Totally stumped on stumble - any gm master techs out there?

[jbwi112]

I have a 2002 trailblazer that hesitates for one second only when you leave a stop light. It is less noticeable when a/c pump is running.

If you accelerate very slowly it does not hesitate, if you punch it it does not hesitate.

It does not hesitate while engine is cold and idle air control valve is open.

It will only hesitate off a dead idle

when you leave a stop light and accelerate, the enigne rpm's drop the engine shutters and stumbles, then it seems like the pcm catches itself and the truck takes right off. idles and runs great otherwise.
I have a check engine light on code p1345 for cam/crank correlation which I could care less about, i just need to fix the stumble

Here is what I have replaced so far, I have serviced my own vehicles all my life but this one im baffled at

fresh gas, oil, ac delco irdium plugs
2 different throttle bodies, both clean
cam positon sensor
crank position sensor
cam vvt selsnoid
gas pedal


could a bad fuel pressure regulator, fuel filter, or failing fuel pump cause this?

 

[djthumper]

Yeah, that title is how you get some ones attention to really want to help you with your first post... I highly recommend that you change it.

Have you put it onto any analyzers? What is the mileage? How long has this been going on?

 

[Mooseman]

The P1345 could be saying that something with the VVT is not jiving. After changing the crank sensor, did you do a CASE (crankshaft angle) relearn? Necessary after chaning the crank sensor. Needs a high end scanner or Tech 2 to do this. Other possibility is that the VVT cam phaser is dead or dying if you were getting this code before changing parts.

A failing fuel pump would give more issues at higher RPM as it can't keep up. Fuel pressure regulator is possible. A check with a fuel pressure gauge would rule those out.

Yeah, I changed the title for you.

 

[jbwi112]

Could a map sensor be the culprit? thanks for changing the foul language in the topic and my apologies to anyone affended


mileage is 165000, I bought the truck last week with no keys and two flat tires as a bank repo for $1000, so I have no idea how long it has been doing this.

I have a bluetooth obdii scanner which should let me look at 02 sensor readings and any other live sensor, what would I watch for?


I believe the phaser is causing the p1345 so I just moved on...

 

[Mooseman]

I doubt the MAP sensor would do that. It either works or it doesn't. However, it could be a vacuum leak but you'd think there would be other symptoms, like increased fuel consumption.

What about the O2 sensor? Was it replaced?

That's great for the OBD2 interface. With Torque, check the fuel trims. They should be as close to 0% as possible.

Repos can be dicey. Usually means no money, leading to no maintenance by the previous owner.

 

[jbwi112]

Fuel economy is right around 17 or 18. The engine starts easy and really runs out nice, very smooth operator. I didnt mess with the 02 sensors i thought they only made a difference while cruising, but if im wrong i can replace them they are cheap. ill start with the scanner tomorrow and go from there thanks

 

[Mooseman]

At that mileage, I would replace the upstream sensor only. Second one is only to report if cat is working correctly, does nothing for running engine at all.

The O2 sensor works all the time so if at idle it's lazy, it might not be reporting proper readings when you just hit the gas. Go with ACDelco only.

 

[gmcman]

I would also look for a vacuum leak, check the intake manifold bolts, do you hear any hissing around the intake anywhere?

How long has the gas been in the tank before you bought it? How many fresh tanks have you ran through it?

My goto everytime I have an issue like this is 1 can of BG44K, available from about any dealer, especially GM. It's not snake oil, this stuff really works. Not knowing the history of a vehicle I would absolutely try it. If that doesn't help after the next tank of fuel then I would try other options.

When you open the throttle from idle, you instantly increase the compression in the engine and if you don't have fresh fuel to compensate then you may get the slight stumble you're experiencing.

 

[gmcman]

Have you also tried to clean the throttle body yet? Pull the intake tube and look at the butterfly, you will know right away. When Cleaning the TB, it's best to remove it and get both sides.

Pull both ECM fuses #10 & 28 prior to starting and when you have it back together reinstall the fuses and allow it to relearn the idle.

