Temperature Gauge Spike/Overheat

I figured I'd ask before tearing into this...

Got this latest XJ about a month and a half ago. It's a 2000, 4.0/AW4 completely stock.

Jeep has been fine up till now although I noticed that the temp gauge seems a touch hotter than my built XJ. The built one sits at exactly 100˚C indicated (~210˚F). This new one sits a touch higher. Gauge isn't that accurate but I'd say it might be 220-230˚F.

Anyway, in the last day suddenly the temperature gauge will spike into the red in less than a second from the steady state temperature.

If I turn off the Jeep for a moment the gauge returns to normal temperature.

If I leave the Jeep on (but engine off) and watch the temperature gauge it will suddenly drop from overheat to its normal temperature.

Looking under the hood the Jeep doesn't 'feel' like it is running hot. No funky smells and such like you get with a overheated engine. There is no exhaust smell in the coolant overflow bottle. Exhaust is not white.

The cooling system has not been opened since I got the Jeep and I've easily put 6-7 tanks of fuel through it before this started happening.

Screwy thermostat? Temperature gauge sending unit? Radiator cap? Air in the system? Other?

TIA

The only way to independently confirm the engine temps is with an Infrared Temp Sensor or by installing a temporary aftermarket temperature gauge and sensor.

If your 0331 head isn't cracked:

The cooling system is a group of related parts that depend on proper function from each of its component parts to keep the engine cool. Service the cooling system and replace any under-performing or suspected weak parts. Any component part of the cooling system that is not fully doing its job will stress the others and your cooling system will overheat. The most important maintenance item is to flush and refill the coolant periodically. Anti-freeze has a number of additives that are designed to prevent corrosion in the cooling system, but they have a limited life span. The corrosion causes scale that eventually builds up and begins to clog the thin flat tubes in the radiator and heater core, causing the engine to eventually overheat. Coolant should be replaced every 36,000 miles or every three years.



-Use a flushing/cleaning solution and then drain and fill the radiator with a fresh 50/50 coolant and water mix. With a neglected cooling system you may have to flush several times.
-Inspect the radiator for mud/bugs/grass clogging the outside and mineral deposits clogging the inside. Clean or replace as needed.
-Replace the thermostat with a STANT or Robertshaw 195* thermostat. Cheap thermostats are cheap for a reason.
-Replace the radiator cap . An old worn out cap will allow boil overs and/or allow the coolant flash over into to steam. You will see the coolant temps suddenly jump from 210* to the Red Zone and back to 210* if your radiator cap is weak.
-Inspect/test or replace the mechanical fan clutch. A worn fan clutch will allow temperature creep at stoplights, in heavy traffic, and on the 4x4 trails. A fan clutch that “looks” OK is not the same as working OK. Consider installing a Heavy Duty fan clutch such as the NAPA #272310.
-Inspect the electric cooling fan and the fan relay. Apply 12 volts and make sure the fan runs. Exchange the cooling fan relay with one of the others similar relays. Confirm that the e-fan starts when engine temps reach 215-218*. Repair or replace the fan or relay as needed.
-Inspect/test or replace the coolant temperature sensor that activates the e-fan.
-Replace the water pump. The pumping fins can deteriorate over time and the pump will not flow enough coolant to keep the temps under control.
-Inspect/replace the radiator hoses. Make sure the coiled wire is installed in the lower hose.

If you have covered all the points listed above and still have overheating issues, inspect the head for cracks and head gasket for leaks. Exhaust gasses entering the coolant can raise the temperature of the coolant or cause steam pockets in the coolant that will temporarily block the flow of coolant

Read more about cooling systems here –

www.offroaders.com/tech/engine-overheating.htm

www.familycar.com/classroom/coolingsystem.htm

Thanks for the advice.

I'll change the simple stuff (i.e. rad cap) and see what happens as that sounds like the immediate symptom.

If I don't get any joy from that I'll R&R the system. The Jeep only has 100k miles on it and according to the previous owner the radiator was replaced last winter (looks new).

I'm wondering if the thermostat is not a 195˚ unit since steady state temperature is higher than normal.

If you see a very quick "spike" in temp that quickly returns to the original temp, this sounds like an electrical issue.

Temperature sending unit is the #1 suspect.

