Hey guys its been a minute since I've posted and ive been looking through past posts for the answer but im not having much luck so im going to just put it out there. I recently purchased a 1993 F150 Supercab 4x4 with the 5.8. It's got 264+ on the clock but its still doing alright. The previous owner had a bunch of crazy wiring running EVRYWHERE through the truck so i carefully traced and removed it all to start. Stereo amps, 12 volt sockets, interior lights, switches to no where, it was a rats nest! Good ideas, poor execution. The major issue is this though... when i start the truck the idle jumps way up, like an old school electric choke would on a carb. After a minute it settles down to where it should be, if i blip the gas pedal itll high idle for a second then go back to normal. well, kind of anyways. As its idiling the oil pressure "gauge" bounces from no reading to normal and the engine has a noticeable miss, like it flat skips a beat then relights like nothing is wrong. It will stay running, its never died while doing all this and once out driving at highway speeds it runs great. At 25 mph and slower the miss can be heard and felt but it still stays running. Ive climbed up and listened to the engine and i swear i hear a vacuum leak but ill be damned if i can find it!! Its got plenty of oil, it does need changed though, and all the plugs and wires look fine. No check engine light ever, which is kind of disturbing, and it runs great down the road. Any ideas are welcome, i really dont know where to start. Thanks for the help!!!

 

The high idle when first started is normal. My guess is the vacuum leak is the IAC opening as the engine tries to recover from the near stall. It doesn’t matter if the engine light is on. The computer can still show active and stored fault codes when running the Key On Engine Off test. That is where you should start troubleshooting.