Overheating at highway speeds

Well I guess i will start with I have a 92 XJ. I replaced the motor 2 years ago with one from a 98 grand. Found out water does not compress to well. The new moter had 55,000 miles on it. Well have not had any problems with it,but after i installed the motor I relized there was not a spot to put in the temp sensor for the guage. It is on the back of the head. Well there was no room to drill it now, so i moved it to the front of the motor on the t-state housing. It has alyays ran about 210 till a few weeks ago. Now I am up to about 225 to 230. The rad. is about 1 year old, new water pump, new t-state, new cap, and new sensor. all replaced with in the last week. I know that my guage will read higher because of the location, but it has never ran this hot before. also checked the rad. out when i changed the pump, did not have any blockage. Any ideas? it i slow down to 55 it cools back down, but at 70 heats up.

70mph =2700 rpm
33" and 4:56

Check your radiator for flow.

I'd replace the thermostat since it is the cheapest part and make sure you have coolant in the thing, sometimes when coolant is low there will be air pockets and other stuff.

I did check the rad. for flow when i replaced the waterpump. also that was the 3rd t-stat

easttxjeeper said:

I did check the rad. for flow when i replaced the waterpump. also that was the 3rd t-stat

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Overheating at highway speeds is almost always caused by insufficient heat transfer thru the rad. That can only happen for two reasons:
1. crud plugging up the tubes, preventing SUFFICIENT flow of coolant thru the rad.
2. bugs, mud, grass, etc. plugging up the FINS on the rad preventing airflow thru the rad.

I am betting you have a rad that "seems" to flow OK, but really is insufficient. Take the rad out, and have a rad shop test the rad for flow. I predict they say it's plugged.

radiator

Does your lower radiator hose have the spring in it ?

I live in california and climb some long hills daily on the freeway and noticed mine heated up just like you said 55 ok....70 to hot. I changed to a cooler thermostat and it made a big differnce. Ofcourse now im sure ill here I went to cool with a 160 thermostat but its working fine for me.

dzraces said:

I live in california and climb some long hills daily on the freeway and noticed mine heated up just like you said 55 ok....70 to hot. I changed to a cooler thermostat and it made a big differnce. Ofcourse now im sure ill here I went to cool with a 160 thermostat but its working fine for me.

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The lower tstat doesn't solve the problem, it just masks it as it's opening up too early. Anyways I am willing to bet $ that if you were to drive a longer climb then you normally do you'd heat up again. Also since you tstat is opening up at 160, your engine runs too cool most of the time which is also not too good of an idea.

Ok I am going out on a limb here and going to dissagree with everyone else saying the rad is the problem.

Overheating while at speed usually means that the coolant is not being slowed down enough to cool properly.. example...when you completly remove the thermostat, at highway speed the vehicle overheats... Now with that said. The first thing I would do, is replace the thermostat...I don't care if its the third or fourth one... buy a good one preferably a 180* at this point with summer comming up. The reason behind this is when a thermostat fails it just doesnt stay closed or open like everyone believes, it will also stay partially open, especially if it has been steamed from getting hot, or air pocket... If the thermostat is suck partially open, you have sufficent cooling at normal driving conditions, and will heat up at speed. The coolant can not get out of the engine fast enough... Also I would like to see a picture of how your temp sender is mounted in the thermostat housing also, you could be getting a false reading as well.... Now this is just my opinion...take it for what its worth...

That is a valid point, but here is how it was explained to me. When you have a properly working cooling system, your t-stat is constantly opening and closing minutely. it is true that a partially blocked t-stat will not flow coolant quickly out of the engine, but it will also keep coolant in your rad longer. This causes the fluid in the rad to cool more. Therefore cooler fluid is now entering your engine. A t-stat is designed to keep your engine at a particular tempurature range. A cooler stat opens quicker, but it also reacts quicker to hotter temps as well. A cooler t-stat will actually start to close off at high temps in an attempt to keep coolant in the rad longer.

Now, to be honest, I thought this was BS. Until I actually changed my t-stat and started to think about it. When I went to change out my t-stat I had found that the spring retaining clip had failed, causing my t-stat to go into full flow mode. I had alot of similar problems as what you folks experience. Nice and cool at low speeds, but overheating at highway speeds. The coolant simply wasn't staying in the rad long enough to be sufficiently cooled. I put a brand new 195 deg stat, and the problem went away. So it makes sense to me.

i had to replace 3 t-stats to get one to work, do yourself a favor the next time you have to do it, test it first, it's way too easy, i know, but 2 bad stats in a row, kinda changed my mind.... throw it into water that is the temp of the stat, ie 185, make sure the waters 185, with a kitchen thermometer. if it opens, its good, then take the water off the heat and see what temp it starts to close.... then you know if its good or not. on the freeway is the wrong place to find out it doesn't work.

