4.0 Temp gauge high but not boiling - JeepForum.com
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post #1 of 3 Old 10-01-2016, 04:06 PM Thread Starter
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1995 ZJ 
Join Date: Oct 2013
Location: Hanover
Posts: 24
4.0 Temp gauge high but not boiling

I'm stuck.

The temperature gauge is reading high at idle, 220-230, but it isn't boiling--the upper radiator hose isn't even hot.

Here's the whole story: the head gasket was leaking coolant into two cylinders, so I replaced it. In the process, I broke the top of the temperature gauge sensor, so I replaced it also.

There was a lot of crud in the cooling system, so when I was done I added a bottle of cooling system flush w/clean water only, and left out the thermostat so I could flush things out w/o the thermostat getting in the way.

I drove it maybe 20 miles yesterday. The gauge was a little higher than it used to be, but I thought maybe I just didn't have enough water in. There was absolutely no heat, even though no thermostat and the gauge said it was about 200-210.

Today I drained the coolant, removed the lower radiator hose from the water pump and the upper hose from the radiator. I used compressed air to try to push the rest of the water out of the engine. I didn't get much out. The air came through very freely. I also disconnected both heater hoses, and blew the core out w/compressed air. I did the same to the radiator.

I put a 195 thermostat in, w/the spring in the head. I added equal parts coolant and distilled water until the radiator was full, then parked it nose uphill and let it idle w/the heater on to try to get the air out. Before long the gauge was high, but there was no heat and the upper hose was not hot. I drove it back in the shop. There was a little heat by then. The upper hose was starting to get hot. When I took the cap off (I had put it back on) a little coolant pushed out.

I thought maybe I put the thermostat in backwards or had a bad one (it's new also), so I drained some coolant and took the thermostat out. The coolant in the bottom of the radiator wasn't hot. I tested the thermostat in water on the stove--it started opening around 195, and opened fully.

1. Do I have a bad temperature gauge sensor? What should it read?
2. Did I damage something by blowing compressed air through? I can see coolant moving through the radiator when the engine's running, but it isn't bubbling.
3. Do I have a buildup of "crud" somewhere that's blocking coolant flow?
4. Did I just have a bad airlock, and panicked?

I'm thinking my new head gasket is also leaking.

I put the thermostat back in, & filled the radiator w/both hoses off the thermostat housing. When started, the gauge went up pretty quickly. The coolant in the radiator looked OK, but then it came all the way up to the filler neck. I put the cap on & let it keep running. The gauge went up to about 240, but the top hose never got warm. Then I saw coolant was trickling out under the cap. I remembered I had seen that last evening, & had just thought it was a bad cap, but I put a new one on today.

My thought: for whatever reason, my new head gasket is leaking, and it's pressurizing the cooling system beyond the cap's limit. I've pulled the spark plugs while it's cooling down; I'll try doing a leak-down test & also see if any coolant leaks into the cylinders while it sits.

Last edited by jbutrip; 10-01-2016 at 05:16 PM. Reason: update
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post #2 of 3 Old 10-02-2016, 09:46 AM
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1998 ZJ 
Join Date: Nov 2015
Location: eau claire
Posts: 2,438
I would suggest a compression test. Start with a dry test and record the numbers. Follow with a wet test (add oil ) and compare the numbers. You want to look for two adjacent cylinders with significantly lower pressure to indicate a head gasket problem.

Good luck,
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post #3 of 3 Old 10-11-2016, 06:48 AM Thread Starter
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1995 ZJ 
Join Date: Oct 2013
Location: Hanover
Posts: 24

I'll follow-up in case someone comes across this in the future.

I did compression and leakdown tests on all cylinders, with piston at both top and bottom (leakdown). Other than valves that aren't sealing very well (185,000 miles & no valve job), was OK.

I took the Jeep to a local cooling/radiator shop. They flushed the system--a lot of rust, other stuff, even after I flushed it. This also took care of any air pockets.

They also replaced my new temp sender. This dropped the temp gauge about 20 degrees. They still didn't think it was correct, so they used an infrared non-contact thermo to check water temperature. Was OK, so they soldered a 1/2 ohm 10-watt resister into the wire going to the temp gauge, bringing it down to within 5 or so degrees of actual water temperature.
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overheating , temperature gauge , thermostat

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