1997 F-150 4.2L
This is one for the experts! I have been trying to troubleshoot this problem for the past year without much luck, in fact it seems to be getting worse.
The main symptoms:
-Rough idle (seems to be when warm, engine vibration almost seems like an occasional miss) you can see the engine shake and feel it with your hand on the engine or while holding the steering wheel.
-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?
-The old autolight platinum plugs were pretty carboned up but the engine doesn't consume any oil.
-No check engine light
Recent mechanical history:
Hydrolocked, coolant in #1 and lower intake gasket replaced last year. (no damage to engine as engine barely turned and did not fire and bend a rod, etc) Coolant temp sensor, idle air control IAC and intake temp sensor changed. Fuel filter changed. Cleaned PVC and checked for leaks. Cleaned throttle body with seafoam multiple times. 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 tested the coil pack. The coil pack secondary resistance (between adjacent coils ie 1-5). The resistance is supposed to be 8,000-11,500 but I get 13,600 and 14,600 when it is warm. 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 (DFPE, coolant temp sensor, EGR control valve, MAF, air intake temp sensor, removed and cleaned EGR) 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.
I tested the fuel pressure at the rail: 38-40 psi with key on engine off. 32 at idle (should be 30-45) and 42 at idle with vacuum line off fuel pressure regulator (should be 40-50 according to haynes manual). It increases slightly when you rev the engine.
Even though I cleaned and tested the MAF several times I figured I would try a new one since it seems this sensor causes a lot of trouble. Zero difference.
One key symptom I am wondering about here and I hope someone can help is that the plugs were in my opinion very dirty after approx 12K miles. here is a picture of them: http://picasaweb.goo...765817055358610
The plugs closest seemed slightly dirtier and are from the drivers side. Most people may say it is burning oil with 89,000 on it but it does not burn any noticeable amount after 3k mi. The exhaust pipe opening also seems excessively carboned up.
Along with the symptom of poor gas mileage, this leads me to think it may be running rich.
Does anyone know what is wrong?