4.0 strange overheating

Rox&Mud

Rox&Mud

CTB
Joined
Nov 3, 2006
Location
Raleigh
My ZJ has had a strange overheating problem for a while now and has been getting worse. I recently found I had a warped cylinder head so that has been fixed but it is still overheating in in the same way. It comes up to 210 normally then it very slowly warms up to the mark between 210 and 260 and holds there for a while, as much as 20 minutes or so, and then suddenly, snap your fingers fast, it jumps to 260+. If you shut it down but keep the key on to watch the gauge after a couple minutes it goes back to the mark between 210 and 260. I have replaced the thermostat and flushed the system. It got a new all aluminum radiator in the fall, new pump and hoses in '09. I thought it may be the sensor but I've never seen it boil until it jumps to 260. The only thing I can think to do now is swap the water pump again...

Anybody have any ideas?
 
It's a electric fan that turns on at 175 and it is pulling strong enough to hold a rag up on the grill. It was suggested that I might have a air pocket somewhere that can't get out. I have flushed the system and positioned the jeep so that the radiator cap is the highest point in the system to try and get any air out and it still goes through the same cycle.
 
No I did not test the new thermostat. I changed it in November, to a 160, for the same reason and again yesterday back to a 195 and it still over heats the exact same strange way.
 
Have you pressure tested the system?
 
Shroud/no shroud?
OEM fan?
What pressure cap, have it checked?
Check flow on radiator, radiator shop can do this
HEad was warped, did you have it magnafluxed?could be a crack. Correct Head gasket, not covering water jackets.
Does it blow out of resevoir?
What gauge are you using? OEM or after market? if after market are you sure the probe is long enough to actually touch the water/coolant
Timing set correct? too much can lead to over heating
Could be something in the water flow, have torn engines down to find old exspansion plugs floating around inside the block, keeping the flow from happening.

just a few thoughts
 
Had the cap checked yesterday / good.
The electric fan kept it cool in the past but maybe it is failing. Just put the mechanical fan and shroud back on. Will drive it after work to see if it makes a difference.
The head was tested and all is good there.
I compared the gasket w/original and it appeared to be the same
it does blow out the reservoir
I'm using the OEM gauge
I haven't adjusted or checked the timing since I've had it, on the list now though
I really hope I don't have to tear the engine down to find something like that

Thanks for the input
 
When I said blow out the reservoir, does it just gradually seep out as it get hotter, or does it do a lot of bubbling? If it does a lot of bubbling, sounds like compression is getting into the water system. That means cracked head, or bad or failing gasket.
Do you know if the shop you took it to to work on the head uses a fly cutter to fix the head or a belt sander. If it's a sander, Very easy to not get it back right. Did the shop tell you how much it was out?
Also, do you notice any steam from exhaust? or any water in oil. Where it looks yellowish milk color.
 
It does not bubble until it jumps to the red line/260. I don't know how they trued the head but I do know they took ten thousandths.
I'm not seeing any steam/smoke from the exhaust or water in the oil, that was one of the first things I thought of when it overheated after putting the head back on.
 
Well, the mechanical fan seems to have helped lengthen how long it take to overheat but not cure the problem.

On to try something else tomorrow...
 
Had a thought this morning that maybe the belt was not tight enough allowing the belt to slip on the fan pulley. I tightened it some and the Jeep still gets hot but did not overheat. I let it idle for about 45 min and then drove it about 15 miles. Before I started driving the temp gauge had moved to the first mark past 210. While driving it got almost to the second mark past 210. I stopped and let it idle again and it actually cooled back to the first mark past 210. It is still running hot and if it was 96 outside like yesterday I'm not so sure it wouldn't have still overheated anyway.
 
Looked for a "rev" on the pump but I can't see any stamp I might need to remove the shroud/fan/pulley to see it though.

I drove it about 50 miles yesterday, mostly highway. While driving at 55-70 mph it heated up to half way between 210 and 260 but did not go over. When I got to where I was going I let it idle for about 30 minutes and it actually cooled back down to one mark past 210. When I headed back it heated back up to the same temp as on the way down but did not go past that point. When I let it idle after getting hot it will cool back down some but not quite back to normal.
 
Yeah, I haven't called you because I didn't want to become that guy who always bugging you when your so busy you don't know which way is up.:lol:


Had the winch on there long before it started overheating.
 
transman731 said:
len, i haven't been ignoring you. as you know, i have nothing to do, and i am just standing around. (insert sarcastic laugh here)
Click to expand...

Lee....just standing around....that is so not true. I saw you playing a game on Facebook the other day.
Well... unless you don't have a chair at your desk anymore. :lol:
 
Fixed:)

First off I'd like to thank everyone for their help and suggestions especially Lee, transman731, for putting up me bugging him.

After changing from a 195 thermostat to a 160 and back to 195, removing the electric fan and re-installing the mechanical fan, cutting the nose out of a thermostat to allow almost free flow, swapping in a new water pump and lots of hand wringing over this issue it turned out to be a lack of air moving through the radiator. Just swapped in the fan clutch from my Wrangler and voila, it didn't overheat. I had checked the clutch before and it seemed to be ok but checked it again last night and it was almost spinning free. Maybe it was intermittent which would explain why it acted the way it did. It's not 90+ outside but I think it is good now. I'm sure I'll know for sure soon.

Thanks again.:beer:
 
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