strange overheating on Gen 2.5

[Dolomiti]

I picked up a low mileage (110k) 2.5 yesterday about 5 hours from home. Vehicle was in good condition and I did a look over before driving it home.

About 35 miles into the return I encountered an odd problem. I was using cruise control at about 75 mph and had the a/c on high when I glanced down and noticed the temp gauge rising. It was a little below the mid way between normal (middle) and high. As I slowed down, the a/c cut off and the temp stopped rising. I pulled over and all things seemed normal, coolant level fine, coolant cycling through radiator, fan and a/c fan operating normally, belts fine, no leaking fluid, no binding of belt when compressor engaged, etc.

As I started driving again, temp returned to normal, maybe even a little below normal, and a/c began working again. So long as I drove about 65-70 with a/c on econ, temp would remain in mid range but would fluctuate mildly between slightly cooler and slightly warmer. If I slowed down while driving in town, it would cool down. If I turned to a/c or drove faster, temp would rise.

I'm stumped. I made it home fine but have no idea. Engine bay is clean and there are no noticeable leaks around the head. I plan on putting in a new cooling system and timing belt anyway, but this problem seems illogical.

 

[mapper]

Any blockages to airflow on radiator? Extra lights, that kind of thing? Radiator could be experiencing some internal blockage too.

I know you mentioned that it was working but I'd double check that the he auxiliary fan that comes on with the A/C turning on when it should? OR for that matter it may be worn/working inefficiently? Maybe it just is worn out and running slow? I'm saying this because your temps seem to be completely normal except with the added heat load of the A/C running..when the aux fan is really needed.

Does it stay Normal if you just let it idle with A/C on?

 

[MonteroLTD]

Aside from checking your cooling system, check the a/c high pressure hose temps with a heat gun. There may be an issue causing the temps to be higher than normal and the condenser can't cool it down fast enough.

 

[Hopeless Diamond]

I have no knowledge of these motors but check to see if the cooling system needs to be "burped". There could be a air pocket in the system causing problems.

 

[Mrknowitall]

You have classic symptoms of a marginal cooling system. I would bite the bullet to have a radiator shop back-flush the radiator. Spring for a new thermostat while the system is open. There is an air bleeder on top of the t-stat housing. you can give that a whirl first, but in your neck of the woods, I'd make sure that radiator is nice and clean. also make sure there isn't debris trapped between the AC condenser and the rad core.

 

[Dolomiti]

I'm planning on doing a new radiator, water pump, thermostat, and timing belt anyway, but it was just strange. Hopefully that will solve it. If not, I'll have to look at the a/c system.

You have classic symptoms of a marginal cooling system. I would bite the bullet to have a radiator shop back-flush the radiator. Spring for a new thermostat while the system is open. There is an air bleeder on top of the t-stat housing. you can give that a whirl first, but in your neck of the woods, I'd make sure that radiator is nice and clean. also make sure there isn't debris trapped between the AC condenser and the rad core.

 

[jeep-N-montero]

I had the exact same issue and symptoms you are describing and it was the condenser fan that needed replacing. Did you check the fan with the engine off but key on? I ask because the main engine fan pulls air through the front causing the condenser fan to turn giving the illusion it's powered on. Do this and report back.

 

[Dolomiti]

I had the exact same issue and symptoms you are describing and it was the condenser fan that needed replacing. Did you check the fan with the engine off but key on? I ask because the main engine fan pulls air through the front causing the condenser fan to turn giving the illusion it's powered on. Do this and report back.

Just tried it with engine off and a/c on. Condenser fan is running without (apparent) issue.

Any blockages to airflow on radiator? Extra lights, that kind of thing? Radiator could be experiencing some internal blockage too.

I know you mentioned that it was working but I'd double check that the he auxiliary fan that comes on with the A/C turning on when it should? OR for that matter it may be worn/working inefficiently? Maybe it just is worn out and running slow? I'm saying this because your temps seem to be completely normal except with the added heat load of the A/C running..when the aux fan is really needed.

Does it stay Normal if you just let it idle with A/C on?

In idle with air on full, temp stays fine.

 

[nwoods]

I had the exact same issue and symptoms you are describing and it was the condenser fan that needed replacing. Did you check the fan with the engine off but key on? I ask because the main engine fan pulls air through the front causing the condenser fan to turn giving the illusion it's powered on. Do this and report back.

Mine did the same thing. I swapped the fan in front of the radiator with another from a salvage yard (thanks Billygoats!), and now all is well.

 

[VenturaMontero]

I had the same issue with my 98 Gen 2.5. The truck is in very good shape even with 160K so I did not suspect anything serious. That being said I replaced the thermostat which I suspected was OEM with a new unit. After refilling and bleeding the cooling system I have NEVER seen the gauge move from "normal" no matter the driving conditions. Just my experience.

 

[nckwltn]

I can cruise and cruise on the freeway at any temps with no problem.... but when I was going up mountains in colorado, even coming up the grapevine back to LA I noticed that if I was pushing higher RPMs (4k+) to keep speed going up a hill, my temp would start to climb. It never went too high, but was up quite a bit from the standard 1/2 mark.

Were your RPMs doing anything different when the temp gauge was going up?

 

[Dolomiti]

Seemed like normal rpm range and no hills. I have driven all over Houston this week, even tow dollied an e30 and temp stays normal.

I can cruise and cruise on the freeway at any temps with no problem.... but when I was going up mountains in colorado, even coming up the grapevine back to LA I noticed that if I was pushing higher RPMs (4k+) to keep speed going up a hill, my temp would start to climb. It never went too high, but was up quite a bit from the standard 1/2 mark.

Were your RPMs doing anything different when the temp gauge was going up?

 

[Dolomiti]

Thanks. I have the parts for a swap, thermostat, waterpump, timing belt kit, radiator, and all new hoses and belts. Will put them in this week and see what happens.

I had the same issue with my 98 Gen 2.5. The truck is in very good shape even with 160K so I did not suspect anything serious. That being said I replaced the thermostat which I suspected was OEM with a new unit. After refilling and bleeding the cooling system I have NEVER seen the gauge move from "normal" no matter the driving conditions. Just my experience.

 

[jeep-N-montero]

I'll be going through my cooling system this week as well, the only thing that saved my butt this weekend on a trail at slow speeds was propping the hood open a few inches with a piece of wood and running the 12 inch auxiliary fan.