Stumble off idle

[jonb8]

My wife's mud racer has a poor mans 4.6 in it with a stock head and camshaft. She picked up a low end miss off idle late in the season last year and I just started messing with it again. I just changed the fuel pump just because it's the last thing that isn't new and it's still there. The jeep is OBD2 1996 and it really comes to life after a couple stumbles but she's really loosing time off the line. She's running open exhaust with no cat or muffler (2 1/2 out the back), 24lbs inj and a 63mm throttle body if that helps...any help would be appreciated cause I'm gonna be in the dog house if she starts the season with this miss and I'm not sure were to go next...

[doublins]

My wife's mud racer has a poor mans 4.6 in it with a stock head and camshaft. She picked up a low end miss off idle late in the season last year and I just started messing with it again. I just changed the fuel pump just because it's the last thing that isn't new and it's still there. The jeep is OBD2 1996 and it really comes to life after a couple stumbles but she's really loosing time off the line. She's running open exhaust with no cat or muffler (2 1/2 out the back), 24lbs inj and a 63mm throttle body if that helps...any help would be appreciated cause I'm gonna be in the dog house if she starts the season with this miss and I'm not sure were to go next...

Both times my motors developed a miss that could not be traced to ignition or injectors, I found the result to be mechanical issues inside of the engine. Pull the covers and have a look- it'll save you the pain of throwing parts and money at the ignition system trying to chase it. If that checks out, then you can start checking out things like CKP, CPS, wires, plugs, injectors, etc. etc. Pulling the covers is free though if you sync it up with a planned oil change- might let you know what's up.

[racing89]

Fuel contamination. Back flush check your filter ,not only for mud, water and such, but small pieces of foam from your fuel cell that overtime will separate and can cause these symptoms. If this is the case, pull your rail and injectors, clean and flush out.

[jonb8]

Just did a compression and fuel pressure check and both came out right on the money, I changed the coil again with a used one and bam the miss was gone when power breaking in garage. I backed it outside and let her loose and it died, had to hold it to the floor to start it (multiple times) and it wouldn't idle at all.... Changed the coil back to the original one and now it seems fixed, takes fuel great under a load and the stumble is gone... I know this isn't gonna be that easy but seems something is going on in the electical system. I also ran the codes cause I had a engine light and it came back a voltage problem and battery temp problem in board. Checked and the sensor in the battery tray was unplugged so now my engine light is finally gone... I'm gonna take it out and run the crap out of it and see how it goes.

[racing89]

Fuel sample ?

[doublins]

Just did a compression and fuel pressure check and both came out right on the money, I changed the coil again with a used one and bam the miss was gone when power breaking in garage. I backed it outside and let her loose and it died, had to hold it to the floor to start it (multiple times) and it wouldn't idle at all.... Changed the coil back to the original one and now it seems fixed, takes fuel great under a load and the stumble is gone... I know this isn't gonna be that easy but seems something is going on in the electical system. I also ran the codes cause I had a engine light and it came back a voltage problem and battery temp problem in board. Checked and the sensor in the battery tray was unplugged so now my engine light is finally gone... I'm gonna take it out and run the crap out of it and see how it goes.

Loose wire in the harness at coil connector? Turn it on let it run and wiggle the harness with your hand to see if it starts missing- I also had this problem that would come and go and traced to a bad connection at the coil, fixed it and problem gone. The fact that it came and went after changing out coils twice would seem to make this a possibility

[racing89]

After you backflush check the fuel filter this weekend ,pull your fuel rail off and let me know how you make out.

[jonb8]

All right, I'm getting back on it today, I'll pull the fuel system down and clean everything... I'm with you, I was thinking fuel delivery till the whole coil thing. I built another motor for this year but this one's going in my AMC Eagle build and I wanna track down it's problem (mecanical or electical) before I pull it.

[jonb8]

Forgot to end this post with the cause. It was the aftermarket tach that I had tied into the coil ground were it plugs into the coil....

[doublins]

Forgot to end this post with the cause. It was the aftermarket tach that I had tied into the coil ground were it plugs into the coil....

Thank you for following up- always appreciated when people go searching for a problem to see resolutions posted at the end.