Coolant temp rising toward red at hiway speed...

1988 XJ 4.0 Auto 4x4 A/C (not working) 95,000 miles

I have the original cooling system. I bought the Jeep during cool weather but the owners said they had a overheating problem and the bottle and hoses were replaced. They suggested I keep an eye on the temp.

The temp has been running just below or right at 210. No problem there. But now the outside air temp is rising as summer approaches and we have had a few days in the upper 80's and low 90's and my coolant temp slowly but steadily rises toward, but not into, the red. This happens on a 25 mile interstate commute at a steady speed of 75mph. Instead of stopping at 210 the gage keeps gradually rising during the entire trip and is just about to reach the red when I get to my destination.

When the weather drops back into the 70's the temp gage stays right at 210 and all is well.

The electric fan will engage when the temp rises. It sounds to me like the radiator may need cleaned or boiled or whatever it's called since the cooling system seems to be doing its job until amabient temp gets high enough that the radiator just can't flow enough to handle it.

I know the XJ's have had a few cooling issues so I thought I'd see if anyone has had this same experience or has any suggestions for what I might check.

thanks in advance,
John

Actually overheating at highways speeds usually is a flow problem. If you are unsure of the waterpumps age i would start there. Replace it it should not be more than 50$.

pete

also check airflow, when I bought mine it had an overheating problem. Turns out there was what looked like a birds nest between the radiator and condenser, and the rubber cowling between the two had fallen, check this before the water pump, because it is cheaper.

It may not be such a problem down there in the south, but here in rust country, another problem is fin rot. Even if the flow is good, when the little copper fins corrode, the thing will overheat, and the only solution is a new radiator.

You can usually some gaps at the ends of the tubes of a rotten radiator, where the fins have blown out. If you can see gaps, it's already worse than it looks, because many other fins have lost their bond to the tubes and aren't conducting any more.

Stuck closed thermostat?Possible air bound cooling system?What happens when you turn the heater on hi?does the temp drop?

Stuck closed thermostat?Possible air bound cooling system

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Not if it only overheats when it's hot outside. that is caused by lack of cooling system efficiency which can be caused by corrosion (inside radiator and outside radiator) or blocked airflow a stuck thermostat will overheat much quicker and at all ambient temps.

True,but it could still be air bound....

I know that this sounds stupid but check you belt tention.... I fought the same problem for to summers. Changed the stat,water pump, clutch fan even bought a fan switch to over ride the temp. It would turn on my elec. fan when the key would turn on.

Bo

That is what mine was doing then my radiator cracked and i decided to go with a swap to a 92's i think so that it had the cap not just the overflow bottle and i live in the desert and no more problems.

What about head gasket? I could be wrong, but a blow headgasket would cause highway overheating.

Mudd said:

1988 XJ 4.0 Auto 4x4 A/C (not working) 95,000 miles

I have the original cooling system.

The temp has been running just below or right at 210. No problem there. But now the outside air temp is rising as summer approaches and we have had a few days in the upper 80's and low 90's and my coolant temp slowly but steadily rises toward, but not into, the red. This happens on a 25 mile interstate commute at a steady speed of 75mph. Instead of stopping at 210 the gage keeps gradually rising during the entire trip and is just about to reach the red when I get to my destination.

When the weather drops back into the 70's the temp gage stays right at 210 and all is well.

Click to expand...

You need a new radiator, pure and simple. It's 18 years old so forget about flushing it, boiling it, chemically cleaning it, or even barbecueing it. Most of the core is probably blocked so anything you do will, at best, be a band aid fix.

This problem could be one of two things...

1) A restricted radiator, either restricted coolant flow or airflow. Make sure all the fins are intact and straight, the grille is not blocked in any way, etc. If all is OK there, backflush the radiator and see if a lot of corrosion and crap comes out. After flushing it, take it for a highway spin and see if the problem is fixed. No need to buy a radiator right off the bat if this one's not leaking.

2) Water pump. I've seen impellers break on the inside of water pumps, causing this problem...it's actually more common than you might think. If backflushing the radiator didn't work, pull the water pump off and inspect it.

Just a note...if you do end up needing a new radiator, do yourself a favor and get one for a 91-01, not a direct fit for a '88.

anony91xj said:

Just a note...if you do end up needing a new radiator, do yourself a favor and get one for a 91-01, not a direct fit for a '88.

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At that age, I would put my money on the radiator.

I disagree with this advice, replace it with a all metal replacment for the same year. There is NO reason to go thru the trouble of converting to an "Open" system.

Rev

Rev Den said:

At that age, I would put my money on the radiator.

I disagree with this advice, replace it with a all metal replacment for the same year. There is NO reason to go thru the trouble of converting to an "Open" system.

Rev

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No reason really, except it's easier to burp the open system. One of my coworkers also has an '89 that he converted to open and he claims he has better heat now. I can't confirm that, but I can say that the heat in my '87 with the closed system leaves something to be desired.

anony91xj said:

No reason really, except it's easier to burp the open system. One of my coworkers also has an '89 that he converted to open and he claims he has better heat now. I can't confirm that, but I can say that the heat in my '87 with the closed system leaves something to be desired.

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The closed system is self burping if you drill a small hole in the thermostat in the right place.....before I drilled my thermo on mine I barley had heat and now I do so I think the system/heater core was air bound.I drove around for a few hours tried the heat and still wasn't hot,then shut the motor off for a few,got back in ran it up to op.temp...suddenly I had good heat.I reccomend a good heater core backflush also..

jeepdeepfreak said:

The closed system is self burping if you drill a small hole in the thermostat in the right place.....before I drilled my thermo on mine I barley had heat and now I do so I think the system/heater core was air bound.I drove around for a few hours tried the heat and still wasn't hot,then shut the motor off for a few,got back in ran it up to op.temp...suddenly I had good heat.I reccomend a good heater core backflush also..

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Yeah I've gone through all that. I actually have a 91+ thermostat housing, so I just bleed the system through the sensor, problem solved

All of this done and the heat works, just doesn't get as hot as any other car I drive, not even as hot as my old '91 XJ. Got significantly better when I bypassed the heater control valve, but it still just doesn't feel up to par on a cold winter day. I really don't care enough to shell out for a new rad just to fix this problem...but if my radiator ever does go south I'll buy a 91-01 rad to replace it with.

Thanks for all the input. I'm sure I'll have no trouble determining the root cause with all the info you have provided. Replacement radiators are pretty cheap anyway, if it turns out to be the problem.

Thanks again,

Now I will post a question about my A/C.....

John