high level of precision. They just aren't.

I have had mine since 2010 and I record every fillup in a spreadsheet (it allows far more possibilities than any app ever will). So I have a good handle on how far a tank will go and I watch the trip miles.

All that aside... if one is interested in a PID for fuel tank level here is what I use for a 2002 with 4.2 LL8.

View attachment 100571


View attachment 100569View attachment 100570

I appreciate that. What app is this?

See the thing is I'm not interested in a precise and gradual lowering of my gauge reading so much as the fact that it says empty when there is still 5-6 gallons left in the tank. I can live with the bad reading ultimately - BUT:

Combining this with some of my other symptoms (namely that most of the time it shifts into second effortlessly for an entire drive - regardless of the amt of throttle applied - but sometimes hesitates to the point of lurching forward at a noticably higher rpm shift than normal) I'm seriously wondering if my pcm might be failing, or maybe have dirty contacts or something.

I've ordered one of these data port inserts and as of now I have my negative terminal disconnected to see if after a reset the gauge is still showing the same. Might pull the connectors from the pcm to check for corrosion for the heck of it.

 

[TJBaker57]

I appreciate that. What app is this?

See the thing is I'm not interested in a precise and gradual lowering of my gauge reading so much as the fact that it says empty when there is still 5-6 gallons left in the tank. I can live with the bad reading ultimately - BUT:

Combining this with some of my other symptoms (namely that most of the time it shifts into second effortlessly for an entire drive - regardless of the amt of throttle applied - but sometimes hesitates to the point of lurching forward at a noticably higher rpm shift than normal) I'm seriously wondering if my pcm might be failing, or maybe have dirty contacts or something.

I've ordered one of these data port inserts and as of now I have my negative terminal disconnected to see if after a reset the gauge is still showing the same. Might pull the connectors from the pcm to check for corrosion for the heck of it.

regarding the late shifts.... I don't know a whole lot about trans shift criteria. I have heard of flaky vss signals maybe causing a late shift.

The app is Torque Pro but any app that allows for custom PIDs can use these parameters. OBD Fusion, Car Scanner for a couple others.

 

[Zmoney]

regarding the late shifts.... I don't know a whole lot about trans shift criteria. I have heard of flaky vss signals maybe causing a late shift.

The app is Torque Pro but any app that allows for custom PIDs can use these parameters. OBD Fusion, Car Scanner for a couple others.

Oh man. You're just a gold mine of tips.

I'll check on that sensor too. Ive never had any related codes or issues with cruise control or anything, but who knows.

Thank you!

 

[budwich]

I believe the "fuel level" pid works reasonably well. I do watch it regularly against the gage and the correlation in my 2008 seems good. I would certainly watch the PID as my first real check if I was looking to determine where a problem might exist in the fuel monitor system well before pointing the finger at the PCM.
Further on your shift problem, that could be related to lots of issues unrelated to any PCM problem. You need to do some significant data gathering to narrow down the possibilities.

 

[Mooseman]

PCM failures are few and far between and are usually an "all or nothing" deal but will throw codes for internal failures when it's a walking wounded.

For the transmission shifts, what has its maintenance been like? Fluid level good? Condition? Doubt that the PCM would send bad shift commands however the tranny might have an issue like a sticking or bad solenoid.

For the fuel level, did you install the correct fuel pump? There are two possibilities, one for SWB and the other for LWB. I think the major difference is for the level sender.

 

[Zmoney]

PCM failures are few and far between and are usually an "all or nothing" deal but will throw codes for internal failures when it's a walking wounded.

For the transmission shifts, what has its maintenance been like? Fluid level good? Condition? Doubt that the PCM would send bad shift commands however the tranny might have an issue like a sticking or bad solenoid.

For the fuel level, did you install the correct fuel pump? There are two possibilities, one for SWB and the other for LWB. I think the major difference is for the level sender.

