Weird fuel trim
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Joined: Oct 2021
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Weird fuel trim
Was driving my 98 F150 5.4l V8 to work and I noticed that the truck was jerking a bit when I would slow down to stop. I used my scan tool to check and noticed my fuel trims are sitting around + and - 10 for the short and around 23 for long bank 1 and 18 for long bank 2.
Ive been dealing with a P0171 code for a bit too. I recently replaced the fuel pump, fuel filter, pcv valve, and pulled the pcv elbows on the bank 1 side and on the back of the intake manifold. I have done every trick to find a vacuum leak from spraying carb cleaner, soapy water, and propane. I even tried the supposed hand over the oil fill port to feel for vacuum.
I don't have a smoke machine but tried the cigar smoke into a vacuum line and nothing. I even pulled my bank 1 fuel injectors and nothing was wrong. So needless to say I'm at a loss. Any info would be great and again this is a 1998 F150 5.4 V8 around 130k miles.
Ive been dealing with a P0171 code for a bit too. I recently replaced the fuel pump, fuel filter, pcv valve, and pulled the pcv elbows on the bank 1 side and on the back of the intake manifold. I have done every trick to find a vacuum leak from spraying carb cleaner, soapy water, and propane. I even tried the supposed hand over the oil fill port to feel for vacuum.
I don't have a smoke machine but tried the cigar smoke into a vacuum line and nothing. I even pulled my bank 1 fuel injectors and nothing was wrong. So needless to say I'm at a loss. Any info would be great and again this is a 1998 F150 5.4 V8 around 130k miles.
Logistics Pro
Joined: Jan 2001
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From: Central NYS
It doesn’t have to be a vacuum leak per se. Your engine is running lean and could be caused by more air being injected after the MAF.
Carefully inspect the snorkel between the MAF and the throttle body for air to be injected without going through the MAF.
Check the PCV system hoses, especially the fresh air side, for places air could be pulled in without going through the MAF.
Check the dip stick tube where it enters into the block. If the o-ring is fubar, air could be pulled in there with out going through the MAF.
Has the MAF ever been cleaned? Maybe it is not reading all the air that is passing through the MAF.
Does the oil fill cap have a good seal?
Carefully inspect the snorkel between the MAF and the throttle body for air to be injected without going through the MAF.
Check the PCV system hoses, especially the fresh air side, for places air could be pulled in without going through the MAF.
Check the dip stick tube where it enters into the block. If the o-ring is fubar, air could be pulled in there with out going through the MAF.
Has the MAF ever been cleaned? Maybe it is not reading all the air that is passing through the MAF.
Does the oil fill cap have a good seal?
More Turbo
Joined: Oct 2019
Posts: 632
Likes: 92
Was driving my 98 F150 5.4l V8 to work and I noticed that the truck was jerking a bit when I would slow down to stop. I used my scan tool to check and noticed my fuel trims are sitting around + and - 10 for the short and around 23 for long bank 1 and 18 for long bank 2.
I've been dealing with a P0171 code for a bit too. I recently replaced the fuel pump, fuel filter, PCV valve, and pulled the PCV elbows on the bank 1 side and on the back of the intake manifold. I have done every trick to find a vacuum leak from spraying carb cleaner, soapy water, and propane. I even tried the supposed hand over the oil fill port to feel for vacuum.
I don't have a smoke machine but tried the cigar smoke into a vacuum line and nothing. I even pulled my bank 1 fuel injectors and nothing was wrong. So needless to say I'm at a loss. Any info would be great and again this is a 1998 F150 5.4 V8 around 130k miles.
I've been dealing with a P0171 code for a bit too. I recently replaced the fuel pump, fuel filter, PCV valve, and pulled the PCV elbows on the bank 1 side and on the back of the intake manifold. I have done every trick to find a vacuum leak from spraying carb cleaner, soapy water, and propane. I even tried the supposed hand over the oil fill port to feel for vacuum.
I don't have a smoke machine but tried the cigar smoke into a vacuum line and nothing. I even pulled my bank 1 fuel injectors and nothing was wrong. So needless to say I'm at a loss. Any info would be great and again this is a 1998 F150 5.4 V8 around 130k miles.
The cheat sheet to these codes:
P0171 and P0174
- Connect your OB scanner to your vehicle. Recheck the trouble codes and find any other related trouble codes that could cause a lean mixture. Follow the diagnose procedure on the other trouble code if you get any. Clear codes.
- Check the live data for the MAF sensor,temperature sensors (rule of thumb is about 5-10 degs hotter than outside air. Now in the very hot states it may be 8-12), and figure out if their signals are logical or way out of range.
- Connect a Smoke machine to your car’s inlet and check for any intake leaks. Also, check the throttle body spring where the cables go to open and close the air gate for smoke leaking out by plugging the intake so no smoke can exscape.
- Check the PCV valve and the EVAP purge valve. You will find a leak or a faulty one-way valve that is causing the P0171/P0174 code in most cases.
- Check the fuel pressure. Connect a fuel pressure gauge to the fuel rail and make sure that the fuel pressure is correct. If it is too low, check the fuel filter, fuel pump, fuel pressure, and regulator. 28-36 PSI is good at idle.
- Check the EGR valve. An EGR valve stuck open can cause the air-fuel mixture to stay lean. If possible, try to check the valve inside it while you open and close it with your diagnostic tool. (Vac hand pump works well here)
- Clean the MAF sensor. (CLEAN ONLY WITH MAF SENSOR CLEANER) A dirty MAF sensor can cause the air-fuel mixture. Inspect it and check for any dirt on the sensor. After it, you should try to clear the trouble codes and check if the code comes back. It might also be damaged, so check its values on idle and on pressure and replace it if faulty.
- Check for exhaust manifold pipe leaks before the O2 sensor. Repair the leaks if you find any.
- Check the inlet air tube for cracks, not sealed correctly, the two inlets pipes into the rubber boot and even at the throttle body on the bottom side where the inlet tube connects.
- If the problem still occurs and you haven’t found any problems with the mentioned parts above, it is time to diagnose your O2 sensor. If you find it giving wrong values – replace it.
Thread Starter
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New User
Joined: Oct 2021
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It doesn’t have to be a vacuum leak per se. Your engine is running lean and could be caused by more air being injected after the MAF.
Carefully inspect the snorkel between the MAF and the throttle body for air to be injected without going through the MAF.
Check the PCV system hoses, especially the fresh air side, for places air could be pulled in without going through the MAF.
Check the dip stick tube where it enters into the block. If the o-ring is fubar, air could be pulled in there with out going through the MAF.
Has the MAF ever been cleaned? Maybe it is not reading all the air that is passing through the MAF.
Does the oil fill cap have a good seal?
Carefully inspect the snorkel between the MAF and the throttle body for air to be injected without going through the MAF.
Check the PCV system hoses, especially the fresh air side, for places air could be pulled in without going through the MAF.
Check the dip stick tube where it enters into the block. If the o-ring is fubar, air could be pulled in there with out going through the MAF.
Has the MAF ever been cleaned? Maybe it is not reading all the air that is passing through the MAF.
Does the oil fill cap have a good seal?
More Turbo
Joined: Sep 2017
Posts: 574
Likes: 146
When I had those codes the problem turned out to be a MAF sending bad barometric pressure readings. I wasted a lot of time chasing non-existent air leaks, fuel problems and things like PCV stuff. Takes a good diagnostic tool to check baro readings.
One mechanic said he always checks MAF baro readings before chasing air leaks, especially on Fords. Saved him many times.
One mechanic said he always checks MAF baro readings before chasing air leaks, especially on Fords. Saved him many times.
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