Hey guys I posted a couple days ago about a intermittent problem I’m having with my 1998 Jeep XJ 4.0. When my jeep gets hot around 215-230 degrees it starts to have a tiny misfire, it’s so small you barely notice it but if you turn it off while it’s this hot and turn it back on again it runs like it’s misfiring on 3 cylinders and struggles to stay on. If I give it gas however and rev it for a min or two it’s starts to smooth out and go away, from what I’ve read and what people tell me it might be a heat soaking problem, but I thought heat soak was only in 2000 and up 4.0 jeeps? I’m just wondering if anyone had had a similar problem or has had heat soak issues in pre 2000s jeep XJs? And yes I have replaced my crank sensor with an OEM mopar part, I’ve also replaced the coolant temp sensor and redone the entire ignition system. Also no codes in the computer pending or current.

 

Thinking I might have read about it here and there in pre '00 XJ's but......
My question is why is your Jeep running so hot?
When you turn it off how long did you let it sit before restarting and had the problem?
I want to say that heat soak takes a bit. Like 20 minutes between shut down and start up.
I would try getting it up to temp, shutting it down, popping the hood, and whatever time you have
let it sit between shut down and start up wait. Then start her and see what happens.
IMO your Jeep should not be running that hot.
Have you verified that temp with an IR gun?

 

Heat soak can happen with any make or model. With the XJ, the 00-01 are more prone to it than older models as thats when they started using a dual precat (some 00, and all 01), which sits directly below the intake manifold. The 4.0L gets hot, and those precats add even more heat. A heat shield was added that sits on top of the intake manifold to help keep heat away from the injectors. Older models did not have a heat shield. While they do not use precats, misfires still happen. Heat soak is not always the problem, tho. It could be a faulty injector, or its electrical connection. If there are no codes, keep driving it. Just keep tabs on it. It may not have happened enough, or long enough for the PCM to see it as a problem. Also, how old are the plugs and wires, cap and rotor? If the ignition has an issue that just happens to be cylinders 3 or whatever, that plug may not be firing when it should be. Misfires can be many things.

When it hits 230*, is the fan coming on? Thats getting a tad toasty for my tastes.

 

temp was read through a scanner, I have a brand new temp sensor in it so it know it’s fairly accurate, and it’ll happen as long as the temp is over 215 anything colder it doesn’t happen so if it does it, as long as I wait for it to drop below that temp it won’t do it , it also won’t do it if I’m driving only when I’m stopped for awhile or going slow like in traffic or doing some 4wheeling. I just bought the jeep and was planning to use it to go off-roading quite a lot this season and into the summer so I’d like to fix it before that hopefully, I did notice the coolant was pretty brown like rust coloured so I’m going to do a system flush this weekend, but that still doesn’t fix the fact that it misses when it gets that hot, id like it to run well even if I’m running through the desert at 80-90 degrees and seems like it’ll do this even when I’m 4wheeling at 60 degrees.

 

I just replaced the entire ignition system last weeks so everything is brand new copper plugs with a brass contact distributor and thicker 8mm wires and Napa ignition coil. All in the stock config, the fan does kick on at around 210 but if I’m out on the trail or in traffic it does not seem like it’s enough to cool it, radiator was recently replaced and same with water pump and thermostat, there’s actually a 180 Tstat in it right now which makes it even more weird, the coolant is pretty brown rust coloured so I’m gunna flush it this weekend and swap a 190 stat in it so it can run more like a stock car but shouldn’t these car still run okay even when they are crazy hot? I wanna run this thing pretty hard through the mountains and desert this season and through the summer for camping and such and I can’t have it running rough while I’m on the trail in the middle of nowhere.

 

Is your electric auxiliary fan working? It should come on around 220°, give or take, depending on the model year. If so, I still recommend wiring up a fan switch like this. Link to the how-to thread is in the video description.
Also, doing a good flush will probably help a bit. All that rust and corrosion tends to prevent heat from transferring the coolant, and the sediment tends to collect in the radiator.

 

yea my fan does turn on, around the 210 mark, the previous owner actually wired up a switch like that for it! It still gets hot and starts to
miss even with the switch constantly on, hopefully the flush cleans the cooling system but still doesn’t remedy my missing problem, like I said before I wanna drive this thing through the desert and while wheeling and don’t want it missing while I’m in the desert or on the trail.

 

How old is your water pump?

 

less than a year old

 

FB97xj1 brought up something.
Are you running the heat shield on top of the manifold?

 

I am not my jeep is a 98 so it did not come with one

 

I was driving my jeep today in about 80 degree weather and my scanner was saying it was running around 235 degrees, no misfires but now I’m sure the heat is effecting something in the engine, it never gets hotter than 235 tho so it’s not over heating it just running really hot, could that be because of my coolant system being kinda rusted, also this morning I tried to use the heater and it barely got hot so I’m assuming it’s clogged

 

I would flush the coolant a good amount of times until you can get clean water coming out.

 

so would the cause of my misfire be this heat?

 

I can’t say for sure but I wouldn’t rule it out.