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...

I second that, a diesel is not a high rev engine, and I would surely not run it at 4500rpm with no load, that is bad! Drive it until it is up to these high temps it is reaching, quick pull over, shut it off, and try to turn the fan by hand, if it is stiff and won't move, then the clutch works. If the engine was running 100C+ and it still spins easily, then the clutch is shot.

On my gasser I can easily hear the clutch engage around 90C....its quite loud with the fan revving up.

OM602 Operating Temperature
 

With:
Good:
Fan Clutch
Correct coolant
properly serviced cooling system
fresh thermostat
operating temperature sensors
fresh surge tank cap (AKA radiator cap)

should be 80 to 84 degrees "C"

How ever if Ambients are over 100 degrees "F" And you're Freezing inside And
on an uphill grade with 800lbs in the trunk...op temps will climb.

Turning heater on helps...
 

He's heading back from Glennwood Springs now - going up the Vail Pass will be the true test - told him not to exceed 95C no matter what.

He says that when he turns on the heat and runs the fan inside the cabin the engine temp goes down somewhat and helps a little - I'm sure keeping the speed down helps the most. It's cool up in high country already - which should help. Let's wish him all luck.

Does that fact with the heater running and cooling the temps help narrow anything else down for him?!

Nah,Not Really
 

'Just lets him know the Heater Core is not totally obstructed.

Cool makes up for thinner air with less heat capacity.

Sixto
87 300D

Coolant replacement...
 

He'll need to service his cooling system - I'm assuming that means flushing the green old fluid out with water and replacing it with MB approved fluid: Zerex G-05 full strength and mix 50/50 with distilled water.

Would he need to do a citrus flush on the system or no?

If there is a good link on the procedure for doing that plz send it this way.

James

Coolant...
 

Could someone confirm that the coolant needs to be 50:50 mix Zerex G-05 with distilled water?!

When is the citrus flush recommended?! Should it be done in this case?

'Suspect long period of Non-Maintenence?
 

Or extended use of ,Verboten, Green coolant?

You have to De-Oil before Citric Flush.

First, I'd Clean the Radiator and Condenser FINS,You may have to remove the
Radiator to get at all the accumulated trash between the two.

50/50 is fine in your climate.
40% Anti-Freeze/ 60% Distilled will offer more heat transfer,
BUT I don't know about that @ 5,000+ feet and near zero temps.

That is the proper coolant and mixture ratio.

I would probably not do a Citrus flush.

Make sure the radiator is extra clean externally, if it is all plugged up with dirt (the fins)...it will pass very little air....and will not dissipate heat well at all. I had this issue on my car but I just replaced the radiator since it was the original.....the new one + new coolant dropped my temps massively....and now it never gets much over 85C no matter what.

Well, why is the green coolant verboten?

Mine has it and I do run warmer than I think I should be running on a hot day.

Also, there is a crack in my surge tank.

It's at the seam and I am pretty sure that it isn't air tight anymore.

How could this affect my cooling system?

His too...
 

Friend's car has the same issue - I'm wondering how that affects the cooling - if at all?!

It depends on how large the crack is. If it won't let the system pressurize, the boiling point is compromised and 120°C. would be very dicey, at best.

If you keep it below 105°C., it's a non-issue.

Wellll....my 2.5 behaved like that when I bought it 4 months ago. Temperatures would go up to 100C when idling or climbing a hill, especially after a highway run or a quick acceleration.

Turned out my fan clutch was bad. I installed a new one and this problem was gone.

Since then I also have replaced the coolant with Zerex G05 and Water Wetter.

Now, the engine rarely goes above 90C, mostly it stays between 80C and about 87C I'd estimate. This is in the summer too.

Running with the AC on will bump temperatures by 5C easy.

Highway cruising with AC off: 82-85C; AC on: about the same.

I went with a 40% Zerex mixture btw.