Hey,

I recently scored a 1985 300sd. On my drive home she started to get hot. The gauge got dangerously close to the red. Before it did, I pulled over and let her cool before I limped my way home.
While ildeing she'll stay at a say temp. but once I go up any hills or on the open road, she gets dangerously hot.

Here's what I did so far and she's still hot:
-I took out the thermostat, I live in hawaii and NEVER need to use the heater. Plus, I figured it'd give more coolant flow. But she still over heats.

-Then I replaced the water pump. Still over heats.

-flushed out the coolant system, took the radiator out (twice) and gave it a good flushing. The radiator appears to be in good shape, I did notice however that when running a water hose through it to flush it, it seemed to not flow through as fast as I hoped.

-I did a block test yesterday to see if I have a faulty gasket seal and the result showed that I do not have a faulty gasket seal.

I'm stumped. I'm not loosing any coolant, only gets hot on freeways and hills. The current radiator looks to be under a year old. I'm trying to do all I can to not buy a $300 radiator unless I absolutely have too.

So, and suggestions? Ideas? Thoughts? Questions? This car started as my summer project car, but now she's become the bane of my existence.

radiator is my guess based on personal driving with both a bad fan clutch and removed thermostat.

removal of the t-stat will cause a cool running condition until periods of high, sustained loads are experienced. (hills, not freeways)

Hawaii is warm, it could certainly be the fan clutch as well. Just not as likely based on what you said. Try searching this site for quick fixes of the fan clutch and testing methods (search 'refill viscous clutch')

thanks, the fan clutch is something I have not looked into yet. I will now...

I know this is highly unusual for a 617, but it sounds like it could be a small head gasket leak to me. When under load, exhaust gasses get into the cooling system and wreak havoc. They make cheap test kits for this to see if exhaust gasses are present. Is there any bubbling or excessive pressure? Have you replaced the pressure caps and made sure the expansion tanks have pinhole leaks.

This is a classic sign among Cadillac Northstar engines, which I am familiar with as well. As a HG means basically throwing away the car, someone finally came up with a bottle repair that is showing alot of promise. Its much cheaper, easier, and economically feasible to do a HG on a 617 though.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

I did the test kit for exhaust leaks in the coolant, but I was parked and mostly at idle speed. It makes sense that the HG could leak at high loads. Is there a specific gasket sealer you recommend if I go that route?

Expansion tank cap could be either not on correctly (venting) or not functioning correctly. I had a scare much like yours and found that I didn't tighten the cap all the way so no pressure was building in the system. All your symptoms/tests point to a bad cap being a very possible cause of your overheating.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

I'm going to give the expansion tank a pressure test today after work, hopefully it's just the cap.

Thermagasket has been a big hit among Jeeps, Toyotas and BMWs. They recently did alot of R&D with Northstars and is really showing promise. I bought a really nice '98 Cadillac STS with the problem just to try the stuff out, so we'll see.

You completely flush the current system until no coolant is present, fill the car with this stuff, drive it around for a short time, and flush and replace with coolant.

It makes sense to try this with northstars, but simple inline motors that are easy to R&R head gaskets doesn't really make sense. They make some very bold claims and I haven't found much negative commentary from folks who tried it.

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1985 500SL Euro w/ AMG bits 130k
1984 300SD Turbodiesel 192k
1980 240D Stick China 188k
2001 CLK55 AMG 101k
2007 S600 Biturbo 149k Overheated Project, IT'S ALIVE!!!

When you refilled the cooling system, did you make sure you burped the system to get the air out of the block. If you have an air pocket at the top of the block, it will cause the engine to overheat. Good luck....

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96 E300d

What about a faulty auxilary water pump? Might be reducing the flow and causing you to overheat.

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Hairline crack in expansion tank, however no loss of coolant.

Quick check of fan clutch is to run car till warm, open hood and press the stop lever. Once the engine is stopped try to turn the fan by hand. It should not freewheel, but should have plenty of resistance.

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----Adam

1985 300SD 322,000

Some folks are making this out to be much more complicated than it needs to be. Think about it - you've already flushed the system and replaced the WP, and you observed there is flow. You don't have any leaks anywhere. So either the flow is reduced or you don't have pressure building in the system, or possibly a combination of both. Flow "not as fast as you had hoped" is difficult for us to gauge, therefore I'd say focus on the pressure. After pressure testing the system then you can narrow it down to flow. The responses here would have you replacing the entire cooling system, head gasket, etc ($$$) without actually diagnosing any problems. I just wanted to tell OP not to get overwhelmed, he is on the right track. Just no need to be jumping to expensive conclusions--it is bad enough he had to replace radiator and WP.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

I'[m surprised noone has pointed this out yet. Put a thermostat back in. MB engines will overheat when the thermostat is removed.

When the engine is cold, the closed thermostat causes coolant to recirculate through the water pump and engine. This is how you get heat before the engine comes up to temperature, before the thermostat opens.

As the engine warms the thermostat opens and directs less coolant through the recirculate path and more to the radiator. If you completely remove the thermostat, there is nothing forcing coolant through the radiator, hence the engine overheats.

You can test the old thermostat by putting it in a pan of water and bringing to a boil. If it does not fully open before the water approaches boiling, then it is bad. A failed thermostat is one of the more common causes of overheating.

Its the fan clutch. Same thing happened to my SDL. Put a new fan clutch in and problem solved.

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1987 SDL. The best there ever was.

I'm going with JCYUHN on this one. Put the thermostat back in it but before you do - drill about 8 holes in it with a 3/16 drill bit. GO CHEAPER FIRST then start throwing $$ at it. This is a free fix. Trust me, I stayed at a Holiday Inn Express last night. HANG LOOSE

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I LOVE CATS....AND FRIES

1981 300sd
1983 300sd
1985 300d
1983 300cd
1991 350sdl