2002 4x4 SCrew 4.6L. Have been experiencing long crank issues intermittently. I did a pressure test and found that the pressure would sometimes need primed to reach 40 PSI, and then would bleed off super fast, never holding anything above zero for more than a minute or two, therefore I have narrowed this issue down to either a bad regulator or a bad check valve in the tank. Pressure reads around 32PSI at idle, and 40-42 PSI with the vacuum line pulled off of the regulator at idle. Oil smells normal and not like fuel. Now for the big puzzler:

My fuel trim data confuses me while driving. Both ST and LT are within acceptable values at idle, but when driving 70mph the LT for bank 2 ranges from +7.8 to +9.4. ST is anywhere from -3.8 to +3.8. Bank 1 appears normal during this time. However, when on a hill or under high RPM while driving, both banks will spike to anywhere from +9 to +15 and remain there until RPMs drop or the truck levels out. Bank 2 generally seeing the highest numbers. I have had the P1071 and 1074 codes before, but replaced the PCV valve, cleaned the MAF sensors, and reset the PCM and they are yet to show back up after probably 700-1000 miles driving

My main question being would I have a weak fuel pump causing the high trim data under heavy load, and is it normal for bank 2 to be higher than bank 1? Or could this also be a result of a failing regulator or check valve? The motor is a year old, fuel filter was replaced when the motor was put in 12K miles ago and replaced again today. My MPG isn't great but isn't terrible. Around 14 highway which I deemed acceptable given extreme cold weather driving, and ethanol in the fuel nowadays. Thanks.

 

I would guess the regulator is leaking thru, or check valve in the tank leaks backward....... Fuel trim don't look all that bad.

 

I just can't understand why the fuel trim spikes at first when under a heavier load such as a hill or harder acceleration, the only thing I can think of would be a weak fuel pump, but with the numbers being in the acceptable range while idling I would think it wouldn't be weak. Since the trim hasn't spiked high enough to retrigger a CEL maybe this is a normal occurance on these trucks since the computer needs a few seconds to readjust for the added fuel consumption.

 

Fuel trim action is very dynamic.
The Ox sensors tell the PCM to richen the fuel when needed or the engine would go lean.
The LT tables are peg count fed and move the averages.
The ST tables are instant reactions. Very different from each other.
Don't let this confuse you. It's all normal unless the LT trims go out of limits and set codes.
The fuel pressure action is the pump check valve not holding or the regulator leaking bypass fuel back to the return line or into the intake manifold vacuum line.
Good luck.

 

 

An exhaust leak was a thought of mine but I haven't been able to locate any other than possibly a very small exhaust manifold leak on the bank 2 side. The manifolds were reused off of the old motor. Bank 1 fuel trims stay around 0 to +/- 1.6 while the bank 2 is in the 7s and 9s. Sometimes bank 1 will have a different number but usually never anything higher than 5. I was thinking maybe I have a pcv elbow or something on the bank 2 side leaking a small amount of air past, as they are a little dry rotted, but I thought if that was the case I'd be seeing the high fuel trims at idle and not while under load because of a vacuum leak being created.

 

 

 

I thought both banks would be affected under that scenario. Since that not the case it obviously isn't the problem. I am planning on correcting it soon, its not in terrible shape. Just keep forgetting to order the right size as no parts stores around here seem to carry one with a large enough diameter.

 

If the manifold really is leaking I'll probably just suck it up and forget about it for now. I told the shop to replace them when the long block was put in if they felt they were unusable and they said they were still in surprisingly good shape. But that was of course a year ago.

 

I should add that the truck doesn't sound like it has an audible exhaust leak at the motor. If there is one it would be hard to locate at the moment. The fuel injectors and fan on these trucks make enough noise to drone out any tiny ticking that might be coming from a manifold. My muffler needs replaced soon as one of the welds is beginning to split on it but I don't think that would be a cause for this issue since there are no O2 sensors that far back in the system.