Only Overheats Under Load

[AL BUNDY]

My wife's clean 91 4.0 Auto 2WD has about 145K and runs xlnt. It does not get hot driving around town, at idle, or on the freeway, but always overheats when climbing long mountain roads, even in winter.
I have flushed the system, replaced the water pump & thermostat, and wired the AC fan to a manual switch.
I searched & read alot of threads that gave me some ideas for things to check but none of the threads only had this problem when under load/climbing long inclines.
Anyone have this same problem? Thanks.

 

[kcjeep6]

Cracked Block, Do a leak down test. Mine did the same thing, when i did the test the water came out of the radiator at 80psi.

 

[hiimmred]

I experienced this with my closed system all last summer every time I headed up the hills to the trail. I'd have to pull over for an hour a couple times. It turned out my radiator which was only a few years old had a ton of blockage at the plastic sides. I switched to an all brass radiator and recently went over an 8500' pass in 90* weather and never got over 210.

 

[AL BUNDY]

Thanks for the ideas.

 

[joe_peters]

I am curious to know the brand of the water pump and the construction of the impeller, cast or sheet metal, or plastic?

 

[AL BUNDY]

Joe: It was about 6 months ago so I can't remember. It was purchased at an auto parts store & I would have gotten the best one they had because I have replaced the water pump on every 4.0 I have had (5). It was a metal impeller, not plastic, but I don't remember if it was sheet or cast.

 

[winterbeater]

It sounds like the radiator can't dissapate the heat while the engine is working that hard. Getting the tranny cooler out of it might help. I wonder if the 2WD's had smaller radiators?

 

[joe_peters]

Joe: It was about 6 months ago so I can't remember. It was purchased at an auto parts store & I would have gotten the best one they had because I have replaced the water pump on every 4.0 I have had (5). It was a metal impeller, not plastic, but I don't remember if it was sheet or cast.

Thanks for the added info. Working on the germ of an idea that poorly made sheet metal and plastic impellers may cause cavitation and lead to overheat.

 

[kcjeep6]

I wish you the best of luck in diagnosing this. I put up with a similar issue for a year, I changed the water pump to a Hesco, Turbo City Thermostat housing with Stewart balance Thermostat 195*, New radiator (3 row copper, now Griffin aluminum), new fan clutch, 97 & up Aux. fan. Hose & belts, replaced the cracked head, converted to open system. Compression was 125-148psi nothing helped. The XJ would run fine, it passed CA smog, and I could run the trails all day long, but as soon as I started making HP the heat just start to build up until it would boil over and loose coolant.
The only cure was to stroke it; I ordered a 4.5 from http://www.motorworksengines.com/ $2300 to my door. I have around 25K on it now and like it more than the V8 in my Grand Cherokee.
The moral of the story is save yourself a lot of headache and do the Leak Down Test
Good Luck

 

[winterbeater]

kcjeep6, do you think he could have cracks or porosity making it overheat under load? Is this what the leak down test could show? Sounds like you did everything that could be done with yours.

 

[joe_peters]

I do remember my sister-in-laws 1980s Mustang II with a 302 in it. It would always overheat on her way home from work, but never on the way to work.

She lived in Scotts Valley, CA and commuted to Silicon Valley. That was something like 4 miles up and 4 miles down over Highway 17.

Put in a heavier duty radiator, high flow thermostat, water pump, even removed the fan clutch and put in a fixed flex flan. Nothing worked.

Well, finally figured out why it only overheated on the way home from work. On the way to work in the morning she always had her headlights turned on. On the way home from work it was broad daylight and she didn't turn on the lights.

The secret was in the grounds. The gauges were not reading correctly when the lights were off.

Just a little info from way back when.

 

[kcjeep6]

kcjeep6, do you think he could have cracks or porosity making it overheat under load? Is this what the leak down test could show? Sounds like you did everything that could be done with yours.

Bingo! When the engine was loaded gases would leak into the water jacket. When doing a leak down test you pressurize each cylinder then listen to where the air is escaping. You must have both valves closed and the radiator cap off. In my case when I put the air to number 4 the water started to be pushed out of the radiator.

 

[AL BUNDY]

kc: That makes alot of sense and is worth checking out. Just kinda weird because my 90 jeep has 217K and does not run half as good but does not overheat.
Joe: I don't remember the water pump making any difference in the overheating. It just went bad and needed to be replaced and it's not just the gauges. It has boiled over and puked out all the coolant doing this before. Thanks for your help.

 

[AL BUNDY]

I removed the AC condensor in front of the radiator just to see if it would make any difference. HUGE difference! The AC condenser looked good and was not clogged up but after removing it, the temp gauge never even reaches the middle with the aux. fan turned off. Anyone having overheating issues, if you do not mind losing the AC, consider trying this modification.

 

[Ecomike]

I removed the AC condensor in front of the radiator just to see if it would make any difference. HUGE difference! The AC condenser looked good and was not clogged up but after removing it, the temp gauge never even reaches the middle with the aux. fan turned off. Anyone having overheating issues, if you do not mind losing the AC, consider trying this modification.

Kinda hard to run my AC in this 100 F heat wave with out an AC condenser.

Do you have an external transmission cooler? If not I would add one. Also you might research the torque converter lock up on the AW4 here. It can be done with an added toggle switch, to eliminate TQ slippage on long steep climbs, which can cause overheating. Also the TC lock up may not be working.

What speed is that mountain climb where it overheats? What gear do you run it in?

Sounds more like the tranny overheating to me, and it transfers excess heat to radiator at the radiator exit!

 

[winterbeater]

I removed the AC condensor in front of the radiator just to see if it would make any difference. HUGE difference! The AC condenser looked good and was not clogged up but after removing it, the temp gauge never even reaches the middle with the aux. fan turned off. Anyone having overheating issues, if you do not mind losing the AC, consider trying this modification.

That is weird to have the condenser create that much air blockage. I could understand a difference between AC on and AC off, but removal of the condenser shouldn't change it much. A little I could see, it does block air no matter what. I believe what you said, but wonder if the condenser didn't have a flaw in the fin forming process to block more air or something like that?????????

 

[Ecomike]

That is weird to have the condenser create that much air blockage. I could understand a difference between AC on and AC off, but removal of the condenser shouldn't change it much. A little I could see, it does block air no matter what. I believe what you said, but wonder if the condenser didn't have a flaw in the fin forming process to block more air or something like that?????????

If not, I wonder if it is letting hot air under the hood, get recirculated? If it is, sealing the gaps between the radiator and condenser might solve the problem. Has me thinking harder about doing that very thing, sealing the huge gaps between the two.

 

[AL BUNDY]

In response to ecomike,
1) "if you do not mind losing the AC, consider trying this modification"
2) Yes, I have an external tranny cooler.
3) I climb mountains in 3rd gear (drive) at 35-45

I just posted this because I have seen alot of overheating threads lately and this is an easy, free modification that made a huge difference.

 

[winterbeater]

From doing OEM design work, I can verify that recircualting hot air from under the hood is a big concern. The design of the seals was given much consideration. Sealing up as many gaps as possible between the front of the radiator and the back around the radiator perimeter and between the hood and radiator was worth spending the extra money for seals. And nothing is put on a vehicle any more unless it is really needed. The fit between the fan shroud and radiator is also often overlooked.

 

[AL BUNDY]

That's an interesting point. I will have to check that too.