ok getting to the niddy griddy I have a 91 F-150 with the I-6 in it. It has a really awful miss only during light throttle and when you push on the gas slightly in Park. This has been going on for about 10K miles or so and it's not gotten more worse nor better. The catalytic converters went to hell, and that didn't affevt the miss. Here's what I have done and what a waste of time and money:
I have done a complete tune-up: plugs, wires, cap, rotor, fuel filter, air filter, breather filter, and adjusted ign, timing to specs. and replaced 02 sensor and reset the computer by leaving the Neg. battery cable unhooked for 30 sec.

This miss my truck has is really pissin me off really bad and I'm about ready to say the hell with it and set it on fire
PLEASE HELP ME!!!!!!!!!

 

BURN IT! I had to get rid of mine to get the head gasket to stop leaking.

My guess is you have something mechanical wrong.....maybe in the valve train that is causing what seems to be a miss. Have you tried un-hooking one plug wire at a time? Do that until removing one has no affect on how it is running....then thats the offending cylinder.

Good luck !

 

check EGR valve, smog pump, any vacuum leaks, spark plugs and cables, distributor cap and rotor, trotle positioning sensor and idle valve if aplicable etc...hope these hints help. Or get a new truck and get into more problems like me.

 

Im not sure if the 300-6's used them but it could be the ignition control module, the throttle position sensor is also a likely possibility. Dont burn it...at least not yet.

Good luck and let us know how it turns out.

 

Try what I told this guy...https://www.f150online.com/forums/sh...0&pagenumber=2
I have a 96 4.9L and I had a vacumm leak and it did the same thing as you described.

 

Ok I just checked out the engine for any leaks, nota, I tried disconnecting one plug wire at a time while running the engine, nota, I was thinking of taking it down to the dealer and have them change my oil and decarbon the entire out (I use Castrol Synthetic) I hope this works out . I do appreicate all your information on this, who knows maybe I just need a new truck!!! But I hate to see my baby go

 

You might try a compression check.
If nothing else it will let you know what
kind of shape your rings and valve train
are in. As suggested earlier i am surprised
you did not find anything pulling the plug
wires one at a time. Maybe fuel system
related.

 

This is an engine I am very familiar with. Disconnect and plug the hose for your EGR valve then take it for a drive. You may be surprised. I do not recommend that you leave it disconnected but I have it repaired now that you have the problem isolated. Let me know what happens.

 

it is mostlikely the EGR system like they say above very common problem on early 90's engines and almost always overlooked!

and remeber it's not just the EGR vavle it's the whole system that needs to be checked and serviced.

also leave the neg bat cable off for more than 30sec that isn't enough time.

good luck-