Ok so I have a 94 f150 with 300 6, m5r2 transmission. Burned up 1st motor and bought a used one, dropped in the new motor and this started happening. The idle is so rough and choppy it dies sometimes. Rpms raise, drop, backfires if I hit the gas. I've replaced dizzy, plugs & wires, copper core motorcraft plugs gapped to specs, cleaned IAC new MAP new PCV, timing was set to 10° btdc with PILL out (or as close to 10° as I could get with it running like this) timing may be slightly off causing backfire but main concern is rough idle. Checked firing order and re-checked, everything is correct. I was thinking leaking vacuum hose but cant seem to find one. Unplug a vacuum line and symptoms do not get worse or better, does this mean it could be a vacuum line? If not, what else should I look into? Thanks.

 

Time for a compression check on that donor engine.

 

That's what I was worried someone was gonna say... guess I just have to try it out and hope for the best. I'll do a fuel pressure test while I'm at it to rule that out as well. I'll post the results of both when I get around to it.

 

Is the check engine light on? Just for fun, try unplugging the MAP sensors electrical plug and see if it runs any different.

 

No check engine but it may not have ran long enough for it to come on. The o2 sensor is disconnected and the light is off which makes me think the truck hasn't run long enough to pick up any codes. I've disconnected the sensor and the truck dies, replaced the sensor and no change to the idle issue.

 

If the engine was able to get up to operating temperature, the check engine light should have come on if the O2 is disconnected. The check engine light should come on immediately if any sensor is unplugged. The truck also shouldn't really die if the MAP is unplugged.

 

Was the replacement motor a '94 as well?
Has any of the original engine wiring been changed? Like did you use the engine harness that came with the donor?

 

No the engine hasn't gotten to operating temperatures and fuel psi is at 50 - 60 fluctuating slightly. The engine was pulled out of a 92 - 96 f150, somewhere in there, and no I stripped it to the block, did a seal kit and dropped it into my truck with all wiring/accessories/vacuum lines from original engine. Havent gotten around to compression test yet, I was rained out. Maybe tomorrow.

 

I'm really stressing the valves. I am pretty sure I seated all the retainer clips properly when I changed the valve seals but maybe one wasn't seated? It sounds like it's running smooth and no loud clattering for the first maybe 5 seconds but after that the fluctuating idle starts happening, so I wouldn't think it's a dropped valve but I just dont know where else to look.

 

Unplug the throttle position sensor, or coolant temperature sensor, and see if the check engine light comes on with the engine running. I suspect you might have a failing computer.

 

Ok I will try that tomorrow. Not something recommended from the junk yard I'm sure? Oh well if that's the problem I'll just be happy just not to swap the motor again.

 

Im betting this capacitor is the culprit

 

I see more corrosion next to that and under the other capacitor to the right. PCM is toast. You may be able to solder in new caps, but the board may be damaged.

 

Yep and on the other end of the board is the same problem. Another capacitor and more corrosion. I'll be getting a new one tomorrow and keep you guys updated

 

Are these things interchangeable between 1992-96 f150s or will it have to be from a 94?