[RobZL1]

I like both the fuel line/check valve restriction and the pressure sensor theories.

I think you could diagnose which might be the most likely by looking at your LTFT's.

If they are extremely negative, then it's likely the pressure sensor.
If they are not extremely negative, then it's likely the fuel line/check valve.

Why do I say that?

If it's the pressure sensor it would be severely overfueled and you'd see it in extremely negative LTFT. In more detail, say it wants 800psi and it's being told it's at 400psi. It's then going to raise the pump cycle to the full 130degrees eventually to try to get to 800psi (though it may already be there or above in reality). In that case, it's also going to be pushing a lot more fuel than it thinks it is, and you'll be seeing maxed-out negative trims to try to maintain commanded lambda.

If you are not seeing extremely negative trims, it's probably the fuel line/check valve. It's not getting enough fuel, and it raises the pump cycle to try and increase the pressure to desired. Since there isn't enough fuel, it only gets part of the way there, and the pressure reading provided by the sensor is accurate the whole time. At this point, it's actually getting the fuel it thinks it is, and you won't see odd LTFT's. If anything, they may be quite positive.

Shoot holes in this, please, but that's how I'd approach it on my car.

[bishopts]

Originally Posted by RobZL1

I like both the fuel line/check valve restriction and the pressure sensor theories.

I think you could diagnose which might be the most likely by looking at your LTFT's.

If they are extremely negative, then it's likely the pressure sensor.
If they are not extremely negative, then it's likely the fuel line/check valve.

Why do I say that?

If it's the pressure sensor it would be severely overfueled and you'd see it in extremely negative LTFT. In more detail, say it wants 800psi and it's being told it's at 400psi. It's then going to raise the pump cycle to the full 130degrees eventually to try to get to 800psi (though it may already be there or above in reality). In that case, it's also going to be pushing a lot more fuel than it thinks it is, and you'll be seeing maxed-out negative trims to try to maintain commanded lambda.

If you are not seeing extremely negative trims, it's probably the fuel line/check valve. It's not getting enough fuel, and it raises the pump cycle to try and increase the pressure to desired. Since there isn't enough fuel, it only gets part of the way there, and the pressure reading provided by the sensor is accurate the whole time. At this point, it's actually getting the fuel it thinks it is, and you won't see odd LTFT's. If anything, they may be quite positive.

Shoot holes in this, please, but that's how I'd approach it on my car.

Its got sound logic, only problem is your fuel trims are different in each fuel trim cell so at idle its easy to see your idle fuel trim cell (i think number6) but none of the others. You would need a scanner that can see cells its not currently in or you would need to force the engine into those cells and check what the numbers are.
But diagnostic of the idle fuel trim cell will more than likely coincide with the others. Especially since it's got half the fuel pressure it wants at idle.

[RobZL1]

Originally Posted by bishopts

Its got sound logic, only problem is your fuel trims are different in each fuel trim cell so at idle its easy to see your idle fuel trim cell (i think number6) but none of the others. You would need a scanner that can see cells its not currently in or you would need to force the engine into those cells and check what the numbers are.
But diagnostic of the idle fuel trim cell will more than likely coincide with the others. Especially since it's got half the fuel pressure it wants at idle.

For what it's worth, it's easy to see both current trim cell and current trim value with HPT scanner while you are driving (or review the log afterwards), which is what he's using.

[bishopts]

Originally Posted by RobZL1

For what it's worth, it's easy to see both current trim cell and current trim value with HPT scanner while you are driving (or review the log afterwards), which is what he's using.

Very true. And if the graphs are setup correctly you can see the entirety of all the cells.

[Rydog2SS]

For an update, I tried the rail sensor and removal of the factory check valve. It was neither of those items.

I installed my old LT1 HPFP and it worked perfect. I have just ordered a new LT4 HPFP, should be in on Wednesday so hopefully when I start it up with that, it will also be perfect. not sure i have it in me to take it apart for anything else again after taking it apart and putting it back together 4 times over the last week and half...

[bishopts]

Originally Posted by Rydog2SS

For an update, I tried the rail sensor and removal of the factory check valve. It was neither of those items.

