Fluctuating Temperature?!?!

91 4.0 aw4 231 177,000
Whenever I'm driving, my heep will heat up to normal temp, little less than 210, then fall to around the middle notch (180?). It does this pretty constantly whenever im driving. However, recently when driving on the highway, ive noticed that it levels out, stays right under 210. This tends to happen when I've had the cruise control on for at least 10 minutes driving on a relatively flat surface. Once I slow down, say for a toll, it drops and immediately begins the same fluctuating routine. I have replaced the radiator with CSF around 2 years ago and have replaced and drilled three different thermostats since then. Heat used to come out of the vents when I had the selector on "OFF", but now it blows whatever the temp is outside. My other friend's jeep also constantly circulates hot air with the switch in the "OFF" position. I am assuming these two gremlins are related?

I did get into an accident in which I skidded into a telephone pole...then proceeded to drive it away from the scene until it overheated and died (hey, what do you expect? I was 16:eeks1 That was 6 years ago. once I rebuilt/replaced the front end, it started to do this.
PLEASE help me with this. The leveling at cruise control has made me come back to this problem after abandoning it a few years ago and leading me to reject my hypothesis of a blown head gasket/warped head. She runs beautifully and love her to death.

Sorry for the long and winded writeup, but i really need some input.

You should still have the vacuum actuated coolant control valve in the engine compartment right where the heater hoses connect and go into the heater core. Check the valves operation.

I would replace the thermostat with a Stant SuperStat Premium 195 degree, that should help stabilize the coolant temperature.

How is your fan clutch? Do you have an efan?

Little confused about your description on the heat issue:

"Heat used to come out of the vents when I had the selector on "OFF", but now it blows whatever the temp is outside."

Thanks for the reply Joe,
If by checking the valves operation, you mean manually flapping it up and down, then yes, I have done this. Since the highway experience, it has led me to think of the fan clutch, since I have not checked it yet. Could the fan grinding into the radiator fins until it died (after the accident) damage the clutch??? I do have an efan, and it works. I dont think teh efan has any bearing on the issue.

I don't know if you've experienced this with the heat, just trying to give as much info as possible (as I believe these problems have something to do with one another since tehy happened at the same time). My heat works perfectly when the blower is on HEAT or VENT on all positions, but with it off it just blows "cold air." By that, I mean it used to blow (circulate?) the same temp (HOT) even when I turned it off (I'm assuming this is what is supposed to happen since the only way air blows out is through the blower motor deriving heat from the heater core.)

Do you think that the accident could've restricted flow, such as from the water pump? I had to replace the radiator, efan, condensor, and had the front piece of teh frame repalced (right in front of the engine).

Sounds like you replaced everything, so I don't see flow as an issue, unless there was a large piece of something broken loose and getting sucked up against the WP.

In the OFF position in the cabin you shouldn't get heat. I'm not all that familiar with the 91 system, but it could be a blend door issue in the heat box. When you put the temperature control in the OFF position that should close the vacuum actuated coolant control valve in the engine compartment, and that should kill the heat completely. So, if your vacuum system is complete and functioning, and that vacuum actuated valve is working, things should get cold. When you talk about flapping something up and down are you talking about the same valve as I am?

You could have damaged the fan clutch, and they really don't last any longer than a WP--about 4-5 years.

I was concerned that your efan might be stuck in the "on" mode.

Now that you mention it Joe, that makes perfect sense. I was having extensive issues with my disconnect dana 30 as well as my cruise control a while back. Found a molested tube coming from the vaccuum canister, replaced it, all is fine. It makes sense because I did that in the summer when I didn't need the heat, so I obviously wouldn't have noticed it. Yes, I am almost 100% positive that the valve I was touching is the heater control valve, but now realize that these problems are in no was correlated because the valve was a vaccuum issue (and working as it should!)

Back to the main issue at hand...

Is there any definitive way of checking for a bad fan clutch? IIRC, it shouldn't turn by hand with the engine off and water cold?
Is there still a possibility that the problem could lie in the fact that it overheated and shut off? (blown head gasket/warped head allowing gasses to pertrude into the system while in operation)?
Some air leaking in some other way?

All suggestions are welcome

Ok, there is a procedure in the FSM, but quick and dirty is this:

Engine stone cold--overnight the silicone fluid will seep out of the reservoir and engage the fan clutch. So, engine stone cold, pop the hood and try and turn the fan by hand--it should be stiff.

Start the engine and let it run for about 5 minutes. That will be plenty of time for the silicone fluid to be thrown back into the reservoir. Shut off the engine. Turn the fan by hand--it should turn easier now.

Start the engine and get it warmed up to about 210 indicated on the gauge. Shut off the engine. Turn the fan by hand--it should be very stiff as it is now fully engaged.

Also, while cold, grab it on both sides about 180 apart and try and wiggle it on the shaft to check the bearings.

