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  #1  
Old 08-21-2004, 04:19 PM
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Overheating '84 300SD

A few weeks ago I noticed that my engine was running hot... at least that's what the temperature gauge was showing me. I'd do a highway run for just 10 minutes at 70mph right after the engine has sat for hours and all of a sudden the temperature would be at 100C, sometimes even 115C. This would happen either with the A/C on or off.

So I decided to do a complete cooling system service. I throroughly flushed the system of the old coolant, removed the thermostat, drained the engine block of coolant and then put in a new OEM thermostat and new engine coolant (MB coolant) in the system. I diluted to coolant to 50/50 with water.
All these I completed yesterday. Some 4 hours later I took the car on the highway, doing 65 - 70 mph with the A/c off and with the outside temperature at 60F and the temperature gauge went from the bottom to 90C within five minutes. Then after that, the temperature gauge kept on shaking between 110C and 90C constantly and after a while settled at 115C. Not wanting to risk damage to the engine, I turned on the heater and right away I saw the gauge go straight down to 80C and then go back up to 90C. It seemed like the speed of the car didn't really matter, as I exited from the highway and drove constantly at 35mph but the gauge stayed at 110C.

I know my water pump works, because prior to my locating and draining the engine block coolant plug, I had flushed and put water in the radiator and when I finally drained the engine block, all that came out was water not coolant. Or is this an incorrect assumption? Anyway at this point I'm really confused. This gauge seems to be behaving badly. I'm suspecting that it is faulty, could be slipping or something similar. Or the coolant temperature sending unit is defective. Or both. Though I think it must be the gauge due to the way the temperature fluctuates.

I've read several pages on this through the search function. The only thread I could find in which a problem similar to mine was discussed was this one: 300SD nearly overheats at high speeds.

However that thread doesn't help me much because I have a radiator that's only a few years old (still looks brand new as a matter of fact). Also found this one: How many diesel owners have had problems with overheating????? in which the fan switch was mentioned as a possible cause, but the fan came on while I left the car idling at around 85C and then went off a little after it had stayed on.

Any suggestions on what I should do would very much be appreciated.

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  #2  
Old 08-21-2004, 05:34 PM
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The water pumps aren't usually the problem on that model. At least not a flow problem. I have seen POS aftermarket pumps with plastic impellors. Some manufacturers BMW, Jag, and Porsche actually put these problems on cars, but MB hasn't sank that low yet. Plastic impellors have been known to slip at certain engine speeds.

But, it really sounds like a radiator problem.
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  #3  
Old 08-21-2004, 08:20 PM
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I wasn't actually suspecting the water pump. I wrote clearly in my post that I believe it to be functioning fully.

Why do you think it might be a radiator problem?
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  #4  
Old 08-21-2004, 09:08 PM
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Mostly because there isn't anything else. The fact that the heater helped actually means the water pump has to be pumping. It also means you didn't have enough radiator at the moment.

Too much heat, too little cooling, or no flow, there just isn't anything else.
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  #5  
Old 08-21-2004, 09:16 PM
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First how many miles on the car? Also you must find out if the car is actually getting that hot any good mechanic should have an infared thermometer in his tool box. Then you can go from there. Checking engine, then checking different spots on the radiator to see if there all equal. Is the any water leaking? especially from the weep hole in the water pump. If while driving and you turn heat up does temp drop? Check fan and belt also.
Hope this helps. A 60/40 mix will let engine run at its coolest.
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  #6  
Old 08-22-2004, 02:01 PM
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The temperature drops while the heater is on. The car has 152,680 miles on the odometer (all accurate). No water leaks from the water pump. I checked that out myself. All fan belts were replaced just 1,000 miles ago.

I'll try and locate a mechanic in my area that has an infrared thermometer so I can determine if the engine is as hot as the gauge shows. In fact, this will be a top priority of mine as I suspect that the gauge may be malfunctioning due to its erratic behavior. Please re-read my original post to get the full details on the behavior of that gauge.

stevebfl, how come you don't suspect the gauge at all? This radiator is pretty new. And it is an original Behr radiator. Would be surprising if it has already stopped working. I know it can't be clogged up inside because I was able to flush it out well and when re-filled, the cooling system took its full capacity of 11 quarts.
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  #7  
Old 08-22-2004, 05:10 PM
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One of your symptoms seems to be rapid flickering of the temperature gauge. On my 240D, I found that the negative battery cable was corroded. Replacing that cable has pretty much eliminated the rapid flickering. Also, on my 300D the temp sensor was leaking a tiny trace of water. This caused the temp sensor terminal to be corroded to the point where the wire broke off when I went to inspect it. Note that with the sensor circuit open, the temp gauge reads zero, so this will not cause high temperature indication, but logically it might cause flickering.
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  #8  
Old 08-22-2004, 09:22 PM
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Had same problem with my 84 300sd when I flushed the system. The fluctuating temperature was caused by air pockets in the system. I ran searches in mercedes shop and the mercedes mailing list and found a procedure for bleeding the air out. Fixed everything. Unfortunately I could not locate the procedure in my files. Do a search and see what you can find.
That IS the problem.
John
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  #9  
Old 08-25-2004, 07:54 PM
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Found a procedure for bleeding air out of the cooling system. I performed it but that didn't solve the problem.