 

[signalnc]

I had a similar issue years ago. I had just cleaned the throttle body and did not do a relearn. Once I did the relearn all was good.

 

[jbwi112]

places here in town only carry bosch or denso, so I ordered the ac delco off ebay as well as some bg44k.
I did try two different clean throttle bodies and cleared pcm after

 

[Mooseman]

By clear PCM, do you mean clearing the codes? That won't do it. You have to cut the power completely from the PCM, either by disconnecting the battery or pulling the fuses for 30 minutes.

 

[gmcman]

Just in case you dont run 87...I would stick with 87 octane until you get your issues solved.

89 is comprised of 87 and 93 and 93 tends to sit longer because of the price...best to get the freshest fuel especially if you don't know if it's fresh.

 

[jbwi112]

Got it, just pulled fuse #28 and #10 to be sure, will let it sit an hour and try it, I assume relearn takes a few on/off cycles with highway speeds?

I run 87 octane in anything fuel injected and 93 in anything with a carb

 

[Mounce]

Most people say to let it idle for something like 10 minutes with AC off and not to touch the gas.

I'm not even certain that the relearn is necessary because I (and others I'm sure) have cleaned it, reset pcm, then got straight on the road without doing the relearn and had no issues. That was before I'd even heard about a relearn procedure.

 

[24v 4.2]

It will slowly relearn by driving too.

 

[jbwi112]

issue remains after pcm reset. My buddy has my scanner and is out of town for the weekend I might swing by a shop in town and see if I can use there scanner quick to look at the fuel trims. The truck idles so smoothly at a stop light you can barely tell its running, a bad 02 sensor is starting to make more sense the more research i do

 

[gmcman]

When did you replace the crank sensor, before or after the stumble?

When did you get a P1345 code, before or after the crank sensor replacement.

 

[jbwi112]

I had a crank relearn done today, just to be sure, didnt fix stumble and p1345 came back.

I finally got my hands on a live data scanner, everything looks good as far as timing advance, throttle position, and engine temp.

Intake temp seems a bit high, its reading 114f and its 80f here today, is this normal?

Also the voltage and readings for 02 sensor 1 seem to jump all over the place consistently. the data for second sensor O2B1S2 fluctuates like sensor one

This data stays the same no matter where the throttle is SHRTFTB1S2(%) 99.2

is this right?

ECT F reads 188.8f and drops to -40.0f on stumble then goes back up to 188.8f

At idle:
SHRTFT1 -12.5%
LONGFT1 -1.6%
LOADPCT 28.2

 

[gmcman]

To test the downstream sensor, you need to be driving at highway speeds after about 5 to 10 min of warmup, then see if it follows the upstream sensor.

-12 on the STFT is alot of fuel to be pulled out.

Weird you get -40 on the ECT, makes me think of a loose battery connection, double check those. 188 is kinda low also, but see what it does when driving at highway speeds. Also, intake temps can reach that high real quick when not moving, after some outside air gets into the tube it will cool down. Using the factory airbox, no mods, 10 deg over ambient is fine at highway speeds.

Get some 44K through there then we can see if those injectors are spraying instead of possibly dribbling.

You didn't say if you replaced the fuel filter yet, I would absolutely change it.

 

[jbwi112]

battery connections are clean and tight, i was thinking bad ground on that sensor, or wait that sensor is grounded in the threads right?

yes fuel filter is getting changed tomorrow

 

[jbwi112]

interesting......

http://forums.trailvoy.com/archive/index.php/t-95543.html

maybe the engine is slighty moving on accel and messing with the wiring to the ect, then ect reads -40 and pcm goes full rich to compensate

would make sense as to why its less noticeable with a/c on cause engine is already under a load, so less twist on acceleration

 

[Mooseman]

The ECT sensor has such a failure rate and being a PITA to replace, it is usually replaced at the same time as the t-stat. Yours may be on its way out.

For the P1345, I had a persistent one that could only be fixed with the replacement of the cam phaser. It's a little involved to replace it as you need a tool or tools prevent the chain from loosening and the chain tensioner from ratcheting out while the cam gear is out.

