temp gauge bottoms out while driving, normal at idle

hi all...

I've worked on lots of cars, trucks and even tractors over the years, but I haven't seen this before:

I'm driving down the road in my 98 xj, 4.0 auto tranny, and the temp gauge bottoms out. If I sit at idle for a minute it goes right to normal. I think at least once when I was going up a fairly steep hill (on the highway) it also went to normal. But I didn't see that, I just *think* I saw it falling as I reached the top.

I've seen plenty of vehicles get hot while driving and then cool off when idling, but never the other way around.

I just replaced my AC compressor and let it sit in the driveway idling a while, and even the hood was hot as hell when I shut it off.

Whatcha think? Maybe a bad water pump? Maybe when it sits still, physics takes over and the water circulates well enough for the temp to be right, but at speed it can't keep up with the heat being generated?

I mean, it could be the sensor, I guess. If the sensor is bad then the efan wouldn't come on enough, or at all. I'm going to let it cool and go see how it behaves next.

Thanks in advance!

Oh, and RIP our 99. My wife had a wreck on the interstate a week ago. The wife is fine, but the xj is totaled. The xj gave us 11.5 years and about 300k miles (bought her with only 16k on it). The only mechanical things she ever needed was one radiator, one starter and one alternator in all that time. Thanks for the memory, old girl.

I'd check with a scan tool to see what the ecu is seeing for temp, it sounds like you might have bad coolant temp sensor or a loose wire.

Agreed, sounds like bad wire, bad sender, or possibly coolant level low enough that the gauge sender is ending up exposed to air once in a while. I doubt the latter since it'd drop much slower, if at all, and you'd notice other issues.

Bad sensor or wire. These things all use variable resistance. If the circuit closes (shorts itself) then resistance falls to zero.

It could also be the thermostat stuck open. The "normal" symptom of a stuck thermostat is slow warmup/cold running in cooler weather.
With the engine at idle, from cold start, the heater hose should get hot while the upper radiator hose stays cold until the temp gauge is up around 180 or so. If both hoses heat up at the same time, the T-stat isn't closing all the way, or stuck completely open.


I was doing some experimenting with dual electric fans on my Comanche and I found out something strange about the I-6. It has a coolant flow issue at idle. Water pump is turning too slow to adequately cool the engine without a lot of air flow through the radiator. With no mechanical fan, and only a single E-fan in the stock position, the truck would quickly overheat at idle. Rev the motor to 1200RPM(vehicle stationary), and the temp would drop back to 210.

If the thermostat isn't closing, it may be over-cooling when there's high airflow through the radiator(at highway speed), then when it drops back to idle, the heat can build back up in the engine.

This matches my own experience.
I had my Jeep idle & warm up halfway decent but stay below 100 degrees water temp on the freeway. It was a really cold drive home - Utah in January if I recall.
Thermostat fixed it.

tbburg said:

It could also be the thermostat stuck open. The "normal" symptom of a stuck thermostat is slow warmup/cold running in cooler weather.
With the engine at idle, from cold start, the heater hose should get hot while the upper radiator hose stays cold until the temp gauge is up around 180 or so. If both hoses heat up at the same time, the T-stat isn't closing all the way, or stuck completely open.


I was doing some experimenting with dual electric fans on my Comanche and I found out something strange about the I-6. It has a coolant flow issue at idle. Water pump is turning too slow to adequately cool the engine without a lot of air flow through the radiator. With no mechanical fan, and only a single E-fan in the stock position, the truck would quickly overheat at idle. Rev the motor to 1200RPM(vehicle stationary), and the temp would drop back to 210.

If the thermostat isn't closing, it may be over-cooling when there's high airflow through the radiator(at highway speed), then when it drops back to idle, the heat can build back up in the engine.

Click to expand...

I'm having the same issue in my 98 4.0 w/aw4 (194k). When I drive on the highway for extended periods of time the coolant gauge will go to full-cold. When I pull off to a stop light at the on-ramp it always seems to come up to normal temperature at the halfway mark. Mine acts erratically though sometimes. It will randomly work properly and then go to full-cold when tooling around from light to light.

I attributed mine to a bad sensor or loose wiring like the other 3 guys are suggesting. Sensors run around $22 online and $42 at the local Napa distribution. I planned on just replacing the sensor because of mileage and if that didn't fix it, I'd start tracing wires. I haven't got to it yet...

Your idle will be about 100 rpm higher, shift points will be wierd, and your gas mileage will drop when this happens regularly. Mine started getting worse over time.

This must be a 98 issue, because it has happened to mine as well. I would get erratic operation with the needle bouncing around periodically during normal driving. The fix was incredibly simple. Remove the connector at the CTS on the thermostat housing. Gently run a small file on the 2 contacts to remove any corrosion. I put a dab of dielectric greese on the contacts to keep any contamination out and re-installed the connector. This fixed my reading problem and has worked perfectly for the last 6 months.

Thanks guys. It's been more random lately, but yesterday it finally did it again on a road that had an emergency lane, so I whipped over and it immediately went to regular temp. Now, I wouldn't think slowing down that quickly would make the thermostat pop open, but who knows. I wiggled the sensor wire and sorta straightened it out, and it ran regular temp the rest of the day. So maybe a bad wire after all. I'll just watch what happens and play it by ear. It wouldn't hurt anything to drop a new thermo in anyway, so I'll probably do that this week just for s#!ts and giggles. With the 99 being totalled, at least I have a source for "try this first" parts. I've already stolen the seats, overhead console and visors out of the 99 for the 98.

Me too. After changing out my rad the temp gauge wouldn't go past 120º.

After cleaning the pins on the temp sensor with contact cleaner all went back to normal.

I think that as the pins on the sensor cycle hot & cold along with the engine, it causes
the contacts in female wire harness connector to lose some of its springiness (=grip)
after a number of years.

I had an issue a few months ago where the gauge wasn't reading accurately. A thin coat of Noalox on the sensor's pins cured it. Think it's a combination of oxidation, vibration, and time in most cases, but he might still have a failing sensor or damaged wiring.