97 F150 4.2 Rough idle, Shakes/stalls at start sometimes
1997 F-150 4.2L
I have been trying to troubleshoot this problem for the past year without much luck, in fact it seems to be getting worse.
Recent mechanical history:
Hydrolocked, coolant in #1 and lower intake gasket replaced last year. Coolant temp sensor, idle air control IAC and intake temp sensor changed. Fuel filter changed. I also tested the throttle position sensor, coolant temp sensor and intake temp sensor with a multimeter and they were all in the normal range. I also replace one of the vac hoses on the intake manifold and inspected the others and they seem fine.
I swapped the coil pack for a new one and it made zero difference. I though I might have a problem with the EGR system so I spent a while troubleshooting that and everything seems to be working perfectly. I checked vacuum pressure at the intake manifold and I am getting a pretty steady 18.5-19.5 in. of pressure which is what a healthy engine should get and this hopefully also means that there aren't any vacuum leaks. I also cleaned the MAF with special CRC electronics cleaner and no difference. New spark plugs and wires. Finally I pulled the vacuum hose off the fuel pressure regulator and no change in how the engine was idling, does this mean the regulator could be bad? Its vacuum line is getting pretty good suction.
-The old autolight platinum plugs were pretty carboned up but the engine doesn't consume any oil so it must be running rich?
-No check engine light
-Rough idle (seems to be when warm, engine vibration almost seems like an occasional miss)
-about 1 out of every ten times I start it, it shakes violently and stalls after about 5 seconds. If you press the gas it usually will not stall.
-Poor gas mileage?
Any ideas? This is driving me crazy, I can't figure it out! Thanks