pww571

Hey all,

I am new to this forum, so apologies if this is in the wrong area.

I recently picked up a 1991 F150 5.0 with the AOD automatic transmission... I think.. (3spd with 4th being OD)

I purchased it at the beginning of the year, and it has some running/idling issues when warmed up that I have been collectively diagnosing over the past few months. I went through the normal changes like changing plugs/wires/cap/rotor - IAC valve - MAP sensor - TPS and ran up and down for vacuum leaks. (still not sure if I have found em all or not - working on it)

I have seemed to get the truck to run, and idle well when starting, warming up, and stopping after warmed up. However, I have a new issue that seems to have me confused.

In an effort to troubleshoot some rough running conditions where the truck had been occasionally skipping/bucking/jerking in drive under a load, I replaced the distributor (including the internal pickup coil) and the ICM that attached to it. After the replacement, I timed it back to about 12 - 14 degrees advance and it seems to like that. However, the skipping/bucking/jerking remains, and has gotten worse. It used to do it occasionally, and only in 3rd and 4th gear between 1500-1800 rpm's - but now, it does it in any gear between 1500 - 1800 rpm's and it will even cut out completely while running (see video)

The truck seems to run fine after we get up to about 2000 rpm's, but the cutting out will happen randomly. It seems to only cut out while I'm cruising without putting a load on the motor. It cuts out, then seems to reset itself, and then turn back on. RPM's drop to 0 then jump back up to whatever rpm I was cruising at. My best guess is that it's something electrical.

It hasn't left me stranded on the side of the road as of yet, but I don't want to push my luck and be stuck pushing my truck.

So, I am reaching out to you all in an effort to pick your brilliant brains.

Video of motor cutout:
Again, it does that randomly while cruising down the road.

Here's a pic of ol' Gordo the Fordo for those who've invested their time.



Thanks for reading.

Punisher1964

Have an 87' f150 with 302 v8 and 4- spd. O/d. Had the exact same problem. Thought it was the icm, replaced, same problem. Replaced coil, problem fixed.

Lazarus-F150

I would also check for a vacuum leak. Even a small leak can cause huge problems...

wde3477

Ignition coil? The more air admitted, the more voltage is needed for the spark to jump the gap. Rough check is a desired blue spark versus a weak yellowish-orange one with a plug pulled and being watched while the engine is rolled over.
There are resistance specs for the primary and secondary side of the coil - don't have my shop manual anymore, but the specs were in there - to check before dropping dollars to replace.

Red96

I agree with this man! Had same issue with another vehicle of mine. Took the coil to Auto zone and had it tested. Tested out fine. Reason being though is because it had time to cool off on the way there. After replacing full pump, fuel pump relay and other various parts, something in my head told me to just replace the ignition coil since I've already wasted money on other parts. Replaced coil, truck never cut out on my again.

Rex Smith

Just to cover your bases... check fuel pressure. I had similar symptoms of rough engine - and even though my fuel pump was new - it was delivering fuel at lower than required pressure from one of my (2) fuel tanks . New pump in the tank fixed the problem... Now - over a year later - it has arisen again - so will be replacing it again. (and double-checking for any wiring corrosion that may be causing a drop in voltage that may manifest to a lower pressure delivered)

(Bought from AutoZone - so is under lifetime parts warranty, and I do my own wrenching when I can).

vjsimone

Distributor - New or Reman?

Hall Effect Switch - New or Reman?

ICM - New or Reman?