An infrared point and shoot thermometer to VERIFY is always a good idea here.

You do NOT want an overheat on an engine with the "infamous" 0331 cylinder head....

Yeah, I'm aware of the 0331 issue. My built XJ is a 2001. It's at 160k miles now and no issues yet.

I should probably get an infrared gun for the tool box.

Root Moose said:

I should probably get an infrared gun for the tool box.

Click to expand...

I bought a Raytek MT6 infrared thermometer for well under $50. A GREAT tool for the tool box, and a quality tool by a tried and true manufacturer.

I'm with birchlakeXJ....a quick spike in temp...as in a few seconds, is definately an electrical/sensor/gauge issue. Engines just don't heat up/cool down that quick. I'd suspect the sensor, gauge, or wireing inbetween.

Bought a rad cap this AM. Just went out to mess with the Jeep. Old one is only 7lbs (versus 16 lbs).

Hmm... makes me wonder why it had a 7 lb one on it... queue twilight zone music with images of 0331 casting numbers please...

We'll see... ran out of time to mess around with it tonight.

7 pound cap? hmmmmmmmmmmmmm

Very strange, as you are correct in that it should have the 16 pound cap!

You could also try removing the connector from the temperature sending unit (on thermostat) cleaning it with electrical contact cleaner and reconnecting it. Sometimes, there is just a problem with contact between the connector and the sensor contact.

My fear at this point is that the 7lb cap was put on there because any more pressure and the head would start weeping through a crack. Maybe I've been taken...

We'll see. Not likely to get to look at it tonight although one of the guys in our club gave me an IR gun.

16 lbs is the correct cap. The cap, or anything else, that doesn't hold correct system pressure allows the coolant to turn to steam at a lower than normal temperature. Steam will stall the flow of coolant or give strange temp readings. The temp gauge can and will flip high to normal as the steam pockets form and decay or as the steam pockets stall coolant flow or passes the thermostat.

So I replaced the cap and took the Jeep for a spin into town tonight (about 20 minutes each way - stopped at the drug store in town so was parked about 10-15 minutes). I got almost all the way home when the temperature spiked again.

The IR temp guage was reading ~260 deg F at the t-stat housing by the time I fumbled around looking for some batteries for it. Top of engine (valve cover was ~180 degrees).

It seems like the gauge is accurate so I don't think it is a temperature gauge sending unit issue.

I think next I'll change the t-stat and coolant and see what that brings.

How do I test the fan clutch? There is resistance when I try to spin it by hand. It does not free-wheel much at all.

You should test the coolant for exhaust gasses. Go to a autoparts store and rent the tester.

The #1 symptom of a cracked 0331 cylinder head is "unexplained coolant loss".

Do you have that??

That head doesn't do well with any overheats.

Here is a link to a good thread on the 0331 cylinder head. Required reading for anyone with an 00-01.

http://www.jeepsunlimited.com/forums/showthread.php?t=391831

If the gauge changes instantly then I'd get a new coolant temp sensor first

I'm not losing coolant. No smell in the overflow.

Mine is doing the same thing. I have a leaking freeze plug and think that may be contributing to my problem. But, I'm also thinking that the T-stat may not be opening when it's supposed to either. The first thing I did was replace the temp sensor.

I'd like to see how you get yours fixed.

Head is cracked. Time to do a replacement.

If the head is cracked you'd smell the burnt coolant coming out of the exhaust right? Also white smoke?
Also, I don't understand what the guage goes up so fast and then comes down really fast. It makes no sense to me, why won't it read the temps in between?

The temperature spike is due to the crack in the head releasing the pressure suddenly once the vehicle gets hot enough. The cooling system is pressurized so that the boiling point is higher than water's normal 212 degrees. If you lose that pressure then you are boiling the water at normal atmospheric pressure which happens to effectively be the operating temperature of the engine.

You need to diagnose the engine to determine if this is the problem versus some flaky t-stat, sensor, rad cap thing. I did that plus visual inspection of the top of the head through the oil fill cap. Mine is borked - yours may not be (but likely is).

fully dressed heads are cheap (~$450-$500). Getting a shop to install it shouldn't be too bad. My Jeep is in good enough shape to be worth doing the fix.