Help understanding fuel trims

[CaptainXL]

The only reason I push this fact is I don't want members to think that 210 is normal temp when 195 actually is.

There is no one set temp that the PCM prefers/accepts. Follow the link below and look at the chart I posted from Alldata. It is the scan tool parameters. Anything within the 190 - 220 temp range is considered normal depending on outside temp. Presumably 220 would be normal if it's hot outside and your towing. The temp would lean more towards 190 in the winter while under no load.

http://gmtnation.com/f25/thermostat-tune-6247/

 

[gmcman]

There is no one set temp that the PCM prefers/accepts. Anything within the 190 - 220 temp range is considered normal

I agree, the "straight up" orientation is for the owners to understand that all is good, when it hangs down a few ticks then we know the stat isn't closing or opening early.

 

[Shdwdrgn]

@gmcman - Good point about the tires... I'm trying to remember if my mileage changed around the same time or not, but I really don't know for sure. I went with some nice all-season's which are more aggressive than what I had, but were still supposed to be very good highway tires. Still, that could certainly be some of the issue, I just hope its not the *whole* reason why my mileage dropped so drastically.

@CaptainXL - I've been programming longer than half the people here have been alive, so the technical details are certainly within my grasp. And yes, I do see the point of not continuing running certain tests if other tests have failed. I'm just not grasping how *this* test would be considered critical enough to impede other emission tests, considering how infrequently this particular system even runs. In my mind, this is kind of like saying I think the oil might be dirty so I'm not going to bother monitoring the pressure any longer. Sure, both items are related to the oil, but a failure in one part does not mean its not worthwhile to also check for failures in the other part, nor does it mean that any such failed testing would be related to each other.

Oh, also of note -- The times when I have pulled my battery and/or manually cleared the codes, it usually takes a couple days before the CEL comes on again. I would assume that during this period (even if there was a P0410 pending but not yet set), the computer would be performing other testing and should have notified me if anything else failed?

 

[CaptainXL]

also of note -- The times when I have pulled my battery and/or manually cleared the codes, it usually takes a couple days before the CEL comes on again. I would assume that during this period (even if there was a P0410 pending but not yet set), the computer would be performing other testing and should have notified me if anything else failed?

Nope. The computer probably sees the problem right away after startup and is withholding further testing until this is fixed. No other tests would be done since the P0410 is encountered upon startup. It waits a few days to see if the problem will resolve itself and then sets the CEL if it doesnt resolve on its own. It's not that the PCM isn't doing any tests that might throw a CEL, it's just that there aren't any tests that have failed which trump the P0410. If they were the same or more important they would show in a scan.

This is similar to when people notice poor gas mileage before they actually get a P0128 for thermostat temp too low. Upon fixing the thermostat many people then get additional codes within a few days after.

You are looking at the situation good, just backwards.



GMCMAN: I would agree with that assesment if the dash gauge being in the middle 12 o clock position means 190-220 degrees.

 

[gmcman]

Shdw:.....what tires do you have?

 

[Shdwdrgn]

Shdw:.....what tires do you have?

Big-O bigfoot all-terrains, 265/70-16. Certainly not as aggressive as some, but they're a little better than standard radials. I really like how the tires handle in the snow, but I'll probably have to look for something cheaper next time (or wait for a good sale).

 

[gmcman]

Big-O bigfoot all-terrains, 265/70-16. Certainly not as aggressive as some, but they're a little better than standard radials.

I'm willing to bet the farm that your tires are the cause of your MPG issue.

Maybe there's somebody close that can swap a good set of highway tires for a couple weeks to compare

 

[Shdwdrgn]

These are highway tires with a little more tread. They're not off-road knobbies. Hell they're rated for 50,000 miles. They're certainly less aggressive than some of the mud&snow tires I used to run on my old car, and I never saw any difference in mileage on it over the years. I could understand maybe affecting my mileage by 1mpg or so... but not by 6.

 

[CaptainXL]

Tires isn't it. Not enough to cause the low mpg on the highway.

There is only one reason why you are getting poor mileage on the highway. A vacuum leak.

When the engine goes into closed loop the PCM regulates the amount of fuel based upon how much oxygen the O2 sensor sees. If the O2 sensor is seeing too much oxygen the PCM will increase fuel flow to keep the A/F ratio as close to 14.7:1 as possible.

