Friend's 1991 overheating on mountain roads...
 

Friend of mine Sylwester has picked up his first MB two weeks back - he's driving on a business trip to the mountains (Silverthorne, CO) and he called to tell me that the engine temps have risen over 110 deg C while driving up the long hills on the way to to the high country - normally his temp has been hovering around 90 in town - he's new to this car and doens't know if this is a normal operating temp of the motor - seems high though. He said in addition, there was a new noise that he could not describe well over the phone. Sorry, not much to go on.

Question - does this model have a scanner port that one could hookup a scanner to and get a head-start at what the problem might be (or if there is one)?! Anything else to check on the car to start the troubleshooting sequence?

Any advice will be welcome -

The problems in order of most probable to least probable in a vehicle of that age are as follows:

1) Radiator
2) Clutch fan
3) Water pump

The clutch fan can be verified for proper operation without removal. The water pump requires removal to verify. The radiator cannot be verified without replacement.

What model of car is it?? It is no help to know its a '91....there are several models and even more engine possibilities that year.

110C is way too high. He likely needs one or more of the following: fan clutch, radiator, coolant changed, thermostat, water pump.

*Edit, darn, I was 30 seconds too late! :D

If it's a '91, the engine is a 602 or a 603...........the issues are the same.

Of course, I'm presuming that he posted in DD for the right reasons.............

True.....

It's a diesel 300D Turbo
 

I believe this is the engine: 1991 300D 2.5 Turbo Engine Type 602.962

What's the penalty for driving around 'as-is' until he gets it home to Denver (90-100 miles away)?!

What is the normal operating temp for a 602 (2.5Turbo)?

What would be the maximum normal operating temp for same engine?
-Like for extremely hot days, hill climbing, wind at your back, etc...

That's a difficult question. But, it's generally accepted that the engine should stay below 105°C. in the worst possible circumstances (maximum load for more than 30 seconds and ambient temperatures of 100°F.)

Personally, I don't think any of these diesels can maintain maximum load at high rpms at high ambients for an unlimited time. They'll all overheat. However, the ones with the new cooling system parts have the benefit of the longest possible time before the temperature climbs to unsustainable levels. Usually, the maximum load on the engine would be reduced by that point in time.

Fan clutch...
 

I told him to keep it under 105C if humanly possible - that was good info. He's got a temporary thermostat from Autozone (hehe) and is installing is right now as we speak.

What's the procedure for verifying the fan clutch is working?! How can he check that? I'm looking at an online manual: http://www.ps2cho.net/downloads/MB%20CD/W124/Index/602_603/20CoolingSystem.htm

Procedure: # 20-310 - Temperature-controlled viscous fan coupling - is this the one...?! It says in there:

"Run engine at 4000-5000 rpm. Once a coolant temperature of approx. 90-95C has been reached, the speed of th eviscous fan coupling must increase, which can be clearly heard"... so if I understand correctly: warm up the engine to normal temp, then rev it up to 4000-5000 rpm and observe the fan - if the fan speed increases then then clutch is working, Ja?!

You'll notice that I did not recommend the thermostat as the problem. The engine ran at the correct operating temperature under normal loads. It's unlikely that the thermostat won't fully open at higher temperatures...........but, not impossible.

When the engine is very hot (above 100°C)..........shut it down and simultaneously observe the fan. If the fan stops with the engine, the clutch is fine. If the fan spins several revolutions after the engine stops, the clutch is toast.

I've never managed to get my 617 powered SD over 95C even when it was 100F outside and I was flogging it on the highway (95mph+)....I have: newer radiator, 9 blade fan + good clutch, newer water pump, and perfect/fresh coolant....99.9% of the time it sits around 82-84C....its almost impossible to get it to go higher.

The gasser goes up to about 105C in standing traffic on an 85 degree day with the A/C on.....but once it hits 105 it powers up both aux fans, and the temp drops rapidly back to 85C or so....

Checking fan clutch now...
 

Right, but someone else did... and it was something he had already purchased by the time I called him - anyway, the Chinese thermoastat does NOT fit :), so he's putting the old one back in.

I told him to run the check on the fan clutch, which he'll do as soon as he re-assembles the old thermostat into the car.

He did mention to me that he recalls that earlier today he had parked the car and left the engine idling for a little while and the fan had kicked on moments later as never before... perhaps that's somehow related.

Nay.

Sorry to disagree with the manual...........but, I'd never run that engine at 4500 rpm for an indefinite period without a load. Furthermore, it's almost impossible to keep the engine from hitting it's rev limiter when you attempt such a procedure.

Finally, the engine is making so much noise that your capability of listening for the fan clutch to engage is just about zero unless you've done it before and know what your listening for.

I've made that exact run half a dozen times with Ol' Turbo. For sustained uphill jumps the temperature does rise, and hints at, but never gets above the century mark. I'd be curious how old and nasty the coolant is before diving into parts.

Coolant...
 

I don't know.... but it looked like the green fluid that everyone around here hates...