Fuel Trim Values on GM4.3l

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George7941

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2006 Sierra with only 14k miles. These are fuel trim values that showed up on my Scan Tool.

LTFT B1 6.2%
LTFT B2 3.1%
STFT B1 0%
STFT B2 0%

Do these values indicate a possible intake leak?

I am getting high Silicon readings (40 at 2500mi oil change interval) in the UOA. I think the high Si is from factory gasketing since my Al values are low, but I am on the lookout for an intake leak as the possible source of the Silicon. So far I have not found any leaks but I wish I had a Smoke Machine to check for leaks.

Thanks in advance for any advice.
 
GM says the values are most likely caused by clogged/restricted fuel injectors. Switch to a top tier fuel if you haven't already.

Sorry, don't know about the high Silicon.
 
All that shows is that under recent conditions, no adjustment was made, and long term use needed a slight adjustment. But this is normal, and is why they have adaptable fuel trim variables.
They are supposed to change a bit to fine tune for widely changing conditions.
 
They are fine.

If you are concerned, disconnect the battery for a bit, and check in a few days.

Like an above poster said, they can change with dirty injectors or different quality gas (depending on alcohol content of gasoline).
 
"If you are concerned, disconnect the battery for a bit, and check in a few days."

Would you please explain how disconnecting the battery(I assume that then we start off fresh with 0 STFT and LTFT values) is going to provide new info or confirm something. I just am not following the reasoning here.
 
It's not a fix, it just is an easy way to see if the trims return to where they are now (which they probably will).

I'm not sure what the range is on GM OBD-II FT values, but when its starts to get +/- 15 or so is when I would be looking for problems.
 
Hi

These positive values seem to indicate that the pcm believes there is a lean condition and is adding fuel to compensate.

The operative word here is "believes...".

The first thing to do is scan for o2 values. The sensor should switch from high to low fairly quickly. If it does not, the sensor could be bad/lazy in which case, the pcm is fed incorrect data.

I would check fuel pressure.. a weak pump can cause a lean condition. A fuel system can fail to deliver fuel by restrictions in the line, pump failure and injector(s) failing.

If only one injector is failing, the pcm will detect, through the 02, the lean condition and richen the mixture to compensate.

I would remove each spark plug to check its condition against its neighbors. One white plug amongst the remainder being black would indicate that cylinder running lean.

Also, once can check for a vacume leak, which can also be responsible.

BTW, a small exhaust leak upstream from the o2 can also introduce unmetered air into the exhaust causing the o2 to read a lean condition as well.

Another thing that comes to mind is the possibility of the MAF sensor not reading correctly.

If you do have a coolant leak, it is possible for the coolant to foul the 02 sensor and afterwards, it would be unable to provide correct data to the pcm.
 
Last edited:
Originally Posted By: tunabreath
Also wanted to add, check the fuel filter if it is an inline type.


Yes, that would be on the short list after doing a pressure test.
 
"I would check fuel pressure.. a weak pump can cause a lean condition."
Fuel pressure is good, it is 65psi at idle.


"Also wanted to add, check the fuel filter if it is an inline type. "
The fuel filter is not serviceable, it is in the tank and part of the pump/sender asssembly.


"would the car be under warranty? "
Yes it is under warranty. It has a three year warranty and the truck is only a year and a half old.
What I am trying to determine is if there is a problem or not, I am getting high Si readings and I want to determine the origin of this Silicon. I will do another UOA this spring and see if the Si values are trending downward, this would indicate factory gasketing as the source of the Si.
Truck has 14k miles and has been thru six oil changes already, I assume it is time to see a downward trend in Si if gasketing is the source of the Si. I have been doing frequent oil changes because of the Si readings.
 
With high silicone and rich mixture, it sounds like a vacuum leak. If you have a spray bottle for water or a can of carb cleaner, you can check for vacuum leaks.
 
Hi

Advanced Auto shows a AC Delco inline gas filter for the 1500 Sierra, P/N GF652.

Scan the truck while going along the road at operating termperature to see if the O2 sensor is switching back and forth properly.

Sensor may just be lazy/slow.
 
Different components can cause the same codes, but when and how the codes are set is the key to the diag.

Vac leaks- lean at idle or anytime absolute vac is high. Generally lean at idle, trending toward normal trims during accel. A good ear and a can of carb cleaner are the quickest way to find a problem. If you have a scan tool, watch the trims/O2 voltages. They will change quickly when the carb cleaner finds the leak.

MAF- exactly opposite symptoms. A bad MAF will tend slightly rich at idle, going lean under accel.

Fuel pump/filter- similar symptoms to a MAF problem except for the rich idle. Trends lean under high fuel demand conditions. It is one reason I laugh at people who suggest "fuel filter" for idle concerns. It was not unusual to see me driving around with a fuel pressure gauge taped to the windshield or tucked under a wiper blade to catch a problem.

Pressure regulator- easy to catch with a pressure gauge. It is either in spec, or it isn't. Returnless/PWM systems can be diagnosed with data from a scan tool as well, but a real pressure gauge to compare the data to is really needed.

O2 sensors- on a multiple bank system a fairly quick diag, as you can compare the two primary O2 sensors against each other. Monitor the crosscount numbers and voltage, if your scantool has graphing functions an overlay will pick up a bad/lazy O2 quickly. An O2 sensor can work great for awhile and suddenly decide to flake out and either flatline or hang on a set voltage. Compare spark plugs on opposing banks. A bad O2 will tend to carbon up plugs on its bank as compared to the other bank.
 
"Vac leaks- lean at idle or anytime absolute vac is high. Generally lean at idle, trending toward normal trims during accel"

Great suggestion, Punisher! I think I will go for a drive with my scan tool hooked up and see if the STFT values are higher at idle than under load. Never thought about it before.
 
Advanced Auto might show an inline filter for my truck but, believe me, my truck did not come with one. I checked after I got the truck, I traced out the fuel lines and there is no inline filter.

I noticed that my Owners' Manual did not list a fuel filter in the section where they list the Delco numbers for the oil filter, air filter, spark plugs etc. I called the Dealer where I bought my truck and the Service Manager told me that my truck did not have an inline filter. GM claims the in-tank filter is self-cleaning, I am a little sceptical about that claim.
 
I have limited experience with testing for vacuum leaks but I would think that large leaks would be picked up by a vacuum gauge but small leaks would not.
 
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