This is a concept that is pretty fundamental to modern day fuel injection systems. Any competent mechanic should be able to diagnose this relatively quickly.

 

[gmcman]

Tires isn't it. Not enough to cause the low mpg on the highway.

There is only one reason why you are getting poor mileage on the highway. A vacuum leak.

A vacuum leak would cause problems, no doubt and I'm not ruling it out. Don't rule out the tires mixed with winter blend fuel.

The tire on the left is what I had, upsized to a 265/60/17 over my stock size 245/65/17 in a Latitude on the right. I lost 40-50 miles per tank and that's 2-3 MPG drop and was closer to 3. The tread is not aggressive but just the design of the tire. Just something to reference as tires definately play a role in MPG.

I was getting 220-230 per tank over 270-280.....huge difference which is why I sold them, handled real well though.

 

[CaptainXL]

Huh? How can any tire (no matter what tread pattern) give you the same city and highway mpg consistently? I think your line of reasoning is way off base here.

Fadyasha had the same issue I think about a year ago. I believe he found his intake manifold was leaking. Might want to get him onboard.

 

[gmcman]

Huh? How can any tire (no matter what tread pattern) give you the same city and highway mpg consistently? I think your line of reasoning is way off base here.

I'm not saying they can, I never said they did..... I'm saying different tires can and will play a role in MPG. Alot of tires are "Low Rolling Resistance" tires, many are not.

I owned both the tires in my above post, 17K miles on the Kumhos, 130K miles combined on cross-terrains and lattitude tours, when my mileage drops from 280 to 230 per tank I know something is wrong, and it was the tires.

Here's another look, I believe the tire on the far left is the OP's tire. Note the more aggressive pattern but not overly.

 

[CaptainXL]

Ok, point well taken. So you are saying it could be a combination of things including winter gas and tires? Ok sounds logical...however the same rolling resistance is encountered in the city and on the highway. So theoretiocally we should see a proportional drop in city as well as highway mpg. If we went by the logic you are proposing we should see both poor city and poor highway mpg. Which he doesn't

280 mpt / 20 gallon tank is roughly 14 mpg. I asssume this is city mileage? So you never drove on the highway to see if it made any difference that way? I am just failing to see how this is helping out troubleshooting.

 

[gmcman]

280 mpt / 20 gallon tank is roughly 14 mpg. I asssume this is city mileage? So you never drove on the highway to see if it made any difference that way? I am just failing to see how this is helping out troubleshooting.

I have the small tank, 17-18 gal. This was 70% HWY...60 MPH and 30% CITY.


SHDW, Did you reconnect the big fat vacuum hose under the intake baffle when you changed the plugs? Also there is another vacuum port for I believe the rear of the EXT models that faces to the PASS side on the front of the intake manifold. There is a rubber cap that tends to crack over time.

 

[gmcman]

Sorry Capt, my 230/280 miles were combined,...Kumho, Michelin respectively. I will get about 340 per tank on road trips banging the fuel light.

 

[Shdwdrgn]

@gmcman - You are correct that the picture above is showing my current tires. As for vacuum, I have a custom intake so everything is exposed... it would certainly be obvious if I had forgotten one of the two vacuum lines connected to it (and I have gone back & forth between the stock intake a couple times, there was no difference in mileage between the two). I don't have an EXT, so the other port I guess is not an issue.

I had originally reported that there was no difference between my highway and city mileage, but I think that was incorrect. There is actually about 1mpg difference... After thinking about it, I realized I had been getting 14.5mpg in town, and my highway mileage is averaging around 15.5mpg. Still a ridiculously small difference between the two. Judging by other's reports, and knowing how I drive, 14.5mpg seems pretty standard for burning off a tank in town, so I had not really questioned it until recently when I began putting on a lot more highway miles again.

FWIW, I pulled my battery cable soon after changing the plugs. My calculated mileage was 13.644 at the end of the last tank (mostly in-town, and right before I blocked off the air solenoid). I've burned through another half a tank since this weekend, mostly highway miles, and the DIC has climbed to 14.6 as of today, however its starting to look like the actual mileage may have really increased. I'm just under half a tank on the gauge, and showing 185 miles so far, which looks promising. I'll probably finish off this tank over the weekend and can get an actual calculation then. At that point I will have gone through nearly two full tanks since replacing the plugs, and the computer should have had plenty of time to adjust itself to the new changes.

