Overheating Issue. Looking for conformation.

Ok. Ive got a 91' xj 4.0. It has the stock cooling system. It overheats at highway speeds. Wheelin' it stays at or below 210. When I drive on the highway it gets into the red. I have pulled over and checked the t-stat neck with a laser temp gun and it reads 240ish when my inside guage is in the red near the 260 mark. I know the temp guage sensor is in the back. It has a new 195 t-stat in it and the waterpump does not leak, no coolant in the oil either. I have not burped it or done the burp hole on the t-stat. I plan on doing that today. From what I have found searching it seems like a radiator. Just waning to confirm this before I ordered one.

Cracked block or Head. I spent the last year trying to correct my overheating issue. Replaced everything in the cooling system and it would still overheat at highway speeds or pulling a hill, but I could run the Rubicon with no issues.
The best way to know for sure is take your XS to a Smog station and have them stick the Exhaust Gas Analyzer in the radiator. Mine took about 10 minutes to show 4% Hydrocarbons.
You can also do a Leak Down test, When I but 80 psi in #4 the water began to flow out of the radiator.
The fix was a 4.5 Motorworks!

Try burping it first, then go on to the other ideas, I would say that it does have something major happening....

A partially clogged radiator will cause overheating, especially at highway speeds. Take it to a radiator shop, who can test for clogging.

xjbubba said:

A partially clogged radiator will cause overheating, especially at highway speeds. Take it to a radiator shop, who can test for clogging.

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X2 on radiator an/or restricted flow of coolant. If it's only doing it at highway speeds I'd be looking at the effectiveness of either the radiator or water pump.

Remember the temperature of the coolant (not the engine) is what you're getting from the gauge. When was the last time you flushed it.

kcjeep6 said:

Cracked block or Head. I spent the last year trying to correct my overheating issue. Replaced everything in the cooling system and it would still overheat at highway speeds or pulling a hill, but I could run the Rubicon with no issues.
The best way to know for sure is take your XS to a Smog station and have them stick the Exhaust Gas Analyzer in the radiator. Mine took about 10 minutes to show 4% Hydrocarbons.
You can also do a Leak Down test, When I but 80 psi in #4 the water began to flow out of the radiator.
The fix was a 4.5 Motorworks!

Click to expand...

Sorry, but being a chemical engineer I have to question the 4% hydrocarbon in coolant test statement. For one thing the volatile part of antifreeze is about 50/50 water/hydrocarbon since ethylene glycol is a hydrocarbon. Perhaps it was exhaust CO2 they tested for? Or I guess it could have been a test for aromatic hydrocarbons only....antifreeze does not contain aromatic hydrocarbons, Hmm. I would think exhaust gasses being hotter and higher in pressure would be more likely to show up in the coolant than fuel during compression?

Is the electric fan working?????? If yes, then try and expose the radiator surfaces buy pulling the electric fan, then check the tubes top to bottom for any cold spots. A cold spot(s) indicates blocked tubes, no flow, no cooling capacity from those cold tubes and thus a partialy blocked radiator. Mine was 50 + % blocked. New radiators from radiatorbarn.com or completeradiators.com (IIRC) are cheap compared to the cost of rodding out a used radiator these days, and there is no gurantee that a cleaned radiator won't leak after the cleaning. Plugged tubes can only be cleaned by pulling the side tanks off the radiator and shooving a makeshift hacksaw blade like tool through the tubes.

Mike I had mine rebuilt with new tanks and they blew my tubes out with just air pressure. I think the rodding thing is no longer done by any of today's shops. I agree that you should pull the grill out and take a handheld infrared temp reader and check for cold spots in the radiator after it reaches operating temperature. Any cold spots indicate a plugged up section of core. If a shop cant blow out those clogged tubes then the next step is a new radiator. My local guy quoted me a price for a replacement if needed at $120 installed with copper tanks. $110 with plastic tanks also installed price. I think they are being rebuilt in Mexico for that price as I live within a stones throw of the river.

90Blue_XJ said:

Mike I had mine rebuilt with new tanks and they blew my tubes out with just air pressure. I think the rodding thing is no longer done by any of today's shops. I agree that you should pull the grill out and take a handheld infrared temp reader and check for cold spots in the radiator after it reaches operating temperature. Any cold spots indicate a plugged up section of core. If a shop cant blow out those clogged tubes then the next step is a new radiator. My local guy quoted me a price for a replacement if needed at $120 installed with copper tanks. $110 with plastic tanks also installed price. I think they are being rebuilt in Mexico for that price as I live within a stones throw of the river.

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In many cases air pressure won't cut it, just not enoguh force involved. They still rod out the ones that are completely clogged with calcium and magnesium silicate and carbonate solids. If a tube leaks they seal it off. I had it done last year on my 85 diesel Jeep as I could not find a new radiator anywhere that was an exact fit. Paid $70 here in Houston to get it rodded out. I think the new radiators are coming in from Asia.

Have you checked the hose's?
Higher speed making a hose collapse?

I could be wrong about the Hydrocarbons, but I’m sure it was an exhaust gas. I replaced my radiator, water pump, thermostat, lower hose with spring, fan clutch, new style eclectic fan, fan switch, move the B&M trans cooler away from the radiator, installed a new head, all cylinders were 125-150 psi and it passed smog. I spent a lot of time and money trying to stop the overheating with no luck. I have searched and the Exhaust gas and leak down test are not mentioned as tool to diagnose overheating. I learned my lesson the hard way and would like to pass it on. If you cooling system is in good shape at least do a leak down test before you spend any money on the cooling system. Good luck

Thanks for the help. I have done a conpression test and its shows good all the way across the board. Did that when I bought it a few months ago. I will definantly check for cold spots with the temp gun. The electric fan does work, by the way.
Thanks again.