Hey Folks, hope you all are doing well as we approach the holidays.

Im 17 and am currently in the process of restoring a 94 Lightning, which turned 185k this past Sunday. On this truck ive redone the vacuum lines, along with the upper/lower intake gaskets. PCM had a capacitor refresh and ive replaced many of the failing sensors (O2, ECT, ACT, TPS, IAC, along with a tune up with motorcraft plugs and rotor). Truck runs fine 98% of the time but the other 2% of the time I run into issues like I did this morning. I went to start the truck, pumps prime as usual, and it fired right up, but instead of going into regular fast idle of 1300-1500, it went to around 1700, then 1500, but then it had a hard falloff into the 600-700 rpm range. from there it would surge from 700-1200 repeatedly, and would get stuck at around 1200 whenever I put it in gear. What stumps me most about this is that the problem disappeared and it started like normal after I shut off/ turned it on a couple of times.... This also happens when the truck is warm too, but at random times. sometimes it will go days without doing it, and out of nowhere at random times it'll act up. Ive tested voltages and resistances of TPS, ECT, and ACT, and they all come back within spec. im led to believe this could just simply be due to the sensors not being motorcraft, especially the IAC, however I could be drastically wrong. I come back with no codes either, no CEL and I haven't found any vacuum leaks. this issue usually doesn't bother me but when it happens it does. Any ideas on where to start???

(also as an add on, I get horrendous MPG, less than 6 to be exact. I don't know of this is associated with my problem or not)

Thanks

 

Sweet truck!
Have you checked the timing? 10* with SPOUT removed.
Fuel pressure? Check the fuel pressure regulator, pull the vacuum line off and see if it is wet with fuel.
What computer is in it?
Is the engine stock? Any modifications from previous owner.

Random intermittent issues suck.

 

Check your pcv valve..sounds like a random vacuum leak to me. Try capping off all your vacuum lines (especially the big ones) and seeing if the problem goes away.
You can try performing a KOER check to see if it pops any codes.

 

i haven’t checked, but my timing i’m sure is 10 or around 10* with the spout removed.
I have not checked fuel pressure as of yet. Engine is stock and it has a C3P3 PCM unit in it. i am aware that is for a 93 model but nothing was changed from 93-94 models. When i first got the truck it had a regular 5.8 PCM so i had to search for a lightning specific one.

 

The IAC is not a sensor, just a solenoid. A quick test: when its idling high, unplug it, if the rpms immediately drop and maybe stalls, then the IAC is functioning correctly.

How are you measuring MPG? The gauge is more of a guestimate then an exact measure. How does the exhaust smell? Any smoke? Check the condition of the plugs as well.