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Odd Temp problem


Well-Known Member
May 31, 2000
Reaction score
City, State
Tempe, Arizona
Year, Model & Trim Level
1993 XLT 4x4
OK, I have been having some cooling problems so I went ahead and put new of the following:

Water Pump
Upper and Lower Radiator Hoses
New Coolant

Now here is the odd problem that I can't seem to find in the searches. My engine takes a long time to reach operating temp (Needle at the N) but it continues to slowly rise. For example, the temp gauge rises then falls a little bit, then rises more, and fall a little bit and this process continues over and over again. After about 30 min of driving, its the M or the A. God knows how fall it would go if I didn't shut it down. I am a little stumped. Any suggestions?

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Did you check the thermostat before you put it in:D

Clogged radiator? That's the one thing you didn't replace. Have you run a good flush through the cooling system?

On the surface, that temp fluctuation appears to be the thermostat.


Sounds like you have an air bubble in there....there are several ways to get rid of the air bubble but one of the best ways I have seen is to:

1. drain some fluid, a litre or so should do it

2. remove the upper rad hose from the radiator, so it is still attached to the thermostat housing

3. Boil a big pan of water inside

4. Pour the boiling water into the upper hose

5. The hot water will open the thermostat (you'll be able to tell when it opens) and then flow through into the intake, forcing out an air bubble.

from now on, since the thermostats are so easy to replace on our 4.0L engines, anytime you buy a new one test it first before you install it. I usually will buy 3 or 4 180 degree thermostats, take them all home and one by one boil them in a pan with a thermometer. I install the one that actually starts opening at 180 degrees and should be fully open by at least 190. Ihave had ones that didnt open until 215 degrees! Thats too hot for my 4.0L. basically thermostats are junk, you have to make sur you get one that works, I have had the best luck with Robert Shaw thermostats. Oh yeah I always return the other thermostats as defective units, I keep the good ones for spares....

Thanks guys. I will try the air bubble thing and then give a flush and change the thermostat if that doesn't work. Are thermostats really that unpredictable??

Oh baby. Yes the thermostats are mostly crap, especially the card typ ones in the yellow package. Stick with Robert Shaw if you can, they are much better, NAPA carries Shaw.

Here's an example:

out of 6 ea 180 degree stats I tesated I got the following:

1 started to open right at 180 degree's was fully open at approx 186

1 started to open at 180 and was fully open at approx 190

2 started to open at approx 192 degrees and were fully open at 200

1 started to open at 200 degrees and was fully open at 210

1 didnt even begin to open until 215 degrees

The factory 4.0L stat is recommended 195 degree's, I wouldnt even want to tes the 195 stats...

Can you imagine what can happen if your coolant temp gets to 215 degrees before the stat even begins to open? In extreme situations (towing, 4x4ing) there is no way the stock fan, shroud, and radiator (especially if an auto) could bring it back down.....

I have seen 3 4.0L's that had a similar problem to your, turned out to be a air bubble.

Anytime you change the waterpump, flush the colling system, or do any service on teh cooling system the thermostat should be changed, I mean they are $3

DAMN, no wonder they are only $3

I had the exact same problem that you mentioned after buying a brand new t-stat from kragen. They temp gauge would cycle from way hot "A" to way cold below "N".

I spent a couple of hours, bought a jet 180 degree t-stat from performance products.com, installed and flushed system and that totally fixed the problem.

Now, when I am towing about 2,000 lbs going up the grade at about 50 or 60 the temp gauge just barley touches the "R".


Cool, I hope one of these ideas helps.

I had the same problem in my 91. I tried all the things you did and they didn't work. Take it to a radiator shop and have them do a 5 sec. leak check, if it is bad, the head gaskets will have to be changed. The shop checked mine for free. Also thremostat are not as bad as most poeple have said they are. It doesn't matter if thier is a small temp. differents when they open, as long as they open and close right. Thier cheap and easy to change, so they get the blame for all the problems........but on the other hand they should never be over looked either when thier is a problem...

Turns out the problem was because of a clogged radiator (Good call RedBeard). It was clogged bad enough that flushing wasn't helping. I am in the process of installing a new radiator today. SO hopefully this will be the end of the problem. I miss driving my ex.

I believe that I have the same problem. Everything has been changed and still the problem. I've found a place to make me a new radiator.

I figured that since I have my 3" body lift I'll just make the radiator 3" higher. 1 or 2 liter's more in the radiator could only help.

Radiator is in, and the cooling problem is gone. The Ex runs really cool right now. I haven't taken it out for an extended drive; that will have to wait until Monday. But so far, it seems to be the cure. Oh, by the way, NAPA ($155) and Checker ($172) sell Modine radiators (the one I bought). A lot cheaper than I thought they would be. The shop wanted to charge $165 to route my existing radiator, yeah right. Just an option to all those who may be looking at radiator work for the hot summer ahead.