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Old 12-22-2021, 05:47 PM   #1
Petey
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2004 VUE 3.5L
Default Fuel Trim

Over the last couple of years I've notices my fuel economy dropping slightly (from around 30mpg down to 26/27 mostly local) at first I thought it was my trips were shortened by a lot by now they are back up and still its lower.

I started looking at fuel trims today with my scanner.

At operating temp vehicle parked

Idle
Short term - between -2 to +2
Long term - around -18

2K and 3K RPM
Short Term - -2 to +2
long term - around -10

I have not really memorized what trims are when I'm driving but if I recall I think the long term was around -8

Why is the short ok but the long showing rich?

The plugs are NGKs with only 12K on them.
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Old 12-23-2021, 10:46 AM   #2
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Default Re: Fuel Trim

Look for exhaust leaks where the head pipe is clamped to the bracket under the oil pan or the collector flange after the cat.

My LTFT’s are -3 to -10 driving down the road and I still get 42-47mpg.
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Old 12-23-2021, 02:11 PM   #3
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2001 SL1
Default Re: Fuel Trim

Rich fuel mixture can be caused by ...
  • failing ETCS
  • fouled ETCS connector
  • Incorrectly spliced ETCS connector (should be soldered)
  • failed thermostat
  • vacuum leak (lines and intake manifold)
  • exhaust leak before the rear O2 sensor

The tell for the first 4 above would be the temp gauge reading low (1/4 of below for a GEN2). The PCM is receiving a low engine temp reading and is increasing fuel to increase the operating temperature. If not resolved then secondary problems result from increased carbonization in the throttle body, EGR, engine, exhaust manifold and catalytic converter. What is you temp gauge reading?

I'm sure a missed a couple of things on that list.

The correct gauge reading for your 97 model at operating temperature would be similar to the photo below.

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Last edited by trottida; 12-23-2021 at 02:18 PM.
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Old 12-23-2021, 02:45 PM   #4
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2004 VUE 3.5L
Default Re: Fuel Trim

Quote:
Originally Posted by trottida View Post
Rich fuel mixture can be caused by ...
  • failing ETCS
  • fouled ETCS connector
  • Incorrectly spliced ETCS connector (should be soldered)
  • failed thermostat
  • vacuum leak (lines and intake manifold)
  • exhaust leak before the rear O2 sensor

The tell for the first 4 above would be the temp gauge reading low (1/4 of below for a GEN2). The PCM is receiving a low engine temp reading and is increasing fuel to increase the operating temperature. If not resolved then secondary problems result from increased carbonization in the throttle body, EGR, engine, exhaust manifold and catalytic converter. What is you temp gauge reading?

I'm sure a missed a couple of things on that list.

The correct gauge reading for your 97 model at operating temperature would be similar to the photo below.

Thanks!

The CAT assembly good and clean with no leaks and temp runs at 3/8.

I'm going to run Techron through it and take the throttle body off and clean it because once in a while it will idle at 1500 at a stop light unless I blip the throttle. I am not sure if the IAC or the plate is getting stuck, which is the reason why I pugged the reader into it in the first place to discover the fuel trim. I will also check on the MAP sensor too.

I've owned this car for the last 20 years and it always had the typical 97 fuel pressure regulator problem (hard start when warm, unless you leave the key on to prime the pump). I always thought that was a pressure regulator problem but now I find there is one the fuel rail too. Should I look at that also?
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Old 12-23-2021, 03:46 PM   #5
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Default Re: Fuel Trim

Take compression readings first. That is easier to do than anything else, and no amount of "high tech" fussing is going to do any good if one cylinder is sick.
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Old 12-23-2021, 04:05 PM   #6
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Default Re: Fuel Trim

Quote:
Originally Posted by billr View Post
Take compression readings first. That is easier to do than anything else, and no amount of "high tech" fussing is going to do any good if one cylinder is sick.
Will do that, but I doubt that's the problem. Its only got 50K on an engine re-ring job.
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Old 12-23-2021, 08:43 PM   #7
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Default Re: Fuel Trim

Hi Petey-

I bought a '97 DOHC with 150kMi (2008), and drove it until skipped
timing chain (2018) with 230kMi.

I became suspicious of the Fuel Pressure Regulator around 170kMi (2013).

OldNuc commented in a thread, that "the FPReg 'sticks' after they age".

