Been trouble shooting my system for some time.

It's generally in adequate shape. Temp sits at 82C 95% of the time.
On hot days however (hot days defined as anything above 30C/ 85F), highway runs above 65mph will bring me up to around 100C. If I'm running AC, 105C. This goes for extended grades during average temps as well.

Before anyone posts a form letter, I've been through all of the usual steps.
Thermostat is working as intended, water pump is new, hoses are firm, citrus flushed (not much came out), 50/50 coolant mix with G-05, new rad cap, and so on. No visible coolant leaks.

The only thing left is the radiator. But before I ready the parts cannon, I'd like to make sure everything else has been ruled out.

Many have suggested using an IR thermometer to check for cold spots.
With the condenser on the front and a spinning fan on the back, I haven't found this very practical. I'm sure it's also possible to a have a marginal radiator with no "dead spots" per se, but rather a systemic under-performance due to age. I am running the original Behr/MBZ rad...

A buddy suggested testing for a slight pressure leak –one small enough to leak air, but not coolant. I'm considering doing this, but would like to make sure I'm not wasting my time. This got me thinking that the system is already pressurized after a drive, though it's not possible for me to know how much, without a gauge. By hand, the top hose is nice and firm.

* Is there a rule of thumb for how long it should stay this way?

* If I do perform a pressure test (I've a power bleeder I can attach to the radiator via auto zone loaner program) is ear enough to find the leak?

* Am I missing anything here?



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1981 240D 4sp manual. Ivory White.

Make sure that the top radiator hose doesn't go up or rise above the radiator. You want it going down otherwise that can create bad flow or an air bubble.

It happened on my 240D. It was running hot and when I pressed down on the hose then all of the coolant from the radiator went down and I topped it off and it ran cool.

Good call.
Just confirmed hose is running at the correct angle of attack.

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1981 240D 4sp manual. Ivory White.

I am not convinced that there is a problem that deserves troubleshooting. My '82 300D behaves similarly. And it has an oil cooler.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Interesting.

I also have an oil cooler. A side thought was whether the oil thermostat was working as it should, and if not, what impact, if any, that might have on engine cooling.

As an ambient temp, 30C/85F just doesn't feel like it should be affecting temps as much as it is, but perhaps you're right.

Then again, VWnate is always going on about driving his 240 through Death Valley, four deep with AC blasting.

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1981 240D 4sp manual. Ivory White.

At 85f your car should sit at 80c on the highway

When you said "on hot days," I was thinking more along the lines of 100ºF.

Elevation is a factor as well; less dense air cools less effectively.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

At 80ºC, an 82ºC thermostat would be in full bypass with no coolant going to the radiator.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Yeah, I should have clarified that. I also run hot as a person.

Re: elevation, I'm experiencing this around sea level.
All signs point to a radiator.

I just hate throwing parts at a problem. I've done enough of that in the past.

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1981 240D 4sp manual. Ivory White.

Yes you're right and I should have written 82c.

Still 80 or 82 it doesn't matter since the gauge/sender is not that accurate anyway.

I know the struggle. Mine always ran cool (apparently too cool at 80c, bad thermostat I found out), but would overheat easily on long trips. That turned out to be the space between the radiator and condenser being mostly full of leaves and compost. An accident deleted the condenser but a new thermostat made it run at 40c. That was because of a rock jamming the new thermostat open. Fixed that and found out that these are supposed to run at temps that would kill any other engine.

I've always kept an eye out for tips on keeping them cool, but the general consensus is that when the engine block starts to melt is when you should worry and not before.

Jesus.

You've had a time...

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1981 240D 4sp manual. Ivory White.

It does sound like maybe you need a new radiator .

I have a new in the box NISSIN 240D radiator if you have troubles finding one .

I bought it a while back and decided to do some more citric acid flushing and that stopped the warm running .

Let me know if you can't find one, I don't see it for less than I paid as my 240D is hands down my favorite so I ass-U-me I'll use it some day long from now .

Remember : Diesels thrive on heat .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I appreciate that Nate.
Found a nissens on Auto hau(s) for about $180 shipped. Not bad.

I realize I'm technically within spec temp wise, but 105 makes me nervous.
If a new rad brings me under 100, I'd feel a lot better.
Seems like after 41 years of service on the original, a new rad must have some positive effect on things.

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1981 240D 4sp manual. Ivory White.

Good deal ~ I paid a little bit more .

Everyone says the NISSIN radiators are the very best .

My original Behr's upper hose nipple simply fall off as I was driving one fine day.....

Luckily for me I was able to re attach the hose to the bit of stub left and get some water, motor on home .

I forget where I got the radiator in it now but it's been fine .

Most folks never service the cooling system until it fails .

"Permanent" antifreeze doesn't mean forever .

After five years or so antifreeze becomes corrosive, I never wait that long .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better