Jeepforum couldn't help.. temp spikes?!

[XJ_Goat]

I posted this question to jeepforum.com but it was essentially received with crickets, maybe you guys can help.

BLUF: Temp will jump from ~220 to 240+ in about a second then after a couple of minutes pulled over it drops back down to from 240+ to ~215 in about a second.. and I dont know why.


I have a 2000 XJ with 170,000 miles, auto, 33" tires, stock gearing, a winch in front, a new radiator (and cap), I flushed the coolant when I replaced the radiator, new 3x10" electric fan set up from Dirt Bound Offroad. and the T-stat is ~ 1 year old (stock temp).


In more detail here's whats Happening:
Starts and idles fine, first fan kicks on at 185*, other 2 fans kick on at 205* and temp stays constant. - no issues at this point
Shorter drives around town (stop and go) no real issues (a little warm but nothing major).
But on longer hwy drives the temp goes up to ~220* (or whatever that first dash is after the 210* mark) and if I have to travel at a sustained higher load (like up a long hill) the temp will jump to 240+* but here is the thing that I cant figure out; it is not a steady temp climb, but a quick change from ~220* to 240*. I pull over, turn off the engine and let the fans run for a minute or two and the temp will drop back to ~215* and when it drops it does it all at once, from 240+ to ~215* in just a second.

Here are my theories:
1. I may have a dieing water pump that can't keep up with the higher load, but that doesn't explain the quick temp changes
2. I may have a head gasket that is its last legs and is letting exhaust into the coolant and causing the temp gauge to read the temp flux, but I dont think that would be limited to highway driving under load.
3. The winch is blocking airflow, but I have electric fans and they should be able to keep air moving in that center section where the winch is and it doesn't explain the quick temp changes.
4. The head has a small crack and when it's hot and under pressure exhaust is getting into the coolant giving me the temp spike, as it cools the gap closes so the pressure can build again and that drops the temp. I dont have milky oil, I cant see anything blatant through the oil fill cap and I tried the auto parts store's coolant / combustion gas test but I couldn't get the engine up to the questionable temp before the tester blew off the radiator -so I can't confirm (or rule out) this possibility.

I don't want to keep throwing money at parts that may or may not solve the problem (especially a new head). I'd like to accurately diagnose the root cause then fix that.

What do you guys think? Do any of those ideas hold water? Have any better ideas to what may be causing this?

Thanks in advance for any help.

 

[old_man]

Jeeps have to follow the laws of physics. There is no way the real temperature can jump 20 degrees in a couple of seconds.

Things that can cause the reading to fluctuate:
1) You have an air bubble in the head next to the temp sensor and from time to time coolant sloshes back and fort.
2) You have a sensor that is flakey
3) You have a bad/corroded connector
4) You have a wire that is either cracked or corroded internally
5) Your temp meter is defective
6) You might have a bad ground but it would most likely show itself on multiple gages.

My bet is on a bad sensor. If the head gasket is in fact leaking, it will cause the coolant to go acidic and it can eat up a sensor, not to mention a radiator, quick;

 

[XJ_Goat]

I completely agree with the law of physics pot of view, but all those ideas would cause an intermittent temp spike, my jeep is pretty consistent on when and how it overheats. That would lead me to believe that I have a real issue not just a faulty reading.

I think the Temp / Pressure / Boiling point relationship may be tied to the quick changes I'm seeing.

 

[MrShaft696]

x2 on the gauge being a liar basically, have you changed the temp sender?

 

[WB9YZU]

Sounds like a bubble in the coolant. The newer jeeps didn't have the coolant bubble issues the older ones did, but if you just changed coolant, I would look into that. Edit I just re-read that you did.
Did you have this problem before the mods? Does your T-stat have the small bypass hole in it? Many replacement don't.

The other 2 possibilities are a bad head gasket, or a bad head.
The heads crack into the water jacket and most people notice them when they check the oil and find a milkshake, but having it show by blowing air into the water jacket makes sense too as I had a head gasket on a 4 cylinder do that once.

 

[Matthew Currie]

That temperature jump is awfully quick. I've had all sorts of cooling system problems in one thing or another, including consistent overheating from things like bad pressure caps, rotten fins, and other things that happened pretty quickly, but a second still sounds a bit like a bad sensor. If there's a glitch at something like 222 degrees, it could still be very consistent, appearing only when you get a little over normal temp. If you've exhausted all the other obvious options, I'd still be inclined to look at the sensor.

Any chance you could hook up a secondary thermometer or thermocouple to this, for a second reading?

 

[XJ_Goat]

Here is the long story on my coolant system;
About 2 months ago I was on a drive ~an hour from my house and a blade on my electric fan fell apart (just old and brittle) and made some god awful noise. I pulled over and removed the remaining parts of the electric fan. I drove it back home but it did get a little warm, nothing crazy, but higher than normal.
Since I had to replace the electric fan i thought this was a great time for an upgrade! So I picked up the Dirt Bound Offroad 3x 10" electric fan kit. I went to take it on its first wheeling trip and on the way to the trail I experienced the over heating symptoms that is the subject of this thread. After pulling over a few times to let it cool and seeing the temp jump up and back down so quickly I began to think I had air in the system (I had to tap into the heater core hose to add the dual stage temp sensor). While on the trail I went to climb a particular rock and the wheel slid off, the bumper fell on the rock and my engine's accessory system chewed into one of the electric fans. sh!t. Well I took that as a sign that I needed to go back and revise my install (that issue turned out to be warn motor mounts). On the way down the trail (on the breaks, no gas) the temp spiked again. I thought 'stupid air pocket' this isn't a real temp issue, the air pocket will work itself out. BOOM! I was wrong. The side of my radiator blew out, coolant all over the trail - not good. I got towed down to a parking lot and was given a ride back home. The next day I returned and installed a new radiator (and all new coolant). success!?
On the drive home (~1 hour) the same temp issue that happened on the way to the trail happened again (and is continuing to happen). Problem not solved.
That's when I replaced the radiator cap hoping that was the cause, no luck - same issue.

