bad_idea

This one has me scratching my head. Drove the jeep to Uwharrie this past weekend (5 hours one way). The jeep gets hot at higher speeds on the highway for sustained periods of time. Stays cool at lower speeds, even crawling through some trails. Haven't had any issues with overheating daily driving it. Let me lay out the vehicle stats and conditions/temps:

Vehicle: 1990 w/ 235K miles, 4.0L (45K on a rebuild), AW4, 231 TC, 8.8 w/ 4.10s, D30 w/ 4.10s, 33x12.5s.

Cooling System: Stock closed loop system. Water pump, thermostat and radiator replaced 5 years ago when new motor put in. Mech fan clutch replaced last summer. Large B&M trans cooler plumbed inline to the cooler in the radiator. Electric fan comes on when engine hits 210. Mech fan shroud in good shape.

Outside temps were 90 Degrees with low humidity.

Trails (1-10MPH) - 195-210 degrees coolant, 180-200 degree trans.
60 MPH - 195 Degrees coolant, 165 degrees trans.
70 MPH - 210 Degrees coolant, 180 degrees trans.
80 MPH - 220+ Degrees coolant, 180 degrees trans.

These were all at said speed sustained for at least 50 miles. At 80 MPH the temp kept creeping up. I finally pulled off to a rest stop, opened hood and elevated idle to ~1800 RPMs to cool the heep off. The rest of the trip I kept the speed down around 65 and had no cooling issues.

My plan is to flush the coolant system (probably hasn't been flushed since engine was put in by PO). Not sure where to go from there if that doesn't get it.

freegdr

Caps are well known to be problematic. Have it pressure tested.

CurrySoSpicy

I think he said it is a closed loop system.

freegdr

Coorect they have pressure caps that are junk even new from dealer . There is a volvo cap as upgrade.

CurrySoSpicy

I see thanks

extrashaky

Sounds like a head gasket to me. I would do a leak down test and a combustion leak test before throwing very much money at the cooling system.

bad_idea

Please elaborate on the head gasket theory. I see no evidence of coolant and oil mixing. No loss of either fluid.

The pressure cap on the expansion tank leaks, I can hear it hiss when I shut the Jeep down. Are you saying that can cause it to overheat?

Either way, my logic tells me the faster you go the more cooling effect the air across the radiator provides. I would expect it to overheat at lower speeds, not higher speed. What am I missing?

extrashaky

You can get a leak of coolant into the oil, but you can also have a leak of combustion gases into the coolant. If the hot gases are spewing into the coolant, the engine can overheat. This type of leak typically gets worse under heavier load, like when going faster. Overheating only at highway speeds is a typical symptom of this type of blown head gasket or cracked head or block.

It may NOT be that, but testing for it is cheap, and ruling it out prevents you from throwing a lot of money at the cooling system only to find afterwards that you needed to take the head off all along.

CurrySoSpicy

Hey, that's not a "bad_idea" to try. Heh

freegdr

Yes upgrade to volvo cap. Dont over think this......I went thru 5 bottles and numerous caps. During the 20 plus years ive had my XJ. Upgraded to open systems few yearss back.

freegdr

Read this....http://www.jeepforum.com/forum/f11/v...cap-xj-613128/

bad_idea

Thanks for the explanation extrashaky. I have limited experience inside an engine. I did a little inter web research on the combustion leak test. If I understand this test correctly, you are looking for CO2 and CO in the coolant system. Start the vehicle up, and test the air in the radiator or coolant expansion tank in the case of a closed loop system for these gases. I don't have the tester with color changing liquid, but I do have access to a 3 gas analyzer that will show me how many PPM of CO2 or CO are present in atmosphere. Am I on the right path? Will there be an allowable amount of these gases in the coolant system? Or should they be at the same concentrations as open atmosphere?

extrashaky

I don't know how to test it with the instrument you have. However, apparently some of the auto parts stores will let you borrow the color-changing test kit under their tool loan programs. They also have the leak down kits.

The Volvo cap sounds like a good idea and something you ought to try anyway, so maybe you should go ahead and do that. But if you get to the point that you're considering firing the parts cannon at it, do yourself a favor and test for a head leak first.

Willys55

you can use the gas analyzer, I do it all the time, just hold the tip above the coolant with the engine running, we do this directly over the radiator cap with the cap off of course. Just dont touch the coolant with the tip of the wand or your readings will be wonky.

test at idle and above 1800 rpm in neutral or park.

BTW: I have had three trucks through the shop in the last 2 years that had bad fan clutches....you could grab the fan with out injury while in high idle....it is rare, but can cause a serious problem at highway speeds.

extrashaky

How? At highways speeds the fan is already not doing anything because the motion of the vehicle is providing more airflow through the radiator than the fan could ever provide. I wouldn't think a bad fan clutch would make a difference.

OP, one other thought: Have you looked at your radiator to see if you can see through it? They do get caked up with unsavory material. You might try spraying some water back through the radiator to see if cleaning it out helps your cooling at highway speed.