 

[jbwi112]

I am going to boil some water, put it in a rubber made container, and dunk a new coolant temp sensor in there and see if the stumble goes away, thanks gm for putting a pigtail harness at the top of the motor... wonder why they did that


tried different coolant temp sensor, same crap
ran a bottle of seafoam into intake, same crap

I am so (editied) tired of working on this (editied) truck and never getting anywhere, i have 20+ hours invested and hundreds of dollars just trying to track down this stumble. I am willing to give up at this point, my admiration for the 4.2l engine is fading

 

[signalnc]

one more painless thing to try--disconnect battery for 20 - 30 minutes, hook the battery back up, turn the key on ( do not start the truck) and leave it for 5 or 10 minutes, then start the truck and leave it for another 5, drive it see if it clears up.

 

[gmcman]

The stumble is a loss of power. You are likely either getting an incorrect air/fuel ratio, or weak spark. Another culprit could be compression but I wouldn't go there yet.

When the 44K arrives, run the tank to almost empty then add the 44K and fill up. After that tank we can re assess the situation. If that doesn't work we can look elsewhere but I will wait for the results. Generally wait until the next regular tank is almost done to make any decisions. I wouldn't press the issue so much if I didn't swear by it.

Edited to add...I didn't see a specific answer to my 2 questions in post #18

 

[Chickenhawk]

I would still focus on the coolant sensor. It is losing the signal somehow. Instead of just placing another one in a pail of hot water, I would just replace it, along with the thermostat. Your coolant temp is too low.

The coolant temp has likely been low for a while and your cat converter is shot. This is shown by your fuel trims. The upstream O2 sensor should rapidly jump between .2 and .9, several times per second. The downstream O2 sensor should be reasonably steady.

While we don't advocate throwing parts at a problem until you find one that sticks, in my opinion, you need a new coolant sensor, new thermostat and new cat.

 

[jbwi112]

from post #4 "I bought the truck last week with no keys and two flat tires as a bank repo for $1000, so I have no idea how long it has been doing this."

If the truck always stumbled off idle I would agree on loss of power, but if I really slowly accelerate or punch it, it does not stumble at all.

My money is on bad 02, bad ground, or bad connection in the harness somewhere

dunking a new coolant temp sensor into 190f water is no different then spending 2 hours to replace it.
if the cts sensor is going to full cold -40 when i press the gas pedal just right and it stumbles, it would make sense that the pcm would richen the mixture up(hence the stumble) . Can i test the voltage for cts at pcm harness and see if it changes when the truck tumbles?

This is my fault but the fuel trims were checked in the driveway, engine not at full temp. I just need to know what data is needed from the scanner and how to properly test for it, if i need to record highway data or idling etc

new 02 sensor and bg44k will be here tues

thanks to all for the efforts to help me get this solved

would a video uploaded to youtube help?

 

[gmcman]

When did you replace the crank sensor, before or after the stumble?

When did you get a P1345 code, before or after the crank sensor replacement.

?

Sorry but this info helps.

 

[jbwi112]

let me go back to basics here. i just bought the truck two weeks ago, the battery was completely dead, p1345 code came on after two on/off cycles of ignition.

the next day I replaced cam pos sensor, crank pos sensor and cam solenoid with new geniune gm parts, then had crank relearn done. P1345 came back again after 2 on/off cycles.
im not concerned about the p1345 as long as its not related to the stumble



When did you replace the crank sensor, before or after the stumble? there is no before, it has always stumbled

When did you get a P1345 code, before or after the crank sensor replacement. there is no before, it has always had p1345

Long story short the stumble and p1345 has been there since the first time i turned the key two weeks ago and still exists after all the parts i replaced

 

[jbwi112]

one more painless thing to try--disconnect battery for 20 - 30 minutes, hook the battery back up, turn the key on ( do not start the truck) and leave it for 5 or 10 minutes, then start the truck and leave it for another 5, drive it see if it clears up.