So all that makes me think the temp reading is accurate, but a new temp sensor is only ~$20 and I might give it a try, I just feel that it is $20 being thrown at a problem that doesn't match the symptoms.

Thoughts?

 

[Tim_MN]

If the radiator cap is not doing its job and holding 16 lbs of pressure, hot spots in the head can flash to steam in an instant. The steam can stall the flow coolant and will cause temperature spikes on the temp gauge. Some auto parts stores have bad part database info and they will sell you a 13 lbs radiator cap. Some cheaper radiator caps are simply too crappy to correctly holds 16 lbs.

A water pump is routine maintenance item like tires, windshield wipers, or mufflers. When worn out, weak, or faulty, the water pump should be replaced.

 

[XJ_Goat]

I'm on my second radiator cap, i doubt both are bad. As for the water pump, it could be dieing... but as I said on my original post a dieing water wouldnt cause a rapid change in temp like I'm seeing, it would be a steady climb and drop, right?

 

[SanDiegoXJ]

I'm on my second radiator cap, i doubt both are bad. As for the water pump, it could be dieing... but as I said on my original post a dieing water wouldnt cause a rapid change in temp like I'm seeing, it would be a steady climb and drop, right?

Only one thing I can think of that would cause your problem, and that is, that back in the good old days, the lower radiator hose had a metal spring to keep it expanded, because, notoriously, jeeps under load would collapse the lower hose and create a vacuum lock on the coolant system; lowering the load on the engine relieved the vacuum lock and the coolant system would go back to normal.

If that sounds like it fits your bill, you might try changing either the lower or both radiator hoses. Or the lower hose and the pump for that matter.

Edit: or adding a hi flow pump...but if it just materialized, my guess is your lower hose is fatigued and collapses on you.

 

[XJ_Goat]

I was doing some more reading and came across this thread: http://www.naxja.org/forum/showthread.php?t=1101664
see post 4:

on the newer jeeps right past 220 the computer will peg the gauge, kick on the "check gauges" light and sound a chime.

I fought this forever in my jeep then found out it's programmed into the ECU. Your jeep isn't actually running that hot, it's just kind of like a sophisticated idiot light.

I'm pretty sure this is what you were seeing. If I'm guessing right, as soon as you saw it, you backed out of it and it dropped back down to 220 or so just as fast as it spiked...correct?

Has anyone else heard of this? Dr. Moab, do you know what years this may apply to?

If this is true than I think I may have a water pump on its last legs and when it gets to 220* the computer kicks "spiking" the temp.

 

[DrMoab]

Well it would apply to your 00 for sure. If you search around you'll see this is quite common. On the OBD2 jeeps (96 and newer) the temp reading comes from the ecu after it comes from the temp sender. On the latest years...not sure when they started doing this, if they hit a certain temp, about 225-230 the computer kicks the gauge all the way to the right and turns on your "check gauges" light. It's kind of like a fancy idiot gauge.

The problem is, your jeep shouldn't be running at 225-230. Consistent, steady temp climbing was the issue I was running into. After a new CSF radiator and a zj clutch fan I mostly fixed mine.

 

[XJ_Goat]

thanks for the quick reply,
I agree that it shouldn't be running at the elevated temps; it idles fine (~205*). Strait flat runs it gets up to ~215* when I come to a stop the temp drops back down.
But the drive up US 89 past your house up to ogden is enough to get the temp over 220* and spike the gauge.
I dont know what others run at with similar electric fan setups but its steady state is warmer than what I'd prefer.

This theory and the cracked head seem to be the only two that may account for the rapid temp changes.

I think it may be time to research good water pumps.

 

[DrMoab]

Remeber though. The temp isn't changing that rapidly. Only the gauge.

 

[XJ_Goat]

yes, the rapid change of the temp reading.

 

[DrMoab]

From say....210 to the point where it spikes the gauge. Is that a fast change or gradual?

 

[XJ_Goat]

gradual

 

[DrMoab]

Well. I can tell you for sure it's not your water pump. Almost always they are a work or no work deal. Usually when they go bad it's bearing failure which causes them to leak but not affect cooling.

How old is your radiator? It could be plugged. It could be a head gasket, it could also be the dreaded cracked head. I think I would start with a good coolant flush. Check your oil for water contamination and check the coolant for bubbles from a cracked head (this isn't an end all be all test but its a start).

 

[XJ_Goat]

radiator is new (<500 miles on it).
I cant see any head damage from the oil fill cap, and there is nothing on the dipstick but oil.
Water pump isnt leaking.
T-Stat is only ~1 year old I bought the type that 'should' fail open.
I've seen pics of completely corroded vains on the water pumps, maybe that's it?

 

[Tim_MN]

At 13 years old and 170,000 miles, it is probably time for a new water pump. Just because the pump is not making noise, or is not leaking, does not mean that it is working.

By definition, steam is hotter than coolant and it will spike your temp gauge.

.