Delphi FG0516 Fuel Pump Module

The tranny was a rebuild installed within 30k miles ago but that was now almost 6 years ago. I read this and decided to give a check. Fluid looks great, but I had a bitch of a time getting a good reading. I saw a low dry spot and decided to add half a quart. It actually seems to be shifting more smoothly so maybe that's all that was.

So having looked at this again.... and checking my own gauge... I came to realize I just about never pay much attention to the gauge beyond "full,,, around half,,, getting low"!! I just don't. I know these gauges are not designed for a high level of precision. They just aren't.

I have had mine since 2010 and I record every fillup in a spreadsheet (it allows far more possibilities than any app ever will). So I have a good handle on how far a tank will go and I watch the trip miles.

All that aside... if one is interested in a PID for fuel tank level here is what I use for a 2002 with 4.2 LL8.

View attachment 100571


View attachment 100569View attachment 100570

This isnt working for me somehow.

 

[Chickenhawk]

Two things. The Delphi FG0516 is listed for the XL version. Your profile lists a Trailblazer LT. If you have an LT, the correct number is Delphi FG0515.

The other thing is the possibility of a stepper motor. I also believe that the low fuel light works off the gauge position, not the sender unit position. I know that sounds weird, but I found this out when my stepper went bad on my speedometer and I found there is a "vehicle overspeed" warning in the cluster that works off the gauge position, not the output, when I was going 80 KPH and the gauge was registering 210 KPH.

 

[Zmoney]

Two things. The Delphi FG0516 is listed for the XL version. Your profile lists a Trailblazer LT. If you have an LT, the correct number is Delphi FG0515.

The other thing is the possibility of a stepper motor. I also believe that the low fuel light works off the gauge position, not the sender unit position. I know that sounds weird, but I found this out when my stepper went bad on my speedometer and I found there is a "vehicle overspeed" warning in the cluster that works off the gauge position, not the output, when I was going 80 KPH and the gauge was registering 210 KPH.

Can you please show me where you found that listing? I got mine from ebay (new in box) but it shows compatibility being fine with both wheel bases. Rockauto shows the 515 as compatible with the 2005-2006 "Trailblazer" and 516 shows 2005-2007 "Trailblazer" (makes no mention of trim)

Both list alternate/oem part number 19153374 but then also a string of 5 more part numbers that don't match.

As to your second point, I actually just got done plugging in a spare cluster I have, which is giving me the same fuel level reading.

 

[Zmoney]

Two things. The Delphi FG0516 is listed for the XL version. Your profile lists a Trailblazer LT. If you have an LT, the correct number is Delphi FG0515.

The other thing is the possibility of a stepper motor. I also believe that the low fuel light works off the gauge position, not the sender unit position. I know that sounds weird, but I found this out when my stepper went bad on my speedometer and I found there is a "vehicle overspeed" warning in the cluster that works off the gauge position, not the output, when I was going 80 KPH and the gauge was registering 210 KPH.

SON OF A %*$%# you're absolutely right!!!

Wellp. I guess you've all collectively talked me off the edge.

I'll report back in however long it takes for me to get a new pump and get this one refunded!

Thanks everyone

 

[Tankcruiser]

they have been generic with parts lately and taken more time to double or triple check things i did same thing with fuel pump listing said for my ext NOT! so returned and correct fuel pump arrived with 100 bucls added to the billing.... this is an notice to all for obtaining correct parts anywhere ecspecially gmt360 365 platforms.....

 

[Zmoney]

Oh yeah. I just got through a week's long slow reaming of some poor kid in customer service. The fit check chart says its compatible with every single 360 platform model trim type with a 'disclaimer' (nit even a complete sentence) in the notes section (which is collapsed and doesnt show this note until clicked) saying simply '129" wb' which can only be found countained at the end of a generic paragraph psa about the importance of cleaning your tank and a recommendation of a delphi cleaning kit.

It took 4 messages back and forth to explain the redundancy of that chart's setup and they agreed to the refund.

