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Positive LTFT under load

Tiny
SIMPERFI84
  • MEMBER
  • 1996 CHEVROLET 1500
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 600 MILES
Okay, guys. I have the truck listed above c1500. Truck has 325,000 approximate miles. Engine is a new factory GM engine, and transmission is a re-manufactured 4l60e, with 600 miles on both. I notice my exhaust has a strong fuel smell. So hooked scanner up at operating temp. At idle fuel trims are as follows:
STFT BNK 1 - 1.6 TO 0.8
LTFT BNK 1 4.7
STFT BNK 2 -1.6 TO 0.8
LTFT BNK 2 1.6
ECT 194
RPM 650

UNDER LOAD ( HIGHWAY SPEED)
CAL 2.7
ECT 194
MAP15 INHG
RPM 2052
SPEED 66 PER SCAN TOOL 71MPH SPEEDO
SPARK ADV 38
IAT 79 DEGREES
MAF 5.28 LB/MIN
TPS 10.2

STFT BNK 1 -0.8 TO 0.8
LTFT BNK 1 13.3
STFT BNK 2 - 1.6 TO 0.8
LTFT BNK 2 14.8

It has been suggested this could be a dirty or bad MAF sensor or low fuel. Press bad pump. Fuel pump is new psi 62, new set of MPFI INJECTORS, cam retard is at 0. Maf sensor is the old one as well has the air temp sensor on air duct. All other sensors on engine are new but all aftermarket. Engine temperature seems low to me for a 5.7l. I have heard these engines prefer OEM sensors. Thoughts and suggestions please.
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Friday, January 17th, 2020 AT 8:45 AM

6 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
The short answer is yes the MAF can cause this. However, it may not be the sensor itself. It is more likely an air leak after an engine replacement like this. When unmetered air is entering the engine it throws off the o2 sensors so it dumps more fuel. If you notice the long term is positive which means it is a lean exhaust. This means there is more oxygen in the exhaust and so the PCM is adjusting for this and dumps more fuel.

If the MAF is faulty then it can cause this but I would start with trying to find if there is unmetered air coming into the engine.
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Friday, January 17th, 2020 AT 3:40 PM
Tiny
SIMPERFI84
  • MEMBER
  • 14 POSTS
Thanks for the response. By un metered air are you meaning air being pulled in after MAF sensor from say air cleaner duct-work or something like that. As opposed to a vacuum leak. If I am understanding the fuel trims correctly LTFT would get better if it was a vacuum leak at higher rpm's, correct I still have trouble keeping trims straight in my head. It be nice to have a smoke machine may have to build one.
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Friday, January 17th, 2020 AT 8:01 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Unmetered air means that it is air coming into the intake after the sensor so it is not being measured. When there is more air coming in after the sensor then the ECM doesn't know it is coming in so it only adds enough fuel to achieve the proper air/fuel ratio based on what is measured. If more air is coming in then it will lean it out.

I totally understand getting the trims mixed up. Took me years. Here is what helps me. The o2 sensor are oxygen sensors. This means they are measuring the amount of oxygen in the exhaust. People often think that when they are positive it is because there is more fuel. This is not accurate, it doesn't know how much fuel is in the exhaust. It only knows how much oxygen is there. So if it is a positive number, that means it has more oxygen in the exhaust. If this is the case, then ECM sends more fuel (go rich command) so that it will start reading less oxygen.

If the number is negative then that means there is less oxygen in the exhaust and in order to get more oxygen it shorten the injector pulse widths to get less fuel which will increase the oxygen.

Just make sure you keep focused on the oxygen and that the ECM uses the amount of fuel to alter the oxygen content.
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Saturday, January 18th, 2020 AT 6:31 PM
Tiny
SIMPERFI84
  • MEMBER
  • 14 POSTS
Okay, thanks. Pretty good explanation. You have given me a direction to follow. I will clean and check MAF first and work my way to and around the intake.
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Saturday, January 18th, 2020 AT 6:40 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Sounds great. Let us know what you find and we can take the next steps based on your results. Thanks
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Saturday, January 18th, 2020 AT 7:49 PM
Tiny
SIMPERFI84
  • MEMBER
  • 14 POSTS
Will do.
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Sunday, January 19th, 2020 AT 5:55 AM

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