I bought a Harbor Freight fuel pressure gage kit, and tested the pressure
at idle, and while varying vacuum input. The pressure varied with vacuum
within specified range. I remained suspicious, and renewed the FPR.
That part replacement was one of the best things I did for the car -- it
stopped some variation in fuel consumption, and ran better. It was the regulator in/on the fuel rail.
For a while an adjustable version was available -- but i think the wear issue involves
an internal 'rubber' seal ring, and wear on the ID of the steel chamber within which it oscillates.

I think the best test of fuel regulation is While Driving under changing conditions ... which I did not do.

My '97 never had that issue with low fuel pressure at startup....
* *

I recall your rebuild some years back....

Last edited by TomM96; 12-23-2021 at 08:49 PM. Reason: fergat un
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Old 12-23-2021, 09:49 PM   #8
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Default Re: Fuel Trim

Quote:
Originally Posted by TomM96 View Post
Hi Petey-


My '97 never had that issue with low fuel pressure at startup....
* *

I recall your rebuild some years back....

Mine had this problem from the day I brought it with 80K, at 110K I found out that Saturn had extended the warrantee to 100K on the pump it was so common.

Cold starts are fine, and warm starts are fine if the car has not sat for more than 10min. I'd say between 10min to 45min after it was warmed up I have to turn to the key run until I hear the pump relay click before I start it.

Only other problem I have is if I start it cold to just move the car around without at least driving around the block it will flood over the next time I start the car. I then have to put it in clear flood mode to start it and it will miss for a min or so.


I did manage to get out for a higher speed drive today, LTFT at high speeds were down to -4, but as soon as I got into stop and go driving it was back up at -10
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Old 12-26-2021, 06:51 PM   #9
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Default Re: Fuel Trim

Compression on all 4 cylinders are between 190-195.

34psi at idle when I unplugged the regulator it when up to 44psi, then dropped right back down when I plugged it back in.

As soo as the pump is shut off the pressure immediately drops to 0 but the car will start right back up.

While the car is warming up the LFT will be -4 and SFT will be -27 after is warmed up the SFT will drop to -3 or lower but the LFT will hang around -18 if idling or -10 if at anything over an idle. (those #'s are all parked with no engine load)

Do I start looking for a leaking injector, map senor, o2 (Post cat only monitors the CAT or does it control fuel too?) There are no exhaust leaks either.
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Old 12-26-2021, 10:51 PM   #10
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2001 SL1
Default Re: Fuel Trim

Your fuel pressure is low. It should be ....

Quote:
if everything is correct you will see 45-51psi key ON engine of or running any RPM or load. Same for cranking.
Here is the fuel system diagnostic but keep in mind that this may be for a GEN3. I grabbed it from a post that was for a 2002 model and there are no markings indicating if it is for the pre 1998 system like yours or the 98-02 fuel system ....

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Old 12-27-2021, 12:05 PM   #11
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Default Re: Fuel Trim

Petey, discovering fuel pressure went back up to acceptable values when disconnecting the pressure regulator, can you drive your car this way to monitor fuel trims? This may be one way to determine if lower fuel pressure is affecting overall mileage. Lower than normal fuel pressure suggests the injectors have to switch on for longer duration to make up for lower pressures. Since the upstream O2 sensor monitors oxygen in exhaust, the pcm determines a richer mixture needs leaning out with negative numbers. This is a little confusing to me but nevertheless your fuel mileage reflects higher than normal fuel consumption. One or more injector leaks may be a possibility too. Examining spark plugs may reveal if an injector is leaking with dry black deposits. Wet black deposits would be presumed as excess oil burning.

Here's a link to understand fuel trims; https://www.aa1car.com/library/what_is_fuel_trim.htm
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Old 12-27-2021, 02:09 PM   #12
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2004 VUE 3.5L
Default Re: Fuel Trim

Those number are for the 98+ system.

From what I found these are for the 97 and older.
My Chilton's not where I am right now and I have not fired up the old computer to look at my GM SI program to confirm.

http://www.saturnfans.com/forums/sho...90&postcount=2

At Idle:

31 to 36 psi
37 to 45 psi (vacuum hose removed)
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Old 01-02-2022, 03:30 AM   #13
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Default Re: Fuel Trim

It was possible to disassemble the FP-Reg on my '97.
When i examined the interior, it was clear that the seal o-ring
had a certain path of movement ... worn in the steel chamber.
Old Nuc said they stick - rather than continuously adjust - after
10/20 years or 100/200kMi. He suggested that I buy the type of
regulator which were adjustable, as indicated by a screw rather
than rivet on the cannister. My experience was that the adjustable
models were no longer in trade -- probably a robot adjusts pressure
and locks it in.