 

[CaptainXL]

Also it worth to note that if your PCM for Less tune was for 89 or higher octane and you use anything less than that the PCM will retard timing in order to keep the engine from knocking. This will decrease your mileage substantially. Did you notice the reduced mileage before the PCM for Less tune?

In case you aren't confident what tune options you ordered or perhaps there was a mixup on their end, I would run some 93 octane and see if the mileage increases.

Do you still have the old PCM?

You also really need to get a vacuum gauge connected or use a scan tool and get a reading to rule out a vacuum leak.

 

[gmcman]

Shdw, you mention "calculated" miles and that tbe DIC readings, are you basing the MPG fiigures from the DIC or by how many gallons you replace with the pump?

 

[Shdwdrgn]

@CaptainXL - I did request my tune for 85 octane because it was running just fine with that previously. The only time I use anything higher is when I plan on towing. Once I get a good baseline reading for all the changes I've made recently (maybe go through one more tank of gas), I plan on swapping the old PCM back into the truck and seeing what happens. Also, I do have a vacuum gauge... what's the preferred location for grabbing a reading, and what kind of numbers should I be seeing?

@gmcman - I use the DIC to watch for any immediate changes to my mileage, but the hard numbers (like 13.644) are calculated at the pump. Also when I said I was getting between 15.2-15.8 on the highway, those were also calculated at the pump (I made a LOT of trips within a couple of weeks).

 

[CaptainXL]

Do you have a scan tool? If so you should be able to convert the absolute intake pressure into vacuum inhg.

 

[Shdwdrgn]

I have an ELM327 device I can hook up to my computer. I'll look to see if it lists intake pressure anywhere (I gotta find some better software!)

 

[gmcman]

Took this engine pic from the web, arrow is pointing to vacuum port.

I recommend a gauge since you can monitor any needle fluctuations.

 

[Shdwdrgn]

After driving home tonight I hooked up the vacuum gauge and was getting a steady reading of either 15 or 17... something kept clicking (possibly my AC compressor trying to run already) which was switching it back and forth between the two readings about every 30 seconds. That seems like a pretty good reading to me, but I don't know what what 'normal' is on the Trailblazers yet.

 

[CaptainXL]

Normal is 18 - 22 in/ hg. Seems low to me.

 

[Shdwdrgn]

Normal is 18 - 22 in/ hg. Seems low to me.

Damn, really? I assume that is for a hot engine, in park, and running at idle? Guess I'll have to break out the can of starting fluid tomorrow and see if I can pinpoint what might be leaking.

 

[CaptainXL]

Never mind. Noticed you live in colorado. You must be up there.

Inches of Altitude Vacuum

Sea level-1000 ft. 18-22
1000-2000 ft. 17-21
2000-3000 ft. 16-20
3000-4000 ft. 15-19
4000-5000 ft. 14-18
5000-6000 ft. 13-17

MOTOR Magazine Article | MOTOR Information Systems

Also, have you checked fuel pressure, changed fuel filter, 02 sensor, plugs and cleaned throttle body? Checked for leaking fuel pressure regulator?

Assuming the engine is in order, tires are properly inflated and brakes or hubs arent dragging then I think we need to start looking at the transmission. Specifically is it shifting into overdrive and is the torque converter locking like it should.

 

[Shdwdrgn]

Ah I didn't even think about the altitude! Yes I'm right about 5000 feet, so apparently I'm right where I should be. Well that's some good news at least.

The fuel filter may be a good point... I don't even know where mine is, and I've put about 26,000 miles on the vehicle since I bought it. Anyone know off-hand what the replacement schedule is for them?

Forward O2 sensor and plugs have all been replaced, TB has been cleaned recently, and no obvious signs of fuel leaks. Transmission was replaced by the dealership at my insistence because the original was having obvious problems. It definitely shifts into overdrive (I've been discussing in another thread my desire to swap to 4.10 gears because I actually get better mileage in 3rd gear when I'm driving 55 and lower), and the TC is locking properly (which I had originally mistook for overdrive engaging).

My mileage on the DIC is still coming up about 0.1mpg every day... That doesn't seem like much, but keep in mind that this is driving in rush-hour traffic through two construction zones each way with speeds barely reaching 55mph at times -- its a wonder my mileage isn't just dropping like a rock. I'll definitely be refilling on Friday with only about 1/8 tank remaining, so I can get a good reading then, but as far as I can tell there MUST have been an improvement with the recent changes. I am currently at 220 miles on this tank, with about 1/4 tank remaining, and as I mentioned before, I usually refill my tank with only 225 miles on it.