Looks like they then updated the chart but the only change they made was moving '129 wb' to the front of the notes paragraph (so at least you can see it when the notes section is still collapsed.) Still lists more than half of its compatible vehicles as incompatible models that dont come in a 129" wb

On that note, correct module is in. Going for my first commute now. Hopefully this was my issue the whole time.

 

[Tankcruiser]

ohh jeez good luck

 

[Zmoney]

Does rezzing work?

 

[Zmoney]

I'm baaa-ack!

Ok guys here's my status update and plea for more gracious assistance:

After the absolutely correct Delphi sending unit went in, I've still got the issue.

Rough idle was fixed by another TB cleaning, hard 1-2 shift was fixed by new motor mounts.

I got an obd Bluetooth reader and between torque and garagepro hardly any of the live data works (clears codes and works fine on my Jeep so I know the reader itself is good)

Just stopped by my trusted mechanic and his snapon scanner confirms at dash-displayed empty, the sending unit is showing 20% left in-tank.

Ive already tried a cluster swap-out (bought one a while back just for a lens replacement) and that still showed an erroneous fuel level.

Could it be a wiring issue? PCM failure?

My mechanic is a solo guy with work up to his eyeballs and is totally unwilling to invest the time for an issue that doesnt have anything to do with driveability, performance or safety. Can't say I blame him but I've got this thing running like a purring kitten and its eating me alive inside not being able to iron out this last issue.

Also, if anyone can point me to a platform-specific guide for the obd app(s) (or another option that actually works) I'd be incredibly grateful.

 

[Mooseman]

hard 1-2 shift was fixed by new motor mounts.

That's a new one to me but I could see it.

Could it be a wiring issue? PCM failure?

Why do people always go for PCM failure? 99% of the time, it's something else. Wiring or a bad ground? Possible. A bad sending unit? Possible. Delphi has been garbage lately with another member having to replace his new Delphi pump due to no pressure.

However, you would have to check what resistances you're getting from this sending unit. There should be an ohms vs level testing chart in the manual.

One thing that has been found out in the other thread is that programming in 05 PCMs was changed for fuel sender readings and was reversed as the fuel sender was changed. 02 to 04 PCMs will work in an 05 but the fuel level will be reversed. Same thing if you put an 05 PCM in an older truck. So if you swapped your PCM, check on that.

 

[Zmoney]

I haven't swapped the pcm. But this is now the 3rd delphi sending unit (latter two were both brand new factory sealed) that is presenting the same issue. (Scan tool says lots of fuel left, so do my pump receipts) dash cluster (two different assemblies tried) is way off. My money is on some kind of wiring issue from the pcm to the dash, but I don't know where to begin.

Truck didn't always have this issue. It sat for about a year though and then started showing the wrong fuel level.

Recap from earlier in the thread: at fill up it shows full. Then within 25-50 miles it'll nosedive to 3/4 tank then drop at a normal rate. Always says its empty when about 5 gallons are left. Sending unit arms aren't bent and the tank is clean and free of obstructions.

 

[TJBaker57]

Delphi FG0516 Fuel Pump Module

The tranny was a rebuild installed within 30k miles ago but that was now almost 6 years ago. I read this and decided to give a check. Fluid looks great, but I had a bitch of a time getting a good reading. I saw a low dry spot and decided to add half a quart. It actually seems to be shifting more smoothly so maybe that's all that was.

This isnt working for me somehow.

Aha!!!

Since this was posted I had the opportunity to do more research and if I'm right in 2004 they dropped the 1155 PID and instead implemented the equivalent SAE standard PID which is 002F. They kept this in 2005 but then in 2006 went back to the 1155 for that year but then in 2007 changed back again to match the 2005!!

So if you still have that PID definition in Torque Pro go in there and change the "1155" to "002F" and let's see what that gets us.

 

[Zmoney]

Forgive me, but I need a little more hand holding than that. Im very tech inclined but never before used a scantool app.

