hello, i am new as a member, but have been lurking successfully for years(thanks for all ive learned from this site). i have 7 ford trucks ranging from 1969 to 1992, and a new 2016 mustang i am not new to turning a wrench, but this one is baffling me. any help or input is much appreciated...

i have a 1990 f250 2wd 4.9 six with a four speed. the truck has a misfire i cant seem to figure out. i will list some background info below, and then all ive done so we can start from there. thanks in advance for any input...


1990 F250 4.9/t18 2wd 87k miles
when i got the truck it ran flawlessly. i pulled 8k pounds up one of the steepest winding grades in my county without an issue. i decided to sell the truck so took it to smog(thanks CA) and it failed. went home and pulled codes, and ended up get a code 31. some sort of problem with the egr system. which made sense because i had high nox reading on the smog test. so since i have another '90 f250 with a big six i swapped egr's. took care of the code but i developed a misfire somehow. it has a stumble/mis at idle and through the rpm range. even with the misfire, i now have absolutely no codes other than the #11 code. since then i have:

replaced plugs with motorcraft platinums
replaced plug wires
replaced distributor with reman unit including cap, rotor, tfe, and pip
interchanged MAP, TPS, IAC,(both have been cleaned), EGR regulator and sensor
checked CTS and is within spec
replaced all vacuum lines/checked for any clogs, etc..
checked the vapor canister
changed fuel filter
checked fuel pressure and its at 48psi running, bumps up about 10 psi when vacuum hose is disconnected
checked all my injectors with the screwdriver to ear test, all are functioning
pulled every plug wire while running and each one pulled gives a considerable misfire so i know they are firing
pulled the egr line while running to see if affects idle, it does not
checked for ground straps and all seem to be present and in decent shape
cleaned thoroughly the grounds in front of the battery and below the coil on the engine block
run around the entire engine/intake with propane and carb cleaner, no apparent vacuum leaks
put my vacuum gauge on a port from the intake and it has an idle pressure of 18, drops to 0 when throttle quickly opened and snaps back to 23 when throttle closed, then back to 18. it bumps up and down between 17-18 which made me think its an ignition problem(hence all the ignition parts replaced)
pulled 02 sensor to see if it affects misfire and it does not

i am at a loss. i dont think i can even remember all ive done but that is what i can remember... anyone have any ideas? thanks

 

also, forgot to mention that i timed the truck after replacing the dizzy, and yes i set base timing at 10btdc with the spout connector unplugged, and the engine at operating temp. timing seems to be bouncing around at idle. doesn't seem normal to me. i'd think at idle the timing mark would stay in the same place.. before doing the EGR the idle would bump up and down in rpm's. now it settles in at about 700 rpm's but the miss is noticeable to the point that you can feel it through the steering wheel when your hand is on it.. not sure what else to do..

 

Check compression, could have stuck or burned valve.

 

i thought of that but figured the vacuum test would have shown different results higher than 1hg in fluctuation if i had a burnt or stuck valve. am i incorrect?

 

Not always, had a bent push rod that once drove me nuts till I pulled the valve cover.

 

looks like i need to comp check it. thanks for your input

 

I had an egr vac solenoid that leaked on my truck that caused those issues. I think it left the egr slightly open all the time. I had my mighty vac and was testing each line looking for leaks. The egr vac solenoid would leak when I had my hand over the port that goes to the egr.

 

Hhmmm. I swapped out the egr vacuum solenoid between the two trucks I have with the big six's but no change. If the solenoid was leaking wouldn't it be cured temporarily when I pulled the egr line off of the egr? Also, I assume I would get a cel code??

 

Got it up to operating temp, pulled the plugs, and checked compression. All cylinders are between 171-176psi. So thats good. I also pulled out the timing light and took another look. The timing mark bounces back about 5 degrees every 3-5 times around. Even with the spout connector unplugged. This doesn't seem right to me... anyone have an explanation for that?

 

Are you sure what you pulled was indeed the spout?

 

Yes 100%. I know that would seem the obvious thing, but I know 100% what it looks like, and where its located. Makes no sense why it would be bouncing around like that. Even with it out. I've also swapped the ecm's between two trucks and no change. I pulled and capped every vacuum line except the MAP, then ran the engine. No change. Still sputtering and missing at idle

 

With the spout disconnected the engine surges slightly up and down. When I reconnect the spout it goes back to a steady rpm, but continues to miss and sputter

 

Pulling the spout should not cause it to surge. Are the wire in good shape?

 

That's what I was thinking. All the wires seem in good shape. I've checked and cleaned every ground I can find. Short of disecting the wiring harness to look for issues, I am a little lost. Shouldn't the timing marks stay in the same spot with the spout disconnected?

 

I'm starting to think this is a fuel related problem, maybe dirty thottlebody?