Overheat fluctuations - frustrated

[Bender]

Yes, yes, I know... there have been so many damn overheating threads lately that I've stopped reading them as well. And YES, of course I've searched. Please, I appreciate your time in reading on...

99 D1.
Symptons:
Temp gauge remains normal duing steady-state driving, but once the driving "changes", the temp begins to climb. Cools back down with some help.
Example: remains normal during interstate driving, them temp climbs when I exit and hit the stop lights.
Another example: temp remains normal in mild neighborhood driving (30-50 mph) but climbs if I climb a large hill.

In both examples I'm able to get the temps back down to normal by continuiing my drive (getting airflow across the radiator) and keeping my RPMs below 2000. Usually starts climbing again at the next stop light.

Occurs with air-con ON or OFF. Aux/air-con fans work (checked by turning on A/C).

My first thought was to change the viscous fan clutch since I've had this happen before. The difference is that this time the new clutch has not solved the problem (changed it yesterday).

New radiator with less than 3000 miles. Clean and free of debris.

Water pump and belt tensioner changed about 40k ago. Do not seem to be a factor since upper and lower hoses become hot as engine heats. Belt tension checked IAW workshop manual... about 1-2mm of bounce at idle. Belt less than 500 miles old. Voltage output has also remained normal, so another reason I'm not thinking it's the belt tension.

Engine warms up fine. Based on indicated temp fluctuations and that all hoses are warm (also ran heater), I'm not inclined to think it is the thermostat (changed 45-50k ago).

Coolant level remains constant. No apparent leaks.

The new fan clutch spins with some resistance when cold. Of note though, it spins with the same amount of resistance once the engine is warmed to operating temp at idle in the driveway. Will the bi-metal switch sense a change at idle or do I need to be moving?

So I have a couple of thoughts but would appreciate any other opinions:
- New fan clutch (aftermarket) sucks. Replace it and get a refund.
- Temp sensor and/or gauge? I've seen this referenced in several of the other threads but I don't have a full understanding. For everyone who has recommended changing to an aftermarket unit, is the recommendation to change the sensor, the gauge, or both? Recommendations? If the OEM gauge shows slow movements and operates in a manner that seems logical, how likely is it to be a faulty gauge/sensor?

Thanks folks. I can limp back/forth to work but need to get this fixed if I'm going to be able to attend a club event this weekend.

Stephen

 

[Bannon88]

Air in the system?

When the temp shoots up, you'll need to take a temp reading to confirm the actual temperature, not just what the gauge is displaying. Confrim that the temp is actually that hot.

I had a similiar problem to what you are describing, I checked to fan, changed the thermostat, hoses, coolant, my problem was the temp would shoot up and any electrical flucuation would send the gauge up or down. Headlights on temp would go up, tap the brakes temp went down. The sensor was just shot.

Sounds to me like air trapped in the system.

Also how is your expansion tank, is it the black orignal one? They are prone to crack along the seam.

 

[Bender]

Neg on the expansion tank - semi-transparent tank.

I've read a few threads noting air in the system. What's your suggestion for the best way to check the actual temperature? The infrared "gun" that I think I've read about in a few threads?

 

[Darren M.]

I had a similar problem with my truck, and that was after I dropped a new radiator AND clutch in. Turned out the new clutch I had wasn't blowing enough air through the radiator. Slapped on a heavy duty one, and the needle hasn't moved. Drove all of last summer and this summer with air blasting, still no movement fromt he needle. Check first for all the regular stuff like air inthe system, theromstat etc. If you have no leaks, and your temp is fluctuating, I would suspect it's the clutch.

 

[p m]

Steve, I'd hope for the better and do the simplest things first - make sure there's no air pocket, then fan clutch and thermostat. I've used the Chevy HD fan clutch on the 95 LWB (serpentine belt, same as D1) - it took care of running warm at once (at the expense of lot more noise).

 

[Bender]

Thanks gents. I'm going to head to a local auto parts store and see what kind of temp gauges or monitoriing devices they have. If nothing else I'll have additional diagnostic equipment for the trail, and maybe I can narrow down this problem a little more...

I'll post an update later...

 

[p m]

Steve, I have that little IR temp gauge, feel free to borrow. I have to admit to never have been able to interpret its readings right. For instance, what does it tell me if it reads 212F at one side of the radiator and 203F on the other? And each reading changes easily by 5-10F depending on how close the gauge is to the surface.

 

[Bender]

Thanks Peter, I'll let you know if I'd like to take you up on the offer.

Bannon88, I burped the system IAW with this thread:
http://www.discoweb.org/forums/showthread.php?t=186&highlight=burp

and then when on a bit of a test drive. Still having the same problem.

Stopped at NAPA and talked to a gent there who seemed to think the sensor was bad. They were out of them, but I may stop by the dealer later today or tomorrow and see if I can pick one up. Short of a bad replacement fan clutch, I'm not sure what else to try. The engine has that "hot rubber" smell to it more than usual, so I'm inclined to think the motor is getting hotter than usual, but I've been wrong before!