Why is my car running so hot? - PeachParts Mercedes-Benz Forum

Ether · #1 05-09-2003, 10:19 AM

Okay, flushed my coolant and refilled with a 50/50 mix about 2 weeks ago in my 80 300SD.

The weather just started to get really warm here in Atlanta and now my temp goes way, way up when I'm sitting in traffic and the AC is on. Yesterday it hit 250 degrees just below the redline on the thermostat.

I've read some posts about potential air pockets in the cooling system after a refill. However, I believe I read in my CD manual that the cooling system will automatically purge any air. True or no?

Could it be coincidental that I'm having temp problems just after a coolant change since it just started to really get warm around here? Maybe I have a problem that would have shown up anyway when the weather got hot?

Any insight is appreciated.

Fisherman · #2 05-09-2003, 10:20 AM

Is your aux fan coming on with the AC? this wil help a lot to keep things cool.

Ether · #3 05-09-2003, 10:22 AM

Yes. Aux fan is running.

P.E.Haiges · #4 05-09-2003, 10:57 AM

Ether,

If your engine only gets hot in warm weather, its probably not the thermostat.

Take your radiator to a good radiator shop and have it flow tested. If the flow test shows that the tubes are causing restricted flow, have the radiator cleaned or a new core put in it.

Either will cost less than a new radiator, especially if the cleaning only solves your problem. Cleaning only should cost about $30 to $40. New core $150 to $200.

P E H

JimSmith · #5 05-09-2003, 08:20 PM

Ether,

These cars have a pretty complicated cooling system. From the thermostat to the air flow management through the radiatior, it is not simple, but it is pretty robust.

Your problem is very tempting to try to attribute to the coolant flush and refill. Such a solution is not likely unless you have a major air bubble that is adversely affecting the performance of the pump, in which case it would not eventually clear itself, or, if you somehow caused some "collateral damage" during the coolant change that might explain the hot running you are experiencing.

So, I would not dismiss the chance that air is trapped in the all or some of the coolant channels in the block and head, and cabin heat exchanger, and its supply and return lines. To get rid of any air in my systems when I do maintenance, I drive over to a hill at the end of the driveway, and run the car nose up on it. If the car has an expansion tank, I make sure the side of the car with that item is on the high side as well. Then I run the car with the heat on full, and let it warm up with the radiator cap/expansion tank cap off. I add water whenever the level drops until it stays full. It can take a while.

Check your car and find out what kind of clutch mechanism is used to engage and disengage the belt driven fan. Some cars have an electromagnetic device and others have a viscous clutch that is activated when the air flow over the fan hub reaches a predetermined value (70*C I believe). The electromagnetic units can be unplugged or have the control sensors/wires damaged. They are relatively easy to fix. The viscous clutches are not really repairable. They have to be replaced.

If your electrically operated auxilliary fan, mounted on the front side of the radiator/airconditioning condensor is working then that can't be the problem. But it is also electrically controlled and could be adversely affected by some unintended events during other maintenance. It should be a two speed fan, with high speed coming on in the conditions you describe.

The rest of the items that could fail and result in higher operating temperatures have nothing to do with your coolant change, and involve things like the thermostat, radiator flow capacity, air flow blockage, and a damaged water pump.

In general the most likely culprit is the belt driven fan clutch. This is because you seem to report the car is not overheating at highway speed, and the actual heat load on the coolant system is much higher under these circumstances than at idle. This would indicate the radiator and thermostat can pass the flow and the air can be forced through the radiator is sufficient. It is just at a standstill that the system cannot keep up, and the key difference at a standstill is the flow of air.

Air is forced through the system by the main, belt driven fan, and, when that base flow rate is not sufficient to deal with the airconditioning load as well, the low speed auxiliary fan comes on. If the water temperature is not being maintained below the 105*C set point, this fan comes on in fast speed. I do not believe the auxiliary fan or fans alone can manage to keep the air flow through the airconditioning condenser (especially with the airconditioning system operating) and engine coolant radiator high enough to control the coolant temperature within the intended range. I am sure this is the case if the auxiliary fan(s) high speed setting is not working.

So, based on your description of your situation, I would not tie the problem directly to your recent coolant flush and refill. I think it is the main, belt driven fan clutch that is not responding. This can be a wire or sensor if it electromagnetically activated, or it is the viscous clutch innards (either the fluid is gone or the mechanism that releases the fluid at temperature that has gone bad). You can check the fan for operation by listening to the noise this fan makes when you rev the engine when it is over 200*F. The main fan will start to make a real racket as you get over 2,000 rpm, and the noise should increase in direct relationship to the engine speed.

I hope this helps, and good luck, Jim

Ether · #6 05-09-2003, 08:40 PM

Jim,

Thank you very much for all that information. You've laid out my Saturday for me.

FWIW, today on my drive home the outside temp was in the upper 80's and I kept the AC off to see what the coolant temp would do.
My drive consists of about 4-5 miles off stop and go then 12 miles of highway driving and then another 12 or so of secondary road with the occasional stop light. The temp got up to 200*F after the first few miles and pretty much stayed there the entire drive.

Joe

Brian Carlton · #7 05-09-2003, 11:55 PM

Ether:

I had some difficulty with higher than desired temp. on my 1984 300SD. When flushing the system, I also removed the radiator to see how clogged the tubes were. If you try to look through the fins, you see 20 years of "fine debris" in there and you can imagine that very little air is getting through. I took about two hours to try and flush as much out as possible using compressed air and some foaming metal cleanser. When finished the airflow was much improved, as judged by the flow from the compressor. I would like to find a chemical that is specifically designed for this. The a/c service mechanics for commercial refrigeration have such a chemical for the condensers that live outside and suffer the same fate. I just wish I could get some.

Anyway, after installing the radiator and refilling the system, the temperature runs between 85 and 95 C. Only on a very hot day with the AC on and the turbo under heavy load will it get up to 100C.

If all of the other mechanical components are operable (fan clutch, cooling fan, etc,) this might possibly be the cause.

Brian Carlton
1984 300SD

edge · #8 05-10-2003, 12:27 AM

Being in GA, I think you can get away with having a 25% coolant mix whixh would lower your operating temperature as water is more efficient a cooler than the coolant is. Also you can add Water Wetter additive to lower your operating temp.

#9 05-12-2003, 10:27 AM

My '83 300SD had also begun to run a little bit warmer over the last week or so, particularly while running the A/C. It wasn't bad but the 10 or 15 degree higher-than-normal temps caught my attention, as I check my gauges frequently. I checked the radiator cap and noticed that one of the gaskets was badly deteriorated! I put a new radiator cap on it and the temps have been holding steady ever since. You probably should check the gaskets on your cap to see if maybe they aren't deteriorating.