I do have the Helm/OEM FSM for model year '96... i could check the
pressure specs (but your numbers match my bad memory). When mine
measured properly with/without vacuum, i Assumed it was Lying.
Replacement of the FPR was gratifying success.

I haven't yet scanned trim as of yet.
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Old 01-09-2022, 02:26 PM   #14
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Default Re: Fuel Trim

Quote:
Originally Posted by Petey View Post
I did manage to get out for a higher speed drive today, LTFT at high speeds were down to -4, but as soon as I got into stop and go driving it was back up at -10
My LTFT is anywhere from -3 to -10 driving down the road. Idle LTFT is anywhere from -6 to -18 and my manual 2002 SL is getting 42mpg in the dead of winter.

When was the last time you did a tuneup and ran a quality injector cleaner through it?

Too much EGR May also cause your problem…..

When you pulled your spark plugs how did they look? Were all four the same color?
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Old 01-09-2022, 10:23 PM   #15
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Default Re: Fuel Trim

Quote:
Originally Posted by 1996SL11.9L View Post
My LTFT is anywhere from -3 to -10 driving down the road. Idle LTFT is anywhere from -6 to -18 and my manual 2002 SL is getting 42mpg in the dead of winter.

When was the last time you did a tuneup and ran a quality injector cleaner through it?

Too much EGR May also cause your problem…..

When you pulled your spark plugs how did they look? Were all four the same color?
Plugs all looked the same and have 12K on them. It has been a while since I have run injector cleaner in it, but right now it has Cataclean in it. I graphed the front and rear O2 sensors and I see the rear is not a flat line and starting to mimic the front's graph. I'm hoping this will help it, I guess you get what you pay for. When I did the ring job I also replaced the cat because the OE flex was starting to rot. It has about 50k on it.

Maybe the Cataclean will clean the injectors, sensors, and cat.
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Old 01-17-2022, 07:11 PM   #16
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Default Re: Fuel Trim

I was able to take the car on 30min highway drive today after the Cataclean has been in there for 1/2 tank.


When I am on the throttle (anything more than a coast) my LTFTs are at -4,-5 STFTs are at or around 0

If I coasting either down hill or slowing down LTFTs go to -7 to -15 (one time -28) STFTs at 0

I did notice the #2 o2 sensor to be a steady line when cruising on the highway or idling (for the most part). It would bounce around when the RPMs would change though.

It was too cold here this weekend for me to get the throttle body off stick a camera in there to see if I can see (a) dripping injector(s) under pressure. If I have to go new injector route, should I have mine tested and cleaned or a get a rebuilt set from ebay?

Should I get a new FPR either way? I know this one is 25 years old w/ 210K on it.
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Old 01-17-2022, 07:53 PM   #17
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Default Re: Fuel Trim

Are you aware that front and rear O2 sensors operate differently from each other as well as have different priorities? The intake manifold/upstream of the catcon sensor fluctuates/cycles many times per second and seen better with an oscilloscope unless a good digital multimeter with a fast acting bar graph displays the rapid cycling. This sensor is more important since it's the prime sensor used by the pcm to control stoichiometric fuel mixtures for emissions control. Output between 0.4v-0.8v is considered optimum. When cycling correctly, one voltage tells the pcm to enrichen then the other voltage tells the pcm to lean mixtures to maintain as close to 14.7:1 air/fuel mixtures for tight emissions control.

The rear sensor/downstream of the catcon is used to monitor catcon efficiency. Its output is a steady voltage unlike the upstream sensor. When age, contaminants, excessive unburned fuel and other influences occur to diminish catcon efficiency, the rear sensor outputs a signal the pcm sees as a failing one to generate a catcon error code. In effect, the rear sensor is the tattle tale telling on the catcon when it becomes less efficient in converting exhaust gases to harmless pollutants.

https://www.aa1car.com/library/o2sensor.htm

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Old 01-17-2022, 08:03 PM   #18
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Default Re: Fuel Trim

Quote:
Originally Posted by fdryer View Post
Are you aware that front and rear O2 sensors operate differently from each other as well as have different priorities? The intake manifold/upstream of the catcon sensor fluctuates/cycles many times per second and seen better with an oscilloscope unless a good digital multimeter with a fast acting bar graph displays the rapid cycling. This sensor is more important since it's the prime sensor used by the pcm to control stoichiometric fuel mixtures for emissions control. Output between 0.4v-0.8v is considered optimum. When cycling correctly, one voltage tells the pcm to enrichen then the other voltage tells the pcm to lean mixtures to maintain as close to 14.7:1 air/fuel mixtures for tight emissions control.

