Eisenschwein

... winch bumper. I see you have winch and lights up front. Is it possible you are not getting enough airflow because of these things blocking the radiator?

vank

While possible this seems unlikely. A lot of people mention this but I have yet to see someone who says they removed their winch and their problem was solved. The lights are mounted pretty far to the side and aren't in the direct path. I also had this same bumper, winch, and lights on a 92 before this 94 with no issues.

However, I suppose it's easy enough to remove the winch and give it a shot. I'll try this and report back. Seems I've tried everything else...

Eisenschwein

I agree, not my #1 choice of causes but if you've exhausted all other ideas, might be worth checking out. I know it helps to block them in the winter to help warm up, so it might work in reverse.

GregGA

Get an IR gun and shoot the thermostat housing when warm.

A well maintained XJ does not run hot. My 87, with original radiator, runs at 195-200.
It also has the "dreaded" closed cooling system. My dash gauge is actually very accurate. You mileage may vary

GregGA

^yep got to accurately verify what it is running at. Get an IR gun at Harbor Freight. About $30

EZEARL

"fan clutch is newish but since I'm overheating on the highway its not really suspect..What am I missing?"
Mine overheated on the highway or pulling a grade. Replacing only the clutch fan fixed it.
Also what gears you running with those 35's?

94XJAZ

I am dealing with the same issue, I replaced/rebuilt the entire cooling system as you did with the Hesco parts and all. Even a heavy duty fan clutch (ZJ I think) and high flow thermostat. I am starting to think it is a faulty thermostat. I have had thermostats fail in the past on other vehicles, they will open crooked or partially, then when your asking more coolant flow at freeway speeds, you don't get enough cooling regardless the air flow. I also vented my hood(2" spacers on the hinges), drove it two weeks with it running at 195 then poof up to 230-240! I am going to change the thermostat this weekend. Throw parts at it, heck maybe it will run cool LOL.

Firestorm500

Vank, removing the winch won't help.

I believe the head(s) I had had an internal crack in it somewhere that caused the engine to overheat. It was nothing in the block or cooling system that was not up to spec. Prior to the rebuild, I did everything you did, including the cat and muffler.

I did not have milky oil or deposits anywhere. It did not foul plugs. It ran smoothly. Started well. It did not burn or leak coolant. In essence, I went through everything you have done.

The first, original head had cracks around the valve pockets in #5 and #6 cylinders. The replacement head came from a salvage yard Jeep that was one year newer and had 35,000 less miles on it. It blew the rear freeze plug out soon after installation. By the time it blew the third rubber plug out, I had replaced everything else as you have done. I finally pulled that head back off to put a brass freeze plug in it. While the short block was still in the Jeep, I said screw it, I'm going to have the whole thing rebuilt. I was scared of the engine, as I did not trust it. I certainly didn't want my wife or daughter to drive it. I keep all my stuff ready to drive 2,000 miles or more on a trip with no advance warning. Just because.

The head that blew out the freeze plugs was remanned, as I said. It still ran hot with the newly rebuilt long block. The head came back off (again!) and was replaced with a rebuilt one from the machine shop.

Problem solved. It never ran hot again. I live in the South where it is very hot during the summer, and is humid summer and winter. I run the A/C from March through November most years. Stop and go, or freeway driving, it stays cool to normal.

If I ever rebuild a 4.0 again that is running hot, I will just chunk the head and buy a brand new one from Clearwater or another aftermarket outfit.

I'm not telling you to run out an get a brand new head. But since you have gone overboard like I did doing everything else, what else could it be?

I rebuilt my short block because it had 192,000 miles on it and low oil pressure. The kid I bought the Jeep from had little to no money. The radiator leaked, so he let it overheat on numerous occasions. That's probably what cracked the valve pockets on the original head. The back cylinders run hotter.

CCKen

.012 out of flat is quite a bit. How many .xxx was milled off to true it up.
Normally .005 should true up any head. Your combustion chamber temps might be higher than normal due to increased compression. Was any milled off the cylinder block deck?

What spark plugs are you using? RC12LYC? You may want to try a cooler plug like the RC9YC and see what happens. In any case, remove your existing plugs and inspect them. See if they look like they're running hot.


I think you should consider installing a different head.







.

vank

I have already verified this. 220 to 230 at the back of the head. Closer to 210-215 at the thermostat housing.

I can try this but it does not make sense to me. Also it doesn't seem bad. Engine running hood popped the fan feels like it's blowing hurricane force winds. I'm running 4.10s currently (soon to swap 4.88s out of another jeep with the same axles before he sells it).

Firestorm and CCKen, I already pulled the head and had it pressure tested and resurfaced from a machine shop I trust. Wouldn't that rule out the head? My Jeep only has 145,000 on it and runs like a top. Even though it runs hot I've never overheated it but don't know much about it's past life other than it seemed to be very well taken care of and I got a stack of receipts and service records with it.

I did not have the block machined and didn't think about testing it for flatness. I have a long skinny granite slab and feeler gauges I could have used but didn't even think about it.

I'll have to dig up receipts to find what spark plugs I put in last. They were OEM champions I believe though. I'll check a few plugs tonight and see what they look like. Thanks for the chart!

vank

One more piece of info I left out before. When I first start the Jeep and watch the gauge as it heats up, it slowly creeps up and past 210, hits about 230-235, and then drops fairly quick to around 220. To me it's almost like the thermostat is opening late.. I have tried three different thermostats at this point all with the same results (motorad 180, failsafe 195, and a super stant 195). Maybe next step is to try no thermostat. I'd rather try that first than replace the head.

Firestorm500

I feel your pain.

Neither the experts nor I can definitively explain what was wrong with the second (SY) head. I was shown what was wrong with my first, original head.

Replacing the second head did finally solve MY problem. In retrospect, overheating may have led to that Jeep being in the SY in the first place. It wasn't wrecked. This wasn't a pick-n-pull type yard, either.

If no thermostat does not solve your problem, you will have a big decision to make.

vank

So here's a pic of cylinder 6 (and theoretically hottest) spark plug:



FYI they are the RC12LYC Champions.

Next I'll pull the thermostat once the engine cools.

Firestorm500

It matches the "hot or lean" category.

freegdr

What do the rest look like . Number 1 would be the hottest part of engine.