 

[CaptainXL]

Never use the DIC to calculate mileage. It is woefully inadequate and will show confusing results. Use the refill method instead.

My guess is that when you use this method you will find you have no issue at all.

 

[gmcman]

I am currently at 220 miles on this tank, with about 1/4 tank remaining, and as I mentioned before, I usually refill my tank with only 225 miles on it.

220 miles with 1/4 tank remaining, on winter fuel, in mixed hwy/city driving?.....that's almost normal and considering the tires you have I don't think you will improve much, I assume all SWB models have the smaller tank.

I have ALWAYS only made it to 260-270 at the fuel light in mixed driving with good hwy tires, sounds like you are close to that.

 

[Shdwdrgn]

@CaptainXL - I never said I was using the DIC to calculate mileage, and just gave another explanation for my DIC usage a couple days ago. Thanks for playing, but in this case your guess is wrong.

@gmcman - it looks like I'll be at about 260 miles with gas to spare when I fill up tomorrow, so yes, this is very encouraging. If it just comes down to the single vacuum leak around the air solenoid, I can work with that. I'm still planning on getting a block-off plate once I get a chance, and I guess I should make up a new pipe for my intake that doesn't have the 1" hose fitting, just to make sure the intake is also properly sealed up.

You know, I'm starting to wonder... I see all these vehicles that push water out their tailpipe when their engine is running clean. Perhaps all the water everyone sees in their secondary air pump is coming from the exhaust side, and is a symptom of the solenoid not closing fully?

 

[BRomanJr]

@CaptainXL - I never said I was using the DIC to calculate mileage, and just gave another explanation for my DIC usage a couple days ago. Thanks for playing, but in this case your guess is wrong.

@gmcman - it looks like I'll be at about 260 miles with gas to spare when I fill up tomorrow, so yes, this is very encouraging. If it just comes down to the single vacuum leak around the air solenoid, I can work with that. I'm still planning on getting a block-off plate once I get a chance, and I guess I should make up a new pipe for my intake that doesn't have the 1" hose fitting, just to make sure the intake is also properly sealed up.

You know, I'm starting to wonder... I see all these vehicles that push water out their tailpipe when their engine is running clean. Perhaps all the water everyone sees in their secondary air pump is coming from the exhaust side, and is a symptom of the solenoid not closing fully?

Yes, that is a very real situation, (mentioned in in edit of post 24.) One byproduct of combustion is water vapor.

 

[Shdwdrgn]

Had to make another trip today, so ended up filling my tank. The results are not staggering, but they are a little better. So on my last tank I got 13.644 mpg from 222 miles. On this tank I got 15.587 from 262 miles. Considering the type of driving most of this tank saw, I suppose that's not too bad, but I was really hoping to get into the 16's. Ah well, let's see what this next tank brings, and then I'm going to guess this is the best I can get with the mods I have.

 

[CaptainXL]

You still never answered my question that I asked a while ago. Regarding your mpg how many miles is this city and how many highway?

 

[Shdwdrgn]

You still never answered my question that I asked a while ago. Regarding your mpg how many miles is this city and how many highway?

Sorry, I actually missed that one. It's impossible to give an exact figure, however my best reading of around 15.8mpg was mostly on the highway (maybe about 30 miles in-town) when I had to make several trips to Denver one week. For my regular daily commute I have about 10 miles in the 55-60mph range, 15 miles in the 40-45mph range, and a couple miles that can go smoothly or crawl (depending on the day)... There are various stoplights along the way, and I probably get around 30 miles per tank of stop&go city traffic.

It's certainly not a great drive, but its a lot better than driving nothing but city miles.

 

[CaptainXL]

You just need to do a test trip. Im not concerned about you driving to work and such and how many bumps or red light stops or coffee breaks you took, blah...blah...

How about just filling up the tank and driving 20-30 miles on the highway at 70 mph on cruise. Then fill up again and record how many miles you drove and divide the gallons you just filled up with into the miles you just ran? Simple and short test is all thats needed.

 

[Shdwdrgn]

I remember the days when I could fill up my tank for $20 and just go driving all day for fun...

These days I don't make long trips unless there's a reason. I'm sure it won't be too long before I have to make another highway trip though.