The rear sensor/downstream of the catcon is used to monitor catcon efficiency. Its output is a steady voltage unlike the upstream sensor. When age, contaminants, excessive unburned fuel and other influences occur to diminish catcon efficiency, the rear sensor outputs a signal the pcm sees as a failing one to generate a catcon error code. In effect, the rear sensor is tattle telling on the catcon when it becomes less efficient in converting exhaust gases to harmless pollutants.

Yes I know the #1 is used for fuel management and #2 is only there to monitor the Cat. I know an inefficient cat has nothing to do with fuel mixture

I just discovered when I started looking into this problem that they graphs for both matched. I was told once that should not be with a working CAT. If you graph the voltage return of both of them an put them on one graph one should go up and down (rich-lean) and the other should be steady. Is this info wrong?
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Old 01-18-2022, 12:20 AM   #19
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Default Re: Fuel Trim

Using a digital multimeter may not display a very fast cycling precat O2 sensor as the dvm is slower reacting than switching speeds of O2 sensors. This would appear as a steady voltage reading on some/most dvm. A higher quality dvm may have better electronics to measure and display the lower and upper voltages of O2 sensors. Oscilloscopes (tube type or modern solid state ones) are designed to capture sine wave signals for display much more accurately than multimeters. Speeds as high as 100mhz. Not everyone has or is able to use oscilloscopes. The link describes O2 sensor operation and may have a drawing showing the sine wave signal of upstream (from the catcon) O2 sensors. Technically, using any oscilloscope requires understanding setting the time period, voltage level and trigger point for it to capture and display cycling sensors with varying frequency rates (slow to high switching rates).

The rear sensor simply measures catcon output as the converted exhaust gases simply becomes a steady heat source and steady oxygen output left from catalytic reaction/conversion whereas the upstream sensor is measuring raw combustion gases (unused oxygen) as they leave each cylinder in a closed loop feedback between pcm and sensor as the pcm varies fuel mixtures.

In perspective, if understanding O2 sensor operation and fuel trims isn't fully understood then it may be better to examine the easier stuff; engine compression, injector leaks, valve wear, spark plugs, vacuum leaks, etc, contributing to higher fuel consumption.

Last edited by fdryer; 01-18-2022 at 12:25 AM.
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Old 01-18-2022, 12:43 AM   #20
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Default Re: Fuel Trim

Quote:
Originally Posted by fdryer View Post
Using a digital multimeter may not display a very fast cycling precat O2 sensor as the dvm is slower reacting than switching speeds of O2 sensors. This would appear as a steady voltage reading on some/most dvm. A higher quality dvm may have better electronics to measure and display the lower and upper voltages of O2 sensors. Oscilloscopes (tube type or modern solid state ones) are designed to capture sine wave signals for display much more accurately than multimeters. Speeds as high as 100mhz. Not everyone has or is able to use oscilloscopes. The link describes O2 sensor operation and may have a drawing showing the sine wave signal of upstream (from the catcon) O2 sensors. Technically, using any oscilloscope requires understanding setting the time period, voltage level and trigger point for it to capture and display cycling sensors with varying frequency rates (slow to high switching rates).

The rear sensor simply measures catcon output as the converted exhaust gases simply becomes a steady heat source and steady oxygen output left from catalytic reaction/conversion whereas the upstream sensor is measuring raw combustion gases (unused oxygen) as they leave each cylinder in a closed loop feedback between pcm and sensor as the pcm varies fuel mixtures.

In perspective, if understanding O2 sensor operation and fuel trims isn't fully understood then it may be better to examine the easier stuff; engine compression, injector leaks, valve wear, spark plugs, vacuum leaks, etc, contributing to higher fuel consumption.

I am not really sure what you are getting at or where you are going with this post.
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