I have changed plugs, wires, cap, rotor, coil and O2 sensor. Cleaned the IAC valve and verified that its working. Checked MAP sensor and TPS sensor. Truck is running rough through all RPMS. When I changed the plugs they were black like its running rich. Air filter and fuel filter aren't very old. I am pulling my hair out trying to figure this out without going broke buying any more unneeded parts. I am not a mechanic but Im usually able to fix anything on my trucks. Any help would be greatly appreciated. Im starting to think about the EGR and fuel pressure regulator. Am I leaning in the right direction?
I haven't checked compression yet. I checked my oil and it smells like gas. If I pull the vacuum line from the fuel pressure regulator there is no change in the way that the engine runs, and Im smelling gas at the tailpipe. I know these can be symptoms of a bad FPR. Can a bad EGR also cause these things?
Someone else told me that today too. Im gonna try to run a heavy dose of injector cleaner through it and see if that unclogs it. If not I guess Im pulling my intake off this weekend so I can ohm them out.
I dealt with a rough running 1994 4.9 for a year. I replaced everything you mentioned above. It turned out it was the fuel pump. My mechanic replaced it and then it went bad again (never buy Standard Motor Products fuel pumps, they were bad luck for me). Finally, on the third try, we went with a Motorcraft Fuel Pump and problem solved.
If you can, I would check your fuel pressure and see if it is consistent. It might be a good idea to replace the fuel pressure regulator, too. Fixing those two things worked for me.
The Map sensor on my 95 went bad and never set a code. I tried one from a JY and it worked so I went to Napa and got a new one. On my 95 the Map sensor was a frequency type and couldn't be tested with a voltmeter.
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