What does this accomplish? I will try anything at this point...

 

[gmcman]

What does this accomplish? I will try anything at this point...

Basically clears all the codes and allows the PCM to relearn. So if you had the CEL with the P1345 code from day one but just took a couple cycles to show up, then we may be looking at a possible timing issue. I think Mooseman stated a VVT issue earlier and if you keep getting this code after the OE sensor swap, you may need to pull the valve cover to confirm the correct timing.

 

[gmcman]

When you pulled the cam phaser solenoid, did it have a lot of crud in the screens, was there oil in the electrical connector?

You could have a shop with a tech 2 command the VVT and see if it passes the tests. I've never had a phaser apart so I wish I could add more but they do have tight tolerances so it could have been neglected oil changes, jumped a tooth from a weak tensioner...hard to say. I don't think you have a gremlin but I'm sure we'll figure it out.

 

[jbwi112]

The screens on the vvt solenoid I took out were surprisingly clean, however there was oil in the connector, hence I replaced the solenoid and pigtail to it.
If timing was off a tooth it would effect gas mileage, poor idle, lack of power etc right?

The scanner used to do the relearn is a snap-on not a tech ii, unsure if the snap-on can command the phaser.

 

[Mooseman]

If it actually skipped a tooth, it would either run like crap, if at all, or possibly bend valves as this is an interference engine. This is not likely. I had a defective timing chain tensioner and it didn't skip. Just made knocking noise at idle only on the top front of engine.
http://gmtnation.com/forums/topic/12240-how-to-replace-i6-timing-chain-and-tensioner/

 

[jbwi112]

Installed new gm 02 sensor, and pulled fuse #10 and #28 for 1 hr, stumble remains.

I tied into the yellow wire (return signal from cts) just before it goes into the pcm. It reads ~2.5 volts and varies depending on temperature. If I press the gas just right and get it to stumble this voltage fluctuates, by .01v

Here is where I get confused. The yellow wire (from pcm) has 5.02 volts when unhooked from the sensor (shouldn't this be 0v?), to more complicate things I have a dead short from the black reference wire to ground (shouldn't the reference wire be +5v?)

I assumed the reference wire (black) provides 5v+ to the sensor and depending on resistance from temp it sends back a voltage to pcm (yellow wire) and uses the sensor screwed into the engine as a ground to vary this voltage.

I am really confused here


also if i unplug the coolant temp sensor the stumble is 99% gone, but the engine idles much higher which is expected
maybe my pcm is toast

 

[jbwi112]

Nevermind, the pcm supplies 5v to the cts and measures the drop across the low reference (ground)

 

[Mooseman]

Just grasping here. Try leaving the fan unplugged. You will get a code and CEL for it and it shouldn't overheat unless it's really hot out. Try that for a while. There have been instances of bad fan clutches shorting out the 5v reference.

 

[gmcman]

Just grasping here. Try leaving the fan unplugged. You will get a code and CEL for it and it shouldn't overheat unless it's really hot out. Try that for a while. There have been instances of bad fan clutches shorting out the 5v reference.

This sounds plausible, would be great if that's the issue.

 

[jbwi112]

Started truck up this morning, was idling out of driveway, ses light and reduced engine power light came on, started missing badly, any throttle attempt made it worse....
Barely got in back by my garage idling in reverse and me pushing it

I found 6 grounds on driver side block and firewall, removed and cleaned all, unplugged fan clutch, no change

Anyways any new symptom is good, I realized my temp gauge was not working at all now. Checked and found the low reference( for coolant temp sensor) was only -5v, tied into ground at battery and temp gauge started working again and reduced engine power now cleared and truck is back to just a stumble off idle but runs out good.

I think my pcm is bad and the low reference(ground) from the pcm to sensors are failing, maybe a cracked solder post inside the pcm. I also ran and new ground from pcm pin #64 directly to battery.

I will try a new pcm form ebay, and if it fixes it I will open up the bad one and scour it for a crack.

also the bg fuel cleaner arrived 2 days ago, only 70 miles so far but no change