I installed my old LT1 HPFP and it worked perfect. I have just ordered a new LT4 HPFP, should be in on Wednesday so hopefully when I start it up with that, it will also be perfect. not sure i have it in me to take it apart for anything else again after taking it apart and putting it back together 4 times over the last week and half...

Well the one thing we all didn't want it to be...

[Rydog2SS]

Originally Posted by bishopts

Well the one thing we all didn't want it to be...

Yeah for sure... I do appreciate the response from everyone, was hoping it was something else, but we will get it back on the road. a little less than 2 weeks till sick week, got a lot of driving to do and track days coming up!

[wakespeak]

Hey guys I may be having a similar issue. I have an SCT scanner. The LTFT is around .68-.98, fluctuating a lot. The car seems to miss, then clear as the LTFT goes over .9. STFT is around 1.0. Is it the sensor?

[bishopts]

Originally Posted by wakespeak

Hey guys I may be having a similar issue. I have an SCT scanner. The LTFT is around .68-.98, fluctuating a lot. The car seems to miss, then clear as the LTFT goes over .9. STFT is around 1.0. Is it the sensor?

What scale are you using? Fuel trim is percentage. So max will be about +28 to -28,
Normal operation is going to be 0 on long term and between -5 and +5 short term.

[wakespeak]

Originally Posted by bishopts

What scale are you using? Fuel trim is percentage. So max will be about +28 to -28,
Normal operation is going to be 0 on long term and between -5 and +5 short term.

The SCT X4 uses 1.0 as no trim and positive as above and negative as below. So a 0.70 would be -30%. I am seeing around 0.84 at part throttle, drops to low 0.7s at idle, and improves to 0.98 at heavier throttle (through it feels like it is missing, then clears some). At 0 throttle/lift it is 0.98. All LTFT. LTFT will sporadically go to the mid 0.9s at idle.

STFT is usually close to 1.0.

So it seems like there is a relative constant over fueling that is more obvious at idle. It has gotten worse over the last few days. New plugs/wires/evap solenoid. Exhaust has raw fuel smell to it not sure if it is the cats. A lot of track miles.

[bishopts]

Originally Posted by wakespeak

The SCT X4 uses 1.0 as no trim and positive as above and negative as below. So a 0.70 would be -30%. I am seeing around 0.84 at part throttle, drops to low 0.7s at idle, and improves to 0.98 at heavier throttle (through it feels like it is missing, then clears some). At 0 throttle/lift it is 0.98. All LTFT. LTFT will sporadically go to the mid 0.9s at idle.

STFT is usually close to 1.0.

So it seems like there is a relative constant over fueling that is more obvious at idle. It has gotten worse over the last few days. New plugs/wires/evap solenoid. Exhaust has raw fuel smell to it not sure if it is the cats. A lot of track miles.

I wouldn't chase fuel trims at inconsistent throttle. The cars response to changes can be altered by variables that are different from day to day, like barometric pressure, humidity, ambient temperature. On top of engine load and throttle demand. It will take in info and adjust for it but if you're watching it happen in real time then you might be chasing something that is correcting itself already.
So focus more on consistent throttle ranges or smooth steady progressing throttle.
Look in your obd2 mode 6 data for historical misfires, look at left bank vs right bank to see if one of them is more consistently higher or lower than the other. If not try and look at things that are shared across both banks. Oil saturated with fuel, low flow in the air filter, gas additives

[wakespeak]

Originally Posted by bishopts

I wouldn't chase fuel trims at inconsistent throttle. The cars response to changes can be altered by variables that are different from day to day, like barometric pressure, humidity, ambient temperature. On top of engine load and throttle demand. It will take in info and adjust for it but if you're watching it happen in real time then you might be chasing something that is correcting itself already.
So focus more on consistent throttle ranges or smooth steady progressing throttle.
Look in your obd2 mode 6 data for historical misfires, look at left bank vs right bank to see if one of them is more consistently higher or lower than the other. If not try and look at things that are shared across both banks. Oil saturated with fuel, low flow in the air filter, gas additives

Appreciate all the advice. Looks like it was ignition related. Replaced the plugs/wires/coils and it cleaned up. Still a bit guarded until I get somewhere where I can get deeper into the throttle.