What's new
By:
Filters

91 YJ runs rough until warmed up question

Thumping

Thumping

Well-known member
Joined
May 19, 2020
Member Number
255
Messages
1,429
Trying to help a friend diagnose an issue. Its a 91 YJ with a 4.0 5spd. When cold it starts fine but at about 2000 rpm is starts misfiring and back firing. Once warmed up it runs fine. Driving while its warm is fine but if its cold outside and the engine temps drop below around 210 it starts to misfire again but the odd part is, no check engine light which in of itself could be an issue.

New plugs, wires, cap. New throttle position sensor, crank sensor, cam sensor and temperature sensor.

This is where he starts asking me for advise. My first thought was the temperature sensor but he's changed pretty much all of them. So it seems to me its either in the ECM or PCM.

I don't have alot of experience with a jeep 4.0 so I thought I'd ask those that do.

Any advise? Besides an LS swap....:flipoff2:
 
Thumping said:
Trying to help a friend diagnose an issue. Its a 91 YJ with a 4.0 5spd. When cold it starts fine but at about 2000 rpm is starts misfiring and back firing. Once warmed up it runs fine. Driving while its warm is fine but if its cold outside and the engine temps drop below around 210 it starts to misfire again but the odd part is, no check engine light which in of itself could be an issue.

New plugs, wires, cap. New throttle position sensor, crank sensor, cam sensor and temperature sensor.

This is where he starts asking me for advise. My first thought was the temperature sensor but he's changed pretty much all of them. So it seems to me its either in the ECM or PCM.

I don't have alot of experience with a jeep 4.0 so I thought I'd ask those that do.

Any advise? Besides an LS swap....:flipoff2:
Click to expand...
o2 sensor before cat?
 
Thumping said:
I'll have him check it. That could be an issue but for some reason its not throwing a check engine light. But that could be the light doesn't work...lol
Click to expand...
My 97 4.0 did some real weird shit as the front O2 sensor was leaving the chat. Took a few days before it got bad enough that I started to suspect it.
 
Sandy Johnson said:
My 97 4.0 did some real weird shit as the front O2 sensor was leaving the chat. Took a few days before it got bad enough that I started to suspect it.
Click to expand...
Well, that's not the problem. It did finally throw a code. According to the code one of the injectors is an issue. So, down that rabbit hole we shall go. :laughing:
 
Rockwood said:
I tried the multi-hole, whatever injectors… Never ran as well as a good set of OE injectors.
Click to expand...
Yep, I'm going to suggest OEM but that's up to him. I've tried advance injectors before and ended up going oem and just threw the advance ones in the trash. I guess its one of those pay me now or pay me later but either way, you're going to pay me. :laughing:
 
Fuel injector issues won't cause backfiring.

I'm leaning toward valve-train issue. I kind of suspect lifter/push-rod is too full of shit to bleed oil properly and the lifter is pumping up when the oil pressure is high because of cold oil. The fact that it doesn't come on until a good rev forces oil pressure to be high in those "further downstream" parts of the system is what makes me lean in that direction.

Maybe try cutting the oil with some ATF and see if that clears it up over time or do a seafoam treatment or whatever.
 
Last edited:
You must log in or register to reply here.
Top Back Refresh