I'll be interested to hear how you resolve this. My engine temp gage acts exactly the same way yours does. However, my heat works fine. I've gone through several thermostats also and currently have an oem Mopar thermostat. I even had an overheating issue a couple years ago. I just got an infrared thermometer to confirm my gage was working, which it was. My fan clutch is relatively new as well. I think my next step is to check my vacuum system, cylinder compression and head gasket somehow because I'm not sure what else to do.

OP, is your CSF radiator of the 3-row variety? I have the CSF 3-row and have temp flucuations very similar to yours, but only during the colder winter months. It definitely started right after installing the 3-row and I just chalked it up to the increased cooling efficiency causing the coolant to cool a little too much and cycle the thermostat closed and open repeatedly until the engine was FULLY warmed up. In extreme cold weather, mine will cycle like that constantly on the highway regardless of the trip length, though. I'm running a stock 195 t-stat.

96 XJ Sport said:

OP, is your CSF radiator of the 3-row variety? I have the CSF 3-row and have temp flucuations very similar to yours, but only during the colder winter months. It definitely started right after installing the 3-row and I just chalked it up to the increased cooling efficiency causing the coolant to cool a little too much and cycle the thermostat closed and open repeatedly until the engine was FULLY warmed up. In extreme cold weather, mine will cycle like that constantly on the highway regardless of the trip length, though. I'm running a stock 195 t-stat.

Click to expand...

You might consider the Stant SuperStat Premium. Its design is meant to reduce those sudden temp changes: http://www.stant.com/brochure.cfm?brochure=205&location_id=168

You would be a great candidate to test and report back on it for this particular problem.

rockauto.com has them for less than $5.

joe_peters said:

You might consider the Stant SuperStat Premium. Its design is meant to reduce those sudden temp changes:

Click to expand...

I will put that on my list after I fix all of the truly broken items, lol. The 195 t-stat I'm running is one of those kinds that only fail in the open position, but apparently isn't a high precision unit.

96 XJ Sport said:

I will put that on my list after I fix all of the truly broken items, lol. The 195 t-stat I'm running is one of those kinds that only fail in the open position, but apparently isn't a high precision unit.

Click to expand...

I tried a couple of those "Fail Safe" brand I think. Then one broke and failed closed--end of story. Besides, all thermostats are supposed to lock open after an overheat condition.

96 XJ,
My CSF was the SECOND radiator that I put in after the accident. I had replaced it with another one before it, which was cheap and sprouted a leak. I thought that the leak might have had something to do with it, but didn't. The CSF one i have in now is a three row, but I'm not sure what the previous one was. Mine will not fluctuate as severely in the summer, but still is very noticible. The tsats i used were standard $5 ones from Advance (can't remember what they were called). I drilled two holes, up and down, on two of them thinking the problem was trapped air (although it still could be).
Joe,
You think a simple swap of a more sensitive thermostat will have that much of an impact? I am just skepticle because it feels like i have taken the thermostat housing off more times than my tires! The easiest fix just does not seem like it would work, but sure would be sweeeeet and will replace it when I have the time! Would be cool to collaborate and fix this problem :lickout:

Well, it couldn't hurt.

You really don't need to drill thermostats unless you have a closed cooling system.

Worst case with an open system is you might have to park on an incline at an angle to put the radiator cap at the highest point to get the last of the air out. Open systems are self-bleeders (hemophillia?).

I would skip the holes.

Yes, drilling would be for closed systems. I was trying to rule out the culprit being trapped air. Does this mean that it's most likely not air and something else? Or could the possibility still remain that exhaust gasses are seeping through? I may be thinking into too much, or getting stuck on the worst case scenario, but wouldn't it make sense? Seems like it SHOULD be a easy fix. Drilling should, in my theory anyway, rule out air. It does make sense anyway to swap the 'stats. Cheap and easy. I like the "hemophilia" representation lol.

Yeah, I think you can rule out air. And it doesn't sound like compression getting into the cooling system, you would have seen bigger signs of that as the engine keeps pumping air in the coolant eventually has to spew out.

Give the SuperStat a shot and post back.

The t-stat can be removed/replaced w/o draining the system right? I did mine a while back and don't remember.

The t-stat can be removed without COMPLETELY draining the system. However, some will spill out when you take the upper radiator hose off. You can minimize it but holding it up right but you'll still have to purge the air from your cooling system.

Pull the lower hose and drain the coolant into a bucket, it will be less messy in the end.

Replaced t-stat with Stant Superstat and replaced fan clutch. Still fluctuating, but a lot less than it was; only minor change instead of drastic. Haven't tried it on the highway yet (installed SYE, need driveshaft!) I was also wondering if my drivebelt would have anything to do with the fluctuating (i.e. not gripping on the water pump/slipping). I have never replaced the drivebelt and have had the jeep for 6+ years. I'm thinking of replacing it tomorrow to see if maybe it has any relation to the cooling. Anybody?

Driveshaft won't have any affect at all.

It sounds to me like you have air in the system. You need to run the heater on full when changing the fluids so that coolant circulates through the heater core, otherwise you'll trap air in there. The heater core might also be blocked up, which would also lead to air getting trapped in the heater lines and upper head.