Slow temp rise

I've got a 2001 Sport. 4.0/AW4. It only gets driven on weekends and special trips. Last summer it suddenly started doing the heat soak, P0303 bit, along with temps getting a bit high. so I rebuilt the cooling system. I never saw over 230 deg on my Scangauge. So, I installed a new radiator, water pump, hoses, thermostat, radiator cap, etc. All quality parts. I did screw up replacing the fan shroud and the mechanical fan would occasionally hit the shroud during acceleration. I actually didn't figure that out until it killed the fan clutch, which got replaced last fall with a Hayden standard unit. I re-positioned the fan shroud and now the fan sounds normal again.

Now the Jeep runs 198-204 deg as long as I am moving. If I sit anywhere for long, it slowly climbs. If I turn on the A/C it slowly climbs even when I'm moving. If I go up a long hill, the temp slowly climbs. Basically if I do ANYTHING other than very light normal type driving, the temp slowly climbs and keeps climbing until I pull over and let it cool off. It's gotten up in the mid 220's a couple times before I could pull over.

If I get over whatever is causing it to climb and get back to normal driving that doesn't call for more power, everything cools off back to the 198-204 range.

Any ideas what could be causing this? It never did this prior to the first overheats I mentioned. Could it be a small crack in my 0331 head? No visual signs of contamination in either the oil or coolant, but who knows about something smaller. I'm baffled. :banghead:

Does the electric fan come on when you turn on the AC? Does the electric fan come on when the temps get over 210ish? The 2001 still has the electric fan that comes on with the AC, right?

The electric fan should come on before you hit 220. If its not, the fan, relay, fuses, and wiring should be checked. To my knowledge, the 0331 crack will cause unexplained coolant loss (leaks into the oil) but not really overheating. Mildly blown head gaskets can cause odd temperature problems. If you're going to pull the head to change the head gasket you may very well consider changing to something other than a factory original 0331. Many crack, especially when exposed to higher temperatures, but not all heads crack. It was more than 2 years after my thermostat failed shut that I starting seeing elevated potassium levels (indicative of coolant) in my oil - not bad enough for milkshake oil or residue under the valve cover.

The electric fan works once temps get high. 217 deg maybe? I also wired a switch in so I can override it and turn it on manually. I use that to cool it down after shutoff so I don't get the heatsoak problem.

I plan on running a UOA on the oil to see if there are any contaminants. If I do see signs of a head gasket problem or head problems, then yes, I'll probably go with one of the aftermarket 0331 replacements.

Just looking for other suggestions in case I've missed something. Thanks for the replies so far!

peterjford said:

Does the electric fan come on when you turn on the AC? Does the electric fan come on when the temps get over 210ish? The 2001 still has the electric fan that comes on with the AC, right?

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no, the fan comes on based on pressure switches. It doesn't come on all the time like the early models.


Since it's an 01, and has the worlds dumbest exhaust manifold I would really suspect the cats being a big part of your problem if it's got 200K miles on it.

The electric fan should come on at 220 degrees per the FSM. The electric fan will also come on when the A/C is on. How old is the electric fan? I know my fan will come on at 222 degrees every time... It shuts off at 215.

Sorry about the delay in replying. Work interfered.

Forgot to mention the Jeep only has 80,000 miles, so the cat SHOULD be OK, but obviously could be bad. It's also a CA emissions, so has a pre-cat. But, as I said initially, it never had this behavior before last year. Before it just got to 204 or so, and very seldom went higher.

The electric fan got replaced last year when I did the cooling system rebuild. I know it runs and I'm assuming it pushes the same amount of air as the factory one.

I don't think the electric fan automatically switches on with the A/C like the earlier models. I know it does kick in at higher temps, but if I'm in one of the situations I mentioned in my first post, the temps will just keep climbing anyway.

Ordered a oil analysis kit, so that will be one thing I'm checking.

