Hello all,

I was looking around the internet to find a good forum to get some questions answered and this one looked to have the most reasonable guys, glad to be aboard!

I work in a construction company that runs various Ford pick-ups. I don't have much specific Ford wrenching time, but I beat up enough old cars that I have some experience. My question today is about a 1991 F-150 with EFI 4.9 straight six. It has 117,000 miles on it.

For some time now it has what I could best describe as a miss at idle. It seems really noticeable at idle. All other RPM ranges it pulls well and runs smoothly. I know at idle this engine should be very smooth. Engine temperature has no effect, hot idle or cold idle is same. During the episodes, the idle speed wont really fluctuate that much. It seems as if it runs smooth for about 1-1.5 seconds, then will shake noticeably for 3-5 seconds, then be smooth for another 1-1.5 seconds. It will sometimes die if you shift quickly from forward to reverse like when you are parking.

So far all I have done it replace the spark plugs (motor-craft gapped at .044") as they were required anyway by mileage, and long time ago I changed the spark ignition box on the side of the distributor. I am a very strong believer in diagnosing problems before throwing parts at them. My "wallet wrench" is pretty weak and can't afford to replace working components.

My uneducated guess is that I have an electrical problem, because at no time else is running operated. Although it could be missing at higher RPM's but I just don't feel it. I work mostly on old Diesels so these possibly spark related issues are new to me.

What things should I be testing and checking out? Stroboscope? I know the ignition modules are know for a complete "lights out" failure, but could it fail partially giving me a symptom as this? Am I missing it entirely?

Any insight appreciated

Thanks for reading.

John in not so sunny CA

 

One good possibility is a vacuum leak.

 

take off your distributor cap and check the terminals, one could be not conductive enough at idle causing either a miss or very little spark on that cylinder. I had a similar problem with my inline 6 and i replaced the distributor cap and rotor (around $35 for both at the local parts store) and it eliminated that roughness. Now, im not sayin this is the solution to your problem what so ever, but i would recomend checking because as we all know, the small inexpensive things we tend to overlook usually are the problem.