Our truck overheated late this afternoon when our son was driving it home from work. He had been in some bumper-to-bumper traffic due to an accident along the way but the problem became apparent a few miles after that.

The UltraGauge alarm sounded and he pulled over and turned off the engine right away. He said the coolant temp warning light didn't come on.

Unfortunately I hadn't reset the UG alarm since getting my replacement UG last summer. It was still at the factory setting of 250F. I had adjusted the old one to 220F. He said the coolant temp warning light didn't come on, but he probably simply didn't notice it in his panicked state since its threshold is also 250F. He's not as well trained as I to keep an eye on the UG temp.

Luckily I was about a mile behind him in traffic so I was able to check out the situation on the spot. I expected to find a failed hose and no coolant in the reservoir but to my surprise the reservoir was full. Hmmm.

Okay, so what does that mean? The cooling system isn't that complicated. If there is no breach in the system (i.e. a leak) that means the thermostat is stuck shut, the fan is not working, or the radiator is clogged, I think. I suppose the pump could not be pumping but that would mean the impeller separated from that shaft. Not likely, I think.

My son and I swapped cars and after it cooled down I drove it the remaining three miles to our house. I of course kept a close eye on the UG temp reading. It climbed pretty quickly through its usual operating range (190F-195F). I turned the heater onto its max setting and the temp the rest of the way home fluctuated between ~204F and 210F. Not killer by any means, but notably higher than the norm. FWIW the outside temp was ~50F.

Tomorrow I'll check the fan, but I did that only a few hundred miles ago. I'm suspecting the thermostat. I didn't change it a year and a half ago when I flushed the system and refilled with new coolant. It dates from the headgasket change 5-1/2 years ago. I'll probably remove and test it tomorrow or this weekend.

I'm doubtful it's a clogged radiator because the coolant has been changed religiously since the radiator was replaced ten years and 94,000 miles ago. There was no sign of "debris" when I flushed the system in July '14. Unfortunately I don't have an IR thermometer to test it.

All this happened just a few days after we made a 400+ mile round trip to Wilmington pulling a U-Haul trailer both ways. Temps on the trip were in the usual 190F-195F range.

What do you guys think? Am I missing something?

 

If you replaced the fan clutch and are certain that the radiator isn't clogged, it makes sense that the thermostat is the problem. Also (this is a long shot) the UG could be malfunctioning. If you end up replacing the thermostat, drill a small hole in it just in case it gets stuck closed (I've been doing this for years).

 

Have you checked system for air recently? I do with engine cold, reservoir and cap in place by removing bleed screw and top off through T using small funnel.
Minute undetectable leaks can add up overtime resulting in system air and no coolant level drop in the reservoir then eventually cause an overheat condition.

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Hmmm. Interesting. I'm skeptical that's the problem (no waterfall/gurgling, for example), but it's certainly a quick, easy thing to check. Thanks.

 

Uh ... wow. It appears the suggestion offered by number9 has worked.

The truck has been losing very, very small amounts of coolant over the last nine months or so. Don't know where, and it hasn't been a big enough amount or problem to put any effort into solving. Just a half inch or so every several weeks (it hasn't been driven much). I've just been topping it off as needed. When I said in the prior post that the coolant level was okay I was speaking about what I found that evening.

I did just as number9 recommended; with the system stone cold I opened the bleed screw and added coolant to top it off. It didn't take much, no more than a 1/4 or 1/2 cup. I replaced the bleed screw and took it for a series of short test drives. The first was about five miles round trip, which allowed the truck to get to normal operating temp. The second, an hour or so later, was about ten miles. The next morning I drove ten miles to a meeting and another ten miles home a couple of hours later. In all four test drives the temp didn't go above 194F and actually dipped to 188F from time to time. That's a couple of degrees lower than what I typically see.

So thank you number 9. In my 6+ years on this and other LR forums I don't recall ever seeing this situation and solution. I still don't understand how so little air in the system could have caused the high temp readings but I'm happy the problem seems to be solved with so little effort.

 

You're welcome and being a LR newbie I appreciate your positive comments. Glad to hear this seemed to work for somebody else too. It's my "work smarter not harder" solution to a common RV8 issue and a cleaner, less wasteful burp/bleed procedure.

Would also like to recommend you check/top-off the same way several more times to rid system of as much residual air as possible. I've been doing it on a daily basis since my last drain and flush about a week ago. Small quantity to top-off declines each time. When you can no longer add fluid or start noticing a slight increase in reservoir cold level it should be fully purged.

Jury's still out but believe if done as part of preventative mx this would help catch normally unnoticed leaks prior to causing possible overheat conditions and reduce the general harmful effects of air in our cooling systems.

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time to pressure test your cooling system

 

+1 and probably should be done periodically as part of any PM routine.

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I read it somewhere in this forum to lift the water tank up a little (dojt damage the hoses), untighten the philips plug over the T hose and add water/coolant until it starts to leak out from the little plug. Do this when the engine is cold. Which seems logical to me because the water tank is normally below that plug. It worked for me

 

Yes, that's the standard bleed procedure as specified in the RAVE. It should be done with the system stone cold. It's always worked perfectly for me - every time - although many, many others have had trouble bleeding their systems, getting the "waterfall" sound from having air in the heater core.

What baffled me this time is that I hadn't had the system open so there "shouldn't" have been air in the system. And I still don't understand how so little air in the system produced the problem I described above.