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Another Overheating TJ.. what next?

18K views 65 replies 27 participants last post by  eruji 
#1 ·
Another Overheating TJ.. what next? **SOLVED**

2001 TJ sport 4.0

Ever since the weather has been getting into the 90 here in so. cali my jeep overheats while idling. This only happens on my way home from work and after ive been on the frwy then stop at a drive in or let it idle for more than 5 minutes. Also, it will sometimes boil in the reservoir as soon as i turn the motor off.

Whats strange is that the temp guage does not slowly raise it goes from 210-220 to redline instantly. If i can get back out on the road and do at least 40mph it will then quickly go back down to 210-220. Seems like the gauge is acting funny. Ive tried turning on the heater and dont notice any reduction in temp. So im not sure if its actually overheating or the gauge or sensor is screwy.

What ive done so far:
New heater core a couple of years ago
new aluminum radiator one year ago
new water pump since april
new thermostat as of this last weekend
new serp belt last weekend
new fan clutch last weekend
EDIT: new rad cap last weekend

no change. still overheats at idle and boils after shut down.

what next?

runs fine otherwise..

is there any way i can monitor engine temp with another device.. diy temp sensor? something? seems strange to me.

where is the temperature gauge getting its data from? ( i think ive read that its not the coolant temp sensor)
 
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#3 ·
Have you made sure you burped the cooling system adequately so there isn't any air in there? I usually run the Jeep up to normal operating temp with the heat blowing and radiator cap off to burp all the air out. You could also try replacing the radiator cap. They're real cheap. A bad cap will let things boil over. I just replaced mine recently.
 
#11 ·
Not on mine it doesn't. I can tell by the hoses going to and from the heater core. Engine running and heater switch off both hoses get hot. Means coolant is flowing thru the heater core. If it didn't run thru there then the return line would not be hot, it would be cold.
 
#13 ·
In front of your radiator is there any thing obstructing it like leafs or anything like that? If there's enough of anything there it will restrict the air across your cores and cause it to run hotter had it happen to a buddy of mine in his Jeep once he cleaned it hasn't had a problem since.
 
#15 ·
Air pockets can get trapped in the system, causing false temp readings and not allowing correct circulation
 
#18 ·
okay, thanks for all the suggestions.. I forgot to include that i have a new rad cap as of this weekend as well. I did the burp thing this morning just to see. My driveway is fairly steep so i parked with the front raised and started the engine and let it warm up. I also turned on the heater but i believe the hoses that feed the heat coil are always on even when the thermostat is closed its still flowing. No bubbles, but the coolant warms up and expands out the open lid and blows all over the place. Also the fan shroud is in place.

So does the boiling into the reservoir give a positive indication of overheating. because it only happens after i shut the motor off. Makes me think that either 1. Since the fan turns off with the motor, it is sending the temperature of the coolant above boiling. and or 2. pressure is being released somehow and causing the boiling temp to lower.

When the jeep starts overheating i do not get a check engine light or any type of warning and i dont notice the engine running differently. That and the fact that it happens so suddenly is what is making me think something is wrong with whatever is reporting the temp.
 
#19 ·
are you losing coolant besides the boil over?
not that i can tell. ive been burping it so i loose a bit there but nothing noticeable

have you tryed back flushing the system to make sure nothing is in there blocking it up
i havent because everything is fairly new, coolant looks perfect, inside the radiator everything still looks pristine. heater core was brand new not that long ago.

auto or 5sp? Any obstructions in front of the radiator other than the ac condenser?
Automatic with AC and the ac radiator looks as dirty as any other car ive ever owned.. a lot of dead bugs and such. How the heck do i clean that thing anyways.. i hit it with the spray nozzle this weekend while washing it and didnt seem to dislodge anything.

I feel like the next thing is to replace the hoses. Ive heard you can have the system pressure checked for leaks.. and some other test to see if coolant is getting into a cylinder. sound about right?
 
#22 ·
I don't think it's the temp gauge, if it were it wouldn't go up and down from idle to 40 mph.

Really sounds like an air flow issue.

I suppose it's possible that you got a bad fan clutch, it should be fairly hard to turn by hand (engine off of course)

You don't have one of those flex fans or anything like that do you?
 
#30 ·
Whats the best way to verify the leaking head gasket or cracked head?
You've got some options

First warm it up real good, shut it off and check the oil cap and dipstick looking for muddy looking oil, it should be pretty obviously brownish green if it's leaking bad. However you may have a blown gasket and not have these symptoms

If nothing then start pulling spark plugs looking for coolant on them

I've heard some people test it with all plugs out except one, pull off the radiator cap as well and crank the engine, if you see bubbles the gasket is blown (I can't verify this method).

Some people will do compression tests or leakdown tests, but those can easily miss a blown head gasket

The best test (other than seeing obvious signs of coolant in the oil) is to use a chemical test kit, you should be able to find one doing a google search for head gasket test kit. It has a test tube sort of thing that sits on the radiator in the fill spout, and has a color changing fluid that lets you know if engine gasses are entering the coolant.
 
#28 ·
With everything you've done you must have some type of block in there somewhere. I'm not sure if this is proper but I would find a way to reverse flush the thing with a garden hose or something and see what happens. And a blocked radiator would (most likely) not cause an overheat at idle, not much air moving through it anyway. When I say blocked I don't mean liquid block I mean air movement.
 
#31 ·
You said you replaced the radiator a year ago. Was it exactly a year or a little less then a year? ie was it changed while it was still cold out? Does fluid leak out of your radiator cap when you see it boiling over (even a little)? What brand radiator cap did you buy?
 
#32 ·
Was it exactly a year or a little less then a year?
i replaced the stock radiator because it was leaking from the top plastic seam. That was in June of last year. It was replaced with an aftermarket all aluminum rad. Ive tried two different branded radiator caps in the past few months. The first was one from napa, the latest was a "safety" pressure release type from Oreilys. i dont care for the pressure release tab because it feels like another point of failure, but wanted to try another cap. There are no leaks that i can see anywhere in the coolant system. I didnt notice any leaks coming from the cap while boiling over. Im gonna take a look at the oil this weekend to see any signs of coolant in there.
 
#33 ·
I would check for leaks on top of the radiator. Even if its a tiny bit. I would also get an OEM radiator cap. They are around $20 from the dealer. I'm going through the same problem now and it turns out its the radiator. I also just replaced mine in November. Now that its hot, it spikes randomly. The filler neck isn't sealing correctly and causing it to randomly spike the gauge. I noticed a little bit of coolant on top of the radiator groove and when I I looked at the filler neck closer, I noticed it was dripping a little. It was the last thing I checked because I figured that the radiator was new and it couldn't be bad already. I did the chem test for head gasket and there's no coolant in my oil. I also replaced everything you did, including the Temp sensor. New radiator will be here today (under warranty) and I won't be able to get it on until Tuesday when I get back, but I'll let you know if it solves my problem. Maybe you're having the same issue...
 
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