I was pushing my '84 300D up a mountain hard, this was a 5,000 foot climb. About halfway up the mountain, the temperature suddenly climbed and I pulled over just before hitting red line. It took about 2 seconds to go from normal to red line, I saw the needle swinging up.

I checked the oil level, it was fine (I have a bad oil leak so this was my first thought). I couldn't find anything wrong. Coolant level fine, and no boiling. I put on a new water pump two years ago. I gave her a few minutes to cool down a bit, she started up fine, I turned around and coasted back down the mountain basically at idle. Immediately as I started driving down the temperature returned to normal.

Still not finding anything wrong, I went back up, slowly in low gear, and no trouble. This was 500 miles ago, normal driving since then and no problems.

I'm thinking it may have been a stuck thermostat. Any other theories? And assuming it was a stuck thermostat, what could cause that and how can a relapse be prevented?

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1984 300D Turbo 350K 50K WVO (two tank) "Maeby"

Not sure about this. If you were low on water and on a steep incline, could the water level have been below the intake in the reservoir, therefore starving the engine for water? Until you leveled out again? Just a guess.

Tehon pass aka the Grapevine or the 15 toward Vegas? How hot was it outside? Did you have your AC on? The cooling system can handle the heat load until it suddenly can't.

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CENSORED due to not family friendly words

No AC. That's on the ol' to-do list (hah!). Compressor doesn't even have a belt.

This was near Independence, CA, heading up to Onion Valley campground, and it was about 100F outside. 8% grade. Fully loaded car but no trailer. Eastern Sierras – Onion Valley – The Toughest Single Climb in California?

Please do say more about the cooling system's sudden transition to not being able to keep up with the heat load. What is the limiting factor? This seems like a plausible avenue. When I went up the second time, slower, I was spreading out the load over a longer time.

I guess it would be a pretty big coincidence for a thermostat to stick right in the middle of an unusual climb. So maybe we can rule out the thermostat theory. Could elevation (about 7,000 feet) play any role? Could oil circulation have been impaired somehow?

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1984 300D Turbo 350K 50K WVO (two tank) "Maeby"

No the thermostat cannot be totally ruled out. Also behind the condenser may be really dirty. Also check that when the car is pretty hot the oil is flowing through the oil heat exchanger as it is also thermostatically controlled. Use a thermal probe as it can get really hot when in the loop.

Running up a long eight percent grade on a 100 degree day is pretty demanding. I also would inspect that radiator carefully.

To me it sound like you reached the limit of hos much heat your cooling system can reject. 100f heat, long 8% grade loaded car and deep in the throttle. There's also less air mass at 7000 foot elevation to carry heat away from your radiator. Now that you know the limit of your cooling system so you can adjust your driving accordingly. I don't think there's anything wrong with your car. If you did this kind of driving on a daily basis there are modifications you can make to improve the cooling system but I doubt that's necessary.

I think the limiting factor was the size of the radiator. The automatic transmission also dumps a lot of heat into the radiator. You could improve things by adding a separate transmission cooler.

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CENSORED due to not family friendly words

tits, this has the ring of truth. thanks for helping me mature my thinking on this.

barry12345, I'll go forth and understand the oil circulation system and thermostat, but I've not had any reason to suspect it thus far and I'm thinking it's OK.

thanks for the responses and thanks to all for a very valuable DD.

cheers

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1984 300D Turbo 350K 50K WVO (two tank) "Maeby"

I doubt that these cars would overheat in the same circumstance when new. I would flush the radiator, change fluid if it hasn't been done in a while, clean the radiator and evap with the radiator removed and replace or at least test the thermostat. I do't like to be stuck on the road and consequently refurbish the entire system when it goes out.

Make sure that hoses aren't at the end of life while you're at it. Then the thing will cool. Radiator fluid needs to be changed to keep electrolysis from eating the heater core.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

If the cooling system doesn't hold pressure, the coolant can boil and a steam bubble expose the sensor. An air bubble can do the same thing. If the fluid fan clutch isn't working, it can prevent the coolant from cooling sufficiently, allowing still-hot coolant to re-enter the engine. There's a time-delay effect with this one, although you should have been well past that if it was a continuous climb. Especially when going up mountains, the transmission can generate a ton of heat. I'm not sure if it's that, or the engine just working extra hard, maybe both, but whatever, it can also add a lot of heat into the cooling system. There also seems to be a time delay effect to this.

Given that you were able to climb at a slower speed, and especially if you did it in a lower gear, then the transmission might have been a significant source of the heat. I've been learning about torque converters, and some of that slippage to create the torque just turns into heat. The torque converters on these old diesels are not the locking kind, so they always slip some.

Lastly, and most oddly, it seems that the contacts in the electrical system can make the gauge read a bit off (high in my case). They can also cause the reading to quickly jump up (or, in my experience, down too). Apparently, this can also randomly happen at the high temp end (but not the low temp end), even though the gauge typically goes up and down smoothly through that part of the range.

Check the thermostat and fan clutch. Pressure test the system and cap. Bleed the system. Lastly, once all that is in order, clean and put dielectric grease on the electrical connections between the gauge and the sensor. And next time you go to climb a mountain, try to do it in a lower gear so less engine power turns into heat in the torque converter.

All this is based on my trials and tribulations with my cooling system this summer. And a lot of accelerating up a 750ft elevation climb on one of our local "mountains", some in 95F heat, which is part of my daily commute.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

check your radiator connection between plastic and aluminum. I had a long overheating incident where a gap would open under heavy load and start leaking, but would seal up again unloaded.

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I think you can totally eliminate anything to do with oil circulation. The problem is 100 % in the cooling system. Flush it out and make sure it has the full amount of fluid and that it is tight. Next, suspect the fan clutch and belts. If that fan clutch is weak, you will have problems on a big load, and you were moving a big load, up hill!

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

I had been wondering if someone is going to metion the fan clutch.

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84 300D, 82 Volvo 244Gl Diesel

It could be a little bit of all of the stuff that was mentioned of different combinations of some of the items mentioned.

As mentioned cleaning the condernser and radiator fins of crud often cures.

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84 300D, 82 Volvo 244Gl Diesel

My guess. I doubt the engine could actually heat up so fast you see the dash gage jump from 90 to 120 C in 2 sec. But, the sensor itself could get that hot that fast. I have seen this after refilling coolant. If the sensor sits in an air/vapor pocket, it can read much higher than the coolant temperature. Add a little coolant, and it drops to what it should read.

I have driven that road to Onion Valley long ago (not in M-B). The valley floor is so sloped that, coming back, you can put in N and glide 70 mph all the way (straight road) back to Independence. Thus, your front end was tilted up, putting the sensor at the high point.

Even if your engine did briefly tap 120 C, you wouldn't do damage as might happen with an aluminum head (like to warp). You didn't even boil over, so unlikely the coolant even got as hot as the dash gage suggested.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I do not remember one post of a non operational oil cooler thermostat. At the same time that does not mean that there are a few engines out there where they are not operational. A lot of heat is scavenged by the oil. If the oil temperature got abnormally hotter because that oil cooler was not on line. It is so simple to check is why I included it.

I really do not see an issue with running a water wetter especially when you have the aluminium head engines either.