Check your lower rad hose at high rpm. Sometimes they wear out and the pull from the waterpump will make the hose pinch shut. Good call RichP

Hi: I have been following this thread. I have a great interest in anyone else whose Jeep overheats. I have had overheating problems in my '93 XJ for several years. And I have changed it all!

One thing that I don't think was mentioned in your case was the fan clutch, which will totally make you overheat at highway speeds because if it's not engaging the fan you know isn't drawing air at speed.

So, if suddenly you're overheating on the highway and you haven't changed the clutch at least do the test.

Currently I am experimenting with thermostats with holes drilled to increase water flow. I am convinced that my Jeep runs hot because there isn't enough water flow, contrary to what some others think. They will say running without a stat "masks" the problem when the issue is really a plugged rad.

Well, in my case, I have a new rad, pump, clutch, hoses with lower spring, sensors and have tried several stats with the result being that without my electric fan working properly my Jeep would overheat sitting at WhataBurger and on the highway on a 100 degree day.

I live in Irving, so we're close. Bottom line, change every part of the system with the best parts you can afford and if you still have issues, get back in touch. We can talk about thermostats.

Good luck,
Brian

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easttxjeeper said:

Well I guess i will start with I have a 92 XJ. I replaced the motor 2 years ago with one from a 98 grand. Found out water does not compress to well. The new moter had 55,000 miles on it. Well have not had any problems with it,but after i installed the motor I relized there was not a spot to put in the temp sensor for the guage. It is on the back of the head. Well there was no room to drill it now, so i moved it to the front of the motor on the t-state housing. It has alyays ran about 210 till a few weeks ago. Now I am up to about 225 to 230. The rad. is about 1 year old, new water pump, new t-state, new cap, and new sensor. all replaced with in the last week. I know that my guage will read higher because of the location, but it has never ran this hot before. also checked the rad. out when i changed the pump, did not have any blockage. Any ideas? it i slow down to 55 it cools back down, but at 70 heats up.

70mph =2700 rpm
33" and 4:56

Click to expand...

Hiway speeds is not fan clutch, it's from restricted flow, fan clutch only kicks in when at low speeds when there is little or no airflow thru the rad. At hiway speeds the airflow thru the rad is good and the fan clutch disengages. My 98 has over 260,000mi on it, I'm replacing the water pump, tstat, cap and radiator this afternoon, actually I'm heading over to the shop to pick up the rad and pump now and bring it back here. The only reason I'm changing it is that I noticed some puddles this past weekend both on the passenger side and dead center under the rad so *something* is leaking out somewhere.

As a mechanic of 30 years, and autoparts sales agent, I have to tell you get the GOOD t-stat! The cheapies get scrunched and minutely bent in bulk crates while in transit with all sorts of stuff (motor mounts, hammers and other heavy stuff) and they aren't exactly treated nice in the warehouses either.
I'd retorque the head as a precaution and maybe move the temp sensor to a freeze plug in the side of the motor (or add a mechanical one since Jeep gauges are only for amusement anyways!)
If the injectors are running too lean, the motor will run hotter too. Adding a bottle of Redline Water wetter will knock the temps down a long ways too.

easttxjeeper said:

Well I guess i will start with I have a 92 XJ. I replaced the motor 2 years ago with one from a 98 grand. Found out water does not compress to well. The new moter had 55,000 miles on it. Well have not had any problems with it,but after i installed the motor I relized there was not a spot to put in the temp sensor for the guage. It is on the back of the head. Well there was no room to drill it now, so i moved it to the front of the motor on the t-state housing. It has alyays ran about 210 till a few weeks ago. Now I am up to about 225 to 230. The rad. is about 1 year old, new water pump, new t-state, new cap, and new sensor. all replaced with in the last week. I know that my guage will read higher because of the location, but it has never ran this hot before. also checked the rad. out when i changed the pump, did not have any blockage. Any ideas? it i slow down to 55 it cools back down, but at 70 heats up.

Click to expand...

Two thoughts, good radiator flush, and two a good cleaning between the radiator and the condensor.

Excellent thread..

Just curious, are you running a winch and/or oversized lights mounted in front of the rad?

I'm not implying this is the problem, but I've noticed it can impeded air flow to the rad enough to (in some cases) make a bit of a difference.

Kent

Also you might want to try a high-flow thermostat housing / thermostat. (if you are going to bother with replacing it)

I've been having some problems with over heating, though I am 99% sure I know what it is (lacking a mechanical fan) I am still buffing up the cooling system for the summer.

I had my 84 do something similar... turned out my belt was slipping.. but probably not your problem since you have a serpintine belt.