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So I did a whole lot more trouble shooting tonight with the aid of a mechanic friend on the phone. I checked the fuel pressure and although I did not initially see pressure when I just turned the key on, after the truck ran for a few minutes I had pressure up to about 29-30 psi. With the truck running I disconnected the fuel pressure regulator and the pressure went up to about 38-39 psi, then came back down when I plugged in the vacuum line again. When I turned the truck off it would maintain the 30 psi for a lot longer than 5 minutes and in fact the pressure actually would climb gradually once the truck was off. I am thinking there may be something with the gage I was using that affected how the line to the gage filled with fuel and that is why I never detected pressures of 38-42 with key on, or maybe I really do have a fuel pump issue, but it seems to be able to maintain the 30 psi while running no issue. I did also take the throttle to WOT while in park and the pressure did not seem to fluctuate much, so again either I have a gage issue, or maybe the pressure is not being regulated. Anyway, I have a new pressure regulator, but did not have time to install it tonight. Also my buddy mentioned that maybe I had a stuck EGR valve, so I swapped the vacuum line from the FPR to the EGR and that caused the truck to bog down and almost die indicating that the EGR is functioning properly. Then he deduced that maybe I had a stopped up catalytic converter, so I disconnected the exhaust just before the catalyst and still had the same symptoms at idle, very rough almost sounds like I have a cam...but I don't!!!
My only two thoughts right now are fuel pump or fuel pressure regulator, any other ideas, or definitive tests that I can run to confirm one or the other, or something else. I may end up at a shop that has a better scan tool so they can actually diagnosis what is going on in the engine. I figure it is air, fuel or something mechanical, but I am at a loss at this point.
Something is odd with the "few minutes" for fuel pressure to get to idle pressure.
Something is odd that when you power braked the fuel pressure did not go up. Power braking sends vacuum to zero and should have put fuel pressure to about 38 to 40 psi.
I would be looking at fuel filter too.
And leaving fuel pressure tester on over night to see cold start fuel pressures.
Guess I need to secure a vacuum tester next. Also to clarify I did not power brake, it was only WOT with the truck in park, so I can check with power breaking. Also I can leave the gage overnight and see if it drops to zero. Again I am not sure if there is something wrong with how I am connecting the gage or something wrong with my schrader valve on the fuel rail... I did already replace the fuel filter and it was quite dirty so I was hopeful that was part of my problem, but no such luck.
So the truck has been with the mechanic for 2 weeks now. He has ruled out O2 sensors and fuel pump/pressure regulator based on the fact that my O2 sensor voltages appear correct and the short and long term fuel trims as correct. He was thinking maybe a broken valve spring, but then he started to test that MAF and found that the idle seems to smooth out briefly when you unplug the MAF. He replaced that and it still does the same thing. They smoked the engine looking for vacuum leaks, but found nothing. Next step was to check if EGR was partially open. My only other clue is that when I changed the fuel filter the fuel that came out was quite dirty looking. If I ran the tank too low and sucked sediment from the bottom or just got bad fuel, could that cause my seemingly random misfire?
Are you currently running a tune? If so, go back to stock before you mess around with anything else. And yes, updating the tune will clear the KAM (Keep Alive Memory) and wipe any pending or other codes/counters.
And what are the results of the fuel pressure tests? Or weren't they done?
Are you currently running a tune? If so, go back to stock before you mess around with anything else. And yes, updating the tune will clear the KAM (Keep Alive Memory) and wipe any pending or other codes/counters.
And what are the results of the fuel pressure tests? Or weren't they done?
Yes I have a tuner, but I flashed it back to stock several weeks ago when I first started to self diagnose. I have also used the Xcal4 to reset the KAM. All the fuel pressure tests came back positive. I get the right fuel pressures at start up, during idle and if I disconnect the fuel pressure regulator. I need to call the mechanic today and see if he found anything with the EGR valve. If that is not it the only thought I have left is bad fuel. What is the best way to clear the fuel system in case of a bad tank of fuel? Do I just need to drain the entire tank and actually clean the tank?
negative on the EGR valve being slightly stuck :-(. Next step is Idle Air Control valve. I need to call tomorrow and see if that worked. Any other ideas? Myself and the mechanic are at a loss.
I’ve never heard of misfires being more pronounced during coasting than accelerating. Check your u joints.
i realize u joints can’t cause your problems with startup and idle.
But doesn’t anyone find it interesting that the truck gives problems while coasting from 60mph?
I’ve dealt with misfires quite a bit and they are always nonexistent when coasting.
Well after the visit at the second shop they finally diagnosed and repaired the random misfire issue as a slightly stuck open EGR. It was not open enough to throw and CELs or even show up on their scan tool as a parameter that was abnormal. The shop said once they installed the new one that the voltage for the EGR sensor dropped by 0.1 Volts from 1 Volt to 0.9 Volts. Once they resolved the misfire, then they could properly diagnose what was actually a drive-line vibration issue related to both rear tires having separated belts. When I bought the truck it had basically new tires, but they were cheap Nexen brand, although they were load range E. So I assume that they simply couldn't handle the 75 PSI on our hall to Myrtle Beach and/or combined with a few pothole hits. This means on top of the $470 repair bill due to the time they put in to diagnose the misfire, I now have a $900 tire bill as I always put Michelin's on my vehicles so I will have the LTX M/S Load Range Es installed today/tomorrow. Thanks for all the input on here and despite the fact that I couldn't repair it myself I am just glad to have it back on the road!
Yeah the shop said the only way they knew it was the EGR is that when they removed it and literally blew through it with their mouth, backwards, that there was airflow and there shouldn't have been as the EGR is a normally closed device. Wish I had thought to try that one at home, but oh well, at least she is back on the road. Here she is all cleaned up and with the new shoes:
04-12-2018, 10:53 PM
