Friend of mine has a 1991 300D 2.5 (602.96x) that has been plagued with annoying overheating problems from the time he had purchased it (about 8 months ago).

The car is running regularly at about 85F on flat surfaces and about 90F at a slightest incline. It's in 100F and above territory in the mountainous terrain (which naturally he's been avoiding).

Running the heat inside the cabin brings the temps slightly down from the hot zone... but still he can't seem to locate the source of the problem.

Thus far, he's done the following trying to resolve it:
1) Replaced the cracked coolant reservoir with a better one (used, including cap)
2) Brand new water pump
3) Brand new thermostat
4) Used fan clutch but in very good shape

I guess the clogged radiator remains as a possibility... what else should he be checking?!

Also, what is the normal operating temperature for this car (around 80-85 I suspect)?!

Thanks,
James

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1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

Isn't there an aux water pump on these 602's?

Also, you should probably do the full flush of the cooling system. That should remove scale, rust, buildup etc... it the system and allow better cooling. Diesel Giant has an excellent writeup.

How is the compression? No gasket leaks?

__________________
Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

Hmmm... I don't know...?!

__________________
1987 Mercedes 300SDL; SOLD
1985 Mercedes 300D; SOLD
2006 Honda Pilot - wife's ride; 122K;
1995 Toyota Land Cruiser - 3X locked; 182K

4) Used fan clutch but in very good shape

I would get a new one to be sure! cheap insurance.

You do have an aux. coolant pump, that could be leaking or clogged.

also check the electric fan which is on the front of the a/c coil. you can jump out the wires to be sure it works. I believe the fan is on a thermostat switch, so that switch could fail or the relay that controls it could fail.

+1 on doing a full coolant flush and refill

check that electric fan first

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87' 300DT (Grey w/ red leather)
87' 300TDT (silver w/ palamino tex) SOLD

2012 VW JSW TDI (on tdiclub.com all the time "btcost")

-aux coolant pump should not matter, it is in the heater core loop.
-I would flush out the radiator.
-I would remove the shroud and fan and make sure the fins are clean, use degreaser and a steam cleaner.

__________________
2011 e320 cdi (132,000) current
2005 Jeep CRD diesel--2.8L (138,000 miles) wrecked, insurance took
1993 Mercedes benz 300D 2.5 Turbo Frybrid powered. (336,000 miles) wrecked and sold as parts car
1990 Mercedes benz 300D 2.5 Turbo (170,000 miles) wrecked and sold as parts car

Check out my Frybrid Vegtable Oil Conversion at http://1990300dfrybridconversion.shutterfly.com/

I flushed my cooling system and it did not get better, if not worse.

I removed the radiator (10 minute job) and took it into a radiator shop. They boiled it out and backflushed it three times before is was clean. It cost $58 and it was the best money I ever spent.

Backyard backflushing will never clean out a plugged radiator like a shop does. It saved me hours/days of extra grief trying to diagnose overheating problems.

All my DIY flushing left me a barely adequate heater core, which was fine before I started.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)