Useless Information

[George B]

Picked up an Autool SDT-206 smoke machine finally from a pawn shop selling on Ebay. Finally got it cleaned up and tested and put it to work on the Yukon. Disconnected the passenger side PCV hose going into the air plenum before the throttle body and inserted the smoke at about 3 psi. Held it there for a minute or two and nothing, no leaks. Popped the top of the purge solenoid hose on top of the intake right behind the throttle body and inserted the smoke again, nada. So back to square one looking for the source of those high negative fuel trims at idle.

Your looking for a vacuum leak to find a negative trim? Wouldn’t a negative trim mean the pcm is pulling fuel due to an excess? Wouldn’t a positive trim mean a vacuum leak?

 

[swathdiver]

Your looking for a vacuum leak to find a negative trim? Wouldn’t a negative trim mean the pcm is pulling fuel due to an excess? Wouldn’t a positive trim mean a vacuum leak?

I don't think so, not if I am reading people's posts correctly. Everything I read points to a vacuum leak. Maybe I need to run the smoke machine again with a little more pressure?

The unit has another gauge to tell you how much pressure is inside the unit you are testing so I don't blow anything up.

 

[George B]

I don't think so, not if I am reading people's posts correctly. Everything I read points to a vacuum leak. Maybe I need to run the smoke machine again with a little more pressure?

The unit has another gauge to tell you how much pressure is inside the unit you are testing so I don't blow anything up.

I always understood it this way:

“How does a vacuum leak affect a non-turbo engine’s fuel trim values?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This forces the PCM to react to the lean condition with positive valued fuel trim readings. The fuel trims will return to almost normal during cruise and wide-open throttle when the throttle plate is open and there is little vacuum in the intake but lots of air flow into the engine. The small amount of un-metered air creating an idle vacuum leak is hidden or masked when the engine’s airflow increases, so the effect on fuel trim is less evident.”
REF: https://www.autoserviceprofessional...k-and-performance-issues-using-fuel-trim-data

So a vacuum leak of unmetered air causes a lean condition and the pcm adds fuel.

Am I wrong?

 

[swathdiver]

I always understood it this way:

“How does a vacuum leak affect a non-turbo engine’s fuel trim values?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This forces the PCM to react to the lean condition with positive valued fuel trim readings. The fuel trims will return to almost normal during cruise and wide-open throttle when the throttle plate is open and there is little vacuum in the intake but lots of air flow into the engine. The small amount of un-metered air creating an idle vacuum leak is hidden or masked when the engine’s airflow increases, so the effect on fuel trim is less evident.”
REF: https://www.autoserviceprofessional...k-and-performance-issues-using-fuel-trim-data

So a vacuum leak of unmetered air causes a lean condition and the pcm adds fuel.

Am I wrong?

I just read the exact opposite based on our earlier conversation today. So now I'm confused and cannot say for sure what is right!

 

[George B]

I just read the exact opposite based on our earlier conversation today. So now I'm confused and cannot say for sure what is right!

Could you share a link to where you read that?

 

[George B]

You could always create a vacuum leak as a test to see which way the STFT goes. Pull the PCV and see. If I get a chance I will do it to mine tonight and share what I find. I suspect unmetered air will cause a lean condition that the PCM will correct with more fuel.

 

[swathdiver]

Could you share a link to where you read that?

I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.

Negative fuel trims at idle | EricTheCarGuy

Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.

www.ericthecarguy.com

 

[Fless]

I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.

Negative fuel trims at idle | EricTheCarGuy

Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.

www.ericthecarguy.com

Funny, I saw this, too, in a search, except what's being discussed there is a speed density system vs our MAF systems. So it doesn't apply.

EDIT: negative fuel trims (rich condition) could be caused by too much fuel pressure, a leaky injector, or a bad O2 sensor. Maybe some other things I can't think of right now.

 

[swathdiver]

Funny, I saw this, too, in a search, except what's being discussed there is a speed density system vs our MAF systems. So it doesn't apply.

EDIT: negative fuel trims (rich condition) could be caused by too much fuel pressure, a leaky injector, or a bad O2 sensor. Maybe some other things I can't think of right now.

This condition usually became less of an issue every time we torqued the intake manifold bolts down. Then we went and replaced the intake gaskets and along the way have replaced the oxygen sensors, all 4. Fuel pressure is within spec and the pump's fuel trim is 1.54 last time I checked. Spark plugs all look the same, Magnaflow cats have 53K on them now. No codes of any kind and no leaks observed with the fancy smanchy smoke machine. Maybe the motor is just getting tired?

I would like to replace the fuel pump assembly and the injectors at some point. Actually, maybe I ought to pull the injectors and clean them in my sonic machine with simple green? We use the truck too much to send them out to the injector guy for proper cleaning.

I saw another article today where the problem turned out to be two vacuum hoses connected on the wrong parts, in that person's case, different wastegate solenoids. Swapped the hoses and the trims went back to normal at idle.

 

[George B]

I think this was it. Frustrating myself because I could not find the exact articles using the history feature of the browser.

Negative fuel trims at idle | EricTheCarGuy

Eric, I’ve got an 2003 Acura TL-S which exhibits the following: At idle the fuel trims are around -10. When running at highway speeds the trims are around zero as should be expected.

www.ericthecarguy.com

Even the linked article in that article says that negative fuel trims mean too much fuel and positive fuel trims mean a vacuum leak.