gummycarbs

Update: it was the thermostat. I used my IR temperature sensor and noticed that the radiator temperature wasn't getting over about 170 degrees, while the temperature gauge showed 210 or more. If the radiator isn't getting hot, the fan clutch isn't going to engage. I did a Prestone 3-day flush. It improved the situation, but didn't fix it. The temperature differential was still higher than expected. I replaced the thermostat (Stand 195-degree 'economy' thermostat) and it instantly resolved the problem. Now, the top of the radiator shows almost the same temperature as the thermostat housing. I did two more flush cycles and replaced all the rubber components, but the thermostat was what completely changed the behavior. The fan still isn't audible at idle. It is audible, however, if I've been idling for awhile and begin to accelerate. It simply sounds like a slight 'roar' as RPMs approach 2k-ish RPM.

As I said, I replaced the fan clutch last year and it had absolutely no effect on the behavior whatsoever. After replaced the reservoir/expansion chamber cap, the needle hasn't yet passed the 210 mark.

Just a followup: I went ahead and replaced the water pump. Amazingly, there was no change in the idle overheat behavior. The last water pump I can find in the records was at 132k miles in 1997. Now at 279k miles, the old water pump's fins looked just about exactly like the new one! No noticeable corrosion, and no perceptible play in the shaft. Kind of amazing, but maybe water pumps last indefinitely when they don't have to support a radiator fan... Like a fool, I didn't replace the thermostat while I was in there. I've also noticed some coolant around the reservoir, so I think the cap is shot. I ordered a new cap and if that doesn't fix the issue, then I'll replace the thermostat, and if it still overheats, then I'll replace the radiator.

Thanks for the responses. The belt is tight, I topped the oil up last week, and the radiator fins are about as clean as can be. The catalytic converter and exhaust is an interesting idea. The cat has recently started to rattle at low RPMs, so it's breaking loose if not completely broken up. I have no idea how that might be affecting flow. It's never been off-roaded and the exhaust is at least visually in pretty good shape. The Hayden 2737 fan clutch might be a good stop-gap measure while I decide whether or not I want to do an engine swap. There's an 80k mile 1988 motor for sale locally.

Hi everyone, I'm driving a 1989 Comanche 4.0. It's at almost 278k miles and has no modifications, aside from maintenance and collision repair. It's a stripper model, with no A/C, so no auxiliary electric radiator fan. Last summer, it began to overheat when idling for extended periods. I flushed the cooling system and replaced the radiator clutch fan. If I recall correctly, I used the old "push a stick against the fan when it's hot and idling and see if the fan stops spinning easily" test to determine that the old fan clutch was bad. Also, I could see coolant flowing pretty well into the overflow tank, so I assumed the water pump was good. Well, here it is summer again and it's overheating again. One thing I've noticed is that I never hear any kind of roar from the radiator fan. My old 1995 ZJ would roar for a bit right after starting up, then I'd intermittently hear the fan clutch engage and more roaring during prolonged idling. So, should I be able to hear the Comanche's radiator fan? The last time it started overheating, I gave it a bit of throttle and noticed the gauge begin to drop a bit. I held it at maybe 1,500 RPM for twenty seconds or so. I've read here that this indicates a failing water pump, but I chalked it up to increased radiator fan speed from the parasitic drag of a non-engaged fan clutch. Regarding the temperatures, I've seen it get *almost* to the start of the red zone. Now that I know it can happen, I keep a close eye on the gauge and shut the engine off when it gets close to last white tick mark. In years past, it would be rock-solid in the 195-210 range. Seeing it at 210 (the middle mark) was a notable occasion!