 

[TJBaker57]

Forgive me, but I need a little more hand holding than that. Im very tech inclined but never before used a scantool app.

Ok, I see in your screenshot back in post #20 on Jun 9 2021 that you had entered the PID setup in Torque Pro as was suggested by me several posts before that at post #14. It did not work for you then.

Assuming you have not deleted the PID you set up back then you can edit that existing entry. The only thing that needs changing is the field "mode and PID". Change it from "22115501" to "22002F01". Save the setup at the bottom of the page.

Do you remember where to go to do this edit?

 

[budwich]

Ok, I see in your screenshot back in post #20 on Jun 9 2021 that you had entered the PID setup in Torque Pro as was suggested by me several posts before that at post #14. It did not work for you then.

Assuming you have not deleted the PID you set up back then you can edit that existing entry. The only thing that needs changing is the field "mode and PID". Change it from "22115501" to "22002F01". Save the setup at the bottom of the page.

Do you remember where to go to do this edit?

although this might help with the OP getting a "better" value from the PCM, I don't think it address his "visual" on the gage which he seems to have confirmed with another reader. I don't know how the steppers work (checked) but it would appear to be some form of "translation issue" to the gage (which is data driven). Somewhat related, have you ever seen a low fuel warning? IF so, at what level? (another conversation with GM a long while ago... do you see it spec'ed in any documentation? ).

 

[Tankcruiser]

check voltage at plug by tank 5volts for fuel level and wiggle wire loom carefully check and see if that 5volt is used for other sensors via diagrams

 

[Zmoney]

although this might help with the OP getting a "better" value from the PCM, I don't think it address his "visual" on the gage which he seems to have confirmed with another reader. I don't know how the steppers work (checked) but it would appear to be some form of "translation issue" to the gage (which is data driven). Somewhat related, have you ever seen a low fuel warning? IF so, at what level? (another conversation with GM a long while ago... do you see it spec'ed in any documentation? ).

Low fuel light comes on at the moment the needle hits empty (on both clusters)

 

[Zmoney]

Ok, I see in your screenshot back in post #20 on Jun 9 2021 that you had entered the PID setup in Torque Pro as was suggested by me several posts before that at post #14. It did not work for you then.

Assuming you have not deleted the PID you set up back then you can edit that existing entry. The only thing that needs changing is the field "mode and PID". Change it from "22115501" to "22002F01". Save the setup at the bottom of the page.

Do you remember where to go to do this edit?

Tracking. I'll give this a shot.

update:
I doublechecked the form and Im certain I filled it out exactly as you have. Made the change to the mode and pid field and saved it. Readded it to my live data page and it just reads 0.0

 

[Zmoney]

check voltage at plug by tank 5volts for fuel level and wiggle wire loom carefully check and see if that 5volt is used for other sensors via diagrams

I'm assuming from the harness connecting into the sending unit itself?

 

[budwich]

Low fuel light comes on at the moment the needle hits empty (on both clusters)

yes, but I could never find anywhere or anyone where it is "spec'ed" to tell you when that indicator comes on, how much fuel is actually left. So, IF GM set that "point" to be at "15%" then, the gage would show "E" but the PCM would be "barking out"... 15% which might be in the range of 3-4 gallons of fuel left. It would appear to be "design intent".

side note: on my ford explorer, it was "spec'd" for how much fuel was left when the warning was activate (in "round gallons").

 

[Zmoney]

yes, but I could never find anywhere or anyone where it is "spec'ed" to tell you when that indicator comes on, how much fuel is actually left. So, IF GM set that "point" to be at "15%" then, the gage would show "E" but the PCM would be "barking out"... 15% which might be in the range of 3-4 gallons of fuel left. It would appear to be "design intent".

side note: on my ford explorer, it was "spec'd" for how much fuel was left when the warning was activate (in "round gallons").

Fair enough. The reason I first got into this crusade was that it didnt used to come on so early. Either that's true or I'm nuts. I'll figure out which eventually, I hope.