I touch all parts of the radiator after the engine had warmed up and the fan was running just to check for any cold spots that would suggest the radiator isn't functioning properly. I couldn't find any.

Then I put a thermometer in the coolant reservoir and the temperature reading was 78C - this, when the car's temperature gauge was showing 100C. That made me wonder, can someone tell me, which coolant temperature does that gauge show? The coolant in the radiator/hoses, or the one in the engine block?

I have another question. After driving the car around town for 25 minutes, I opened the coolant reservoir with the gauge showing 100C and with the radiator fan on and with the engine running, but there was no pressure present in the system. The reservoir cap just opened without any force? Is this normal? The reservoir cap is brand new. I just purchased it from ***************.

Anyone with any other suggestions?
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  #10  
Old 08-25-2004, 09:36 PM
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No, you would have been scalded by expanding coolant at 100C!

You have a bad temp "sending unit" (ie thermistor). screws into the block below #1 glow plug somewhere if I remember correctly (or is that the Volvo?).

Side of the block, anyway.

You could also have a wire grounding on it's way to the thermistor, too, or a bad guage.

It's also possible that you have a leaking coolant reservoir that won't hold pressure, they crack at the neck sometimes so there is an opening where the pressure cap seats.

A bad visco clutch will also cause the temp to rise some.

Peter
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  #11  
Old 08-26-2004, 03:40 AM
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I checked the reservoir for leaks very well and it doesn't leak at all.

So which coolant temperature does the gauge show? That of the coolant in the radiator/expansion reservoir or the one in the engine block?

I think I'll hand this over to the professionals tomorrow. At least I've tried.
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  #12  
Old 08-26-2004, 12:41 PM
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Dropped off the car at the mechanic's this morning. I'll let him take care of it. Maybe I should just stick to my computers.

Thank you all for your suggestions.
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  #13  
Old 08-26-2004, 04:14 PM
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Gauge measures the temperature of the coolant in the block.
I seem to recall a post on the diesel discussion a while back in which this problem can be caused by the bypass system. I think a thermostat not seating correctly (??) can cause the coolant to be returned to the radiator rather than circulating through the block. It sounds as if intermittantly, this is happening to your motor.
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  #14  
Old 10-31-2004, 10:06 AM
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Quote:
Originally Posted by kerry edwards
I think a thermostat not seating correctly (??) can cause the coolant to be returned to the radiator rather than circulating through the block. It sounds as if intermittantly, this is happening to your motor.
That can't quite be the case because sometimes when I just put the key in ignition without cranking the engine, I see the temperature gauge flickering a bit.

My mechanic replaced the sending unit and it sort of improved. The flickering reduced, and typically the car runs at 100 C which I still think is too much considering other folks here have the same car running at 90 C. Anyway, one day I hit the highway and the gauge was almost touching the red mark. I couldn't smell any burnt antifreeze or feel a reduction in performance. I was around my mechanic's shop so I drove it straight to him. He popped the hood and the aux fan on the radiator wasn't even on!! Then he checked the temperature of the coolant and it was just 78 C, at temperature at which the thermostat wouldn't even have opened. At that point, he told me not to worry about it and concluded that I need a new temperature gauge.

I still drive the car the way it is, though I've never seen it go very far up the gauge past 105 C. I sent in an oil sample about a month ago. Once I get the results I'll determine if any antifreeze is getting into the oil, in which case I'll know that maybe my gauge is right and I am over heating the engine, though I really doubt it. I followed the procedures located at http://instrument.articles.mbz.org/temp/ to clean my sensor and it still flickers a little bit. Furthermore, my fuel level light doesn't come on when it should. I'm getting more and more convinced that those gauges - all in the same part of the cluster - are faulty.

I have one question though: at what temperature is the aux fan on the radiator supposed to kick in? I know I've heard it running sometimes, and while I know something is wrong with my gauge, I was kind of worried that after driving on the highway at 80mph for 20 minutes, when I got home the temperature gauge was showing 100 C and the fan wasn't on. Shouldn't it be on after such a highway run?
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  #15  
Old 10-31-2004, 12:42 PM
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Gaage is bad, or the wire to the gauge is shorted somewhere. Similar problem on the Volvo, guage will suddenly peg for no reason.

Aux fan should come at 212F, switched by a temp sensor, either the main on or a separate switch on the thermostat housing depending on year. You can test by grounding the wire to the switch on the thermostat housing or by unplugging the temp sensor. Fan must come on full speed.

If it runs once in a while, it's working correctly and you need to find the bad wire or replace the gauge -- sooner or later, you are going to have a real problem and seriously overheat the engine thinking the guage is just acting up. This is about the only way (other than running with no oil) to kill that aluminum block.

Peter

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