I could never drive behind semi trucks on the highway or run ac on the highway with out over heating. Put in a 3 row radiator and now I can drive up some pretty steep grades with the ac in the summer weather and my gauge will sometimes reach 210. Only downside is it runs so cold in the winter. My jeep is a 93 4.0 ax15 btw. I got my 3 row from radiator barn

The FSM shows that on 2000-2001 models, the electric fan comes on when A/C pressure is at 300 +/- 20 PSI... The computer turns the fan on at 223 degrees and turns off at 217 without A/C dialed up... I just verified the numbers. I watch my coolant temps like a hawk! I also flush my coolant every 2 years on the 4.0 whether it needs it or not. Has your water pump ever been changed?

with 80K you'd hope the cats in the manifold wren't bad.

Have you looked at the fuel trims? Is it running lean/rich?

Probably not a bad idea to check the oil out, but it may not have coolant in the oil. You may be pushing combustion gasses into the coolant, which would hot spot the head. I'd run a hydrocarbon sniff test on the coolant too if you really suspect the 0331 has gone tits up.

FWIW I've got an 01 with 230K on it and it's original 0331 head, so they truly are a crap shoot.

Call me crazy but I think you are running too cool at 198-204, I always read that 210 is normal operating temp. Maybe you need to normalize your temp to around 210 first. What thermostat are you running, any high flow cooling additions and how many core radiator?

I am having a similar slow heat issue and I keep suspecting exhaust or blow head gasket, for what its worth.

iwannadie said:

Call me crazy but I think you are running too cool at 198-204, I always read that 210 is normal operating temp. Maybe you need to normalize your temp to around 210 first. What thermostat are you running, any high flow cooling additions and how many core radiator?

I am having a similar slow heat issue and I keep suspecting exhaust or blow head gasket, for what its worth.

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no.
closed loop happens at 170ish, the thermostat is designed to keep the operating temperature at 195. You'd be aghast to see what my 5 speed renix jeeps run at. With no trans heat they get to 190 and stay there. Still managing 25 MPG on the highway.
My wife's 01 runs at 200 and is equally happy.

The thermostats job is to keep heat in the motor. If your junk runs above the thermostats designed temp then your cooling system is inadequate. You should have more cooling than you need and let the thermostat keep the motor warm.

87manche said:

with 80K you'd hope the cats in the manifold wren't bad.

Have you looked at the fuel trims? Is it running lean/rich?

Probably not a bad idea to check the oil out, but it may not have coolant in the oil. You may be pushing combustion gasses into the coolant, which would hot spot the head. I'd run a hydrocarbon sniff test on the coolant too if you really suspect the 0331 has gone tits up.

FWIW I've got an 01 with 230K on it and it's original 0331 head, so they truly are a crap shoot.

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Good suggestions. I do have OBDII software for my laptop so I will set it up and take some readings this weekend on fuel trim etc.

Hadn't heard of a hydrocarbon sniff test. I'll look into that.

For another poster above, yes, new 3 row radiator and new water pump (not reman.) when I rebuilt the cooling system. Also new 195 deg thermostat.

You might want to try replacing your thermostat, or a quick test without one. I know someone that went through their entire cooling system only to find that the new thermostat was bad.

https://www.youtube.com/watch?v=ON_RtokUohI

autozone will rent you the tool. You have to buy the fluid for like 10 bucks.

http://www.autozone.com/autozone/accessories/Tools-Garage-and-Equipment/Block-Tester/_/N-25dh

If there is a Napa autocare center near you they may do the hydrocarbon test for you. They did mine for free. A member on the forum told me that anything above 5ppm is indicative of exhaust gases in the coolant.
The guy at napa told me not to worry about anything under 10ppm. I lean to the guys on here that know XJ's over the other guy.

I have the block tester mentioned above, however if your leak is very small, the results may come back a little non conclusive. Mine didn't turn yellow, just slightly green.

Any updates on this? I have the same exact issue and replaced the same parts. Still does the same thing. Thanks.