I've been having cooling issues on hot days(80F or above) ever since I bought the 300SD a year and a half ago. The hottest the engine ever got was 105C. It's getting warm again so I decided to tackle the issue today.

I drained the engine block and radiator, removed the t-stat and thought, "great replacing this piece of junk is going to solve my problem." The car actually runs about 6 degrees hotter now.

After searching through some threads, I read something about a t-stat break in period, any truth to that? It's a Behr t-stat. Has anyone had problems with those. I thought about the fan clutch, and it does have some play in it. When I shut the car down after the engine has warmed up to about 85C, the fan stops after about two revolutions. Is this normal or should it immediately stop?

I filled the system through the upper radiator hose with a 50/50 mix of Zerex G-5 and distilled water. I don't think there's any air in the system. The reservoir tank has just a little coolant in it while the engine is running, then fills back up when the engine is off. I noticed that there wasn't any coolant dribbling down from the top reservoir hose while the engine was running. Any advice? Thanks

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'81 300SD

Is your radiator clean? You can force the aux fan on to see if it is an airflow problem.

I didn't use a Behr in my 606, but I've noticed it's been running a hair warmer now with the new Wahler t-stat and fresh coolant. I made sure to clean the exterior of the radiator when I installed a new fan clutch, and I also used a flushing agent when replacing coolant but it's still running a tad warmer. It was over 80f today and it would creep up to 95c or so in traffic with the AC running on high, but run around 85c (the t-stat's rated temp) when rolling down flat portions of the freeway at 75mph. I've been reading reports on the forums here and there about new t-stats running a bit warmer than the old units people pulled out, but as long as it's in the safe zone you shouldnt' worry. Double check your coolant/water mixture, I used a Prestone tester and discovered I was at around 60% antifreeze, so I will be draining a little bit tomorrow and topping off with more pure distilled water to get closer to 50/50. Your coolant should be a bit over the arrow on the side of the expansion tank when hot, you may be running a little low on cooalnt if it's vanishing when running, or air is infact trapped within.

As far as your fan clutch, it "sounds" ok but it's impossible to tell over the web. Search for the newspaper test, you simply roll up some newspaper and try and stop the fan when at idle, there are a number of youtube demostraions of this. It's fairly crude but it did allow me to see my clutch was very bad

Okay, so if the coolant is supposed to be at the arrow while at operating temp, then that means you should have coolant all the way to the cap with the engine shut down? The cap is fairly new by the way, and I was surprised that there was no steam or pressure at the cap when I removed it after bringing the operating temp up to 90C on the highway.

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'81 300SD

No it should be at the arrow when cold and on the level ground, may not have been as clear as I could have there. But IIRC my manual states ~1 inch above the arrow when hot, right at it when cold (or about half way up the tank and the seam). It shoudln't be rising when the engine shuts down, it should do the opposite, I suspect air is in the system. By the way, in your climate 90c on the highway is A-ok and definitely no need for concern at all. It should run quite well and effeciently at those temps in fact, our diesels do enjoy a little more heat to run their best.

Perhaps the coolant system hasn't got all of the air out of it yet?

Where were your heater controls when you drained the system - closed or open?

There are also thermostats that have different ratings - what was the rating on the new one you fitted?

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Thanks for the quick responses. Any tips on how to burp the system? The coolant in the reservoir tank has always behaved as hot=down,cool=up. Maybe I've been driving around with air in the system all this time.

Yeah, 90C is nice when it's 60F at 5:00 P.M., but my problems occur when it's 80F at 1:00 P.M. and I'm running a little over 100C

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'81 300SD

I think that the monovalve defaults to open with the engine off. I turned the heater on when I first started the car so I could get an accurate fluid level in the reserve tank.

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'81 300SD

That still really isn't too bad, it's still well within the safe zone but perhaps a wee bit higher than it should be if you're rolling; in traffic with AC you can expect those temps though. In any case, try lifting the front of your car to make it as high as possible and, with the cap off, run the car up to operating temp with the heater on max and give it a shake from time to time, it should bubble up eventually. I am a bit impatient and have found putting a piece of cardboard over the front of the radiator can heat it up faster and hopefully move the air out quicker.

Oh crap that monovalve thing - I've heard of it but never experienced it.

Search in google for

burp coolant monovalve site:peachparts.com

and I'm sure after a bit of reading you'll find the answer. I think I remember that some people do exotic things with axle stands...

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

and to be perfectly clear...

105 is totally fine all day long in these cars.

hitting 120 is NOT fine, but anything safely below that will do no harm to the motor.

it's not NORMAL, but it's ok on the car.

what is also not normal, is your drive should have pressurized the coolant. since it did not, I would have your cooling system pressure tested. you MUST have a leak. if a pressure test of the system reveals a tight system, you should replace your cap.
the cooling system needs to hold at least 15 psi, preferably 22. (100 to 140 KPA cap)

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

Check the accuracy of the gauge with a non-contact thermometer.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I would recommend that you calibrate your Prestone tester with a known 50/50 mix before you rely on it. I have one that reports an 80/20 mix as 50/50.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

That's what is known as a "highly profitable" Prestone tester.

Thanks for the heads up on the coolant tester, I will give it a test with a bottle of factory pre-dilluted 50/50 Honda blue coolant I have sitting in my garage this afternoon. I did go out and check my bottle of MB gold coolant and, based on my capacity and how much I used of bottle 1 and bottle 2, I'm a hair over 60% af/water, could also explain why it won't wick away heat quite as fast as before too.