300D cooling enigma - PeachParts Mercedes-Benz Forum

LJ DERBY · #1 05-15-2005, 10:51 AM

Hi everyone,
On my 83 300D I have a cooling problem that almost defies description. This is only an example of the odd behavior. Yesterday I drove about 6 miles to fill up the tank and 6 miles back. All this time the temp gauge read half way between 80 and 100. As I was turning down my street the gauge started to climb and stopped at 100. I let it idle in the drive and it didn't cool down but stayed at the 100 mark. I have had cooling problems off and on for the last year so I have changed the thermostat three times, put in new coolant, ran water through the radiator under pressure and it ran good, checked the viscuous clutch, new pressure cap, etc. etc. I'm sure I forgot other things I did. My question is, Can the impellor blades on the water pump come loose from the shaft and randomly catch and work like they are supposed to and then break loose and not pump much?

Lee

Brian Carlton · #2 05-15-2005, 11:04 AM

This doesn't sound all that good. Erratic temperature issues, where you can't seem to repeat the results with the same ambient temperatures may point to a head gasket issue.

After you let the vehicle sit overnight, open the hood and squeeze the upper radiator hose. See if you can pinch your fingers together. Then open the cap on the expansion tank and try to pinch the hose, again. If the hose is now much softer than the first time your tried it, the odds are that you have a head gasket "issue".

You also might want to send in an oil sample at the next oil change. If the vehicle is using any coolant, the oil sample will show it. Don't ask how I know.

#3 05-15-2005, 12:21 PM

A new temp sensor would not be very expensive. I'd swap it out to be sure your gauge is not being sent a bad signal.

jake1222 · #4 05-15-2005, 03:19 PM

When I got my 81 300SD it was having some overheating problems also. I went through the entire system and even changed out the head gasket. The last and only thing that I changed out was the plastic overflow tank on the fender wall. It was cracked on the bottom where I couldn't see it and was releasing coolant making it look like it was coming from the overflow hose or the cap. Check your tank and see if that may be a problem area.

LJ DERBY · #5 05-15-2005, 05:14 PM

Thanks All,

I'm not loosing coolant and I did change out the temp sender with a new one, maybe I have a defective new sender? The car doesn't overheat so I have some leeway to trouble shoot. I bought a new pump but haven't put it in yet. Maybe that's the next step. I hate it when there is no clear path to take. A infrared temp sensor would be handy for this wouldn't it? I've always wanted one, maybe this will be the excuse to splurge. Lee

nhdoc · #6 05-15-2005, 05:32 PM

Don't forget the gauge...I had a 240D where the temp would climb and drop, but it was just the dash gauge that was irratic. A gentle "tap" on the side of the console brought it back down...you might want to confirm that what you have is not a simple as a faulty gauge or bad wiring to it.

diesel don · #7 05-15-2005, 09:33 PM

The technical guys will not like my answer, but all my 123's run hot. I obsessed with the first one even though it NEVER overheated. After a while, I noticed how common the problem is (or is reported to be).

So I said to myself, if I overheat one and warp it, I'll go out back and grab another car. That was 4 years ago and I haven't had another worry on 'running hot'. All the guages 'run hot' but I don't lose sleep over it.

My 2 cents

t walgamuth · #8 05-16-2005, 01:01 AM

i agree with the last post... it is not clear to me that you actually are overheating, just running hotter sometimes than others.

that said,

i would probably change the water pump since that is the only thing you have not. i am not sure exactly how they tend to fail but only that they only tend to last about 40,000 and when they do the car runs progressively hotter. a clear sign is leaking from the relief hole in the pump casting.

pawoSD · #9 05-16-2005, 01:57 AM

Mine does it too, 95% of the time the gauge sits pointing straight across, but at random it will go up to the 100C line, sometimes even a little above it, then it will flitter around between straight across and above the line, then it will randomly drop back to the normal spot. So I've taken to believing that my gauge is just somewhat erratic, mine's never overheated either, even in 90+ degree weather in highway and stop and go. The '83 will always sit on the 80C line, never ever moves.

Brian Carlton · #10 05-16-2005, 09:37 AM

Quote:

Originally Posted by diesel don

The technical guys will not like my answer, but all my 123's run hot. I obsessed with the first one even though it NEVER overheated. After a while, I noticed how common the problem is (or is reported to be).

It's not like the answer is incorrect, but it's not the whole truth and nothing but the truth.

I also agree that you can run a 617 at 110°C. all day long and it won't care a whit about it.

However, folks should realize that this shows evidence of worn components. It could be anything from an old radiator to a bad temperature sending unit to a corroded water pump.

If you want the vehicle to have the reliability to go 600 miles in the summer at the drop of a hat, you need to address these "issues" before they become more severe.

However, they tend to degrade very slowly and you can go quite a long way before you have a major problem.

Palangi · #11 05-16-2005, 10:33 AM

I went through the "Temp Gauge Twich" problem in both cars.

In researching old threads, I found that most successful fixes were related to resolving poor grounds for the instrument cluster. I looked at that, and cleaned up the ground points, but found it didn't help with either car. I also found an ancient obscure post pointing to the negative battery cable. On the 240D, I found the heavy wires to be deteriorating from corrosion inside the vinyl jacket, and extending into the terminals. On the300D, it looked OK, but I replaced it anyway. Result was, on both cars about a 90% reduction in temp gauge twitching by replacing the nrgative battery cable. I think most old MB's seem to show this problem to some degree. I don't know on what percentage the battery cable is the culprit, I only know that it seemed to work on these 2 cars.

Also, just as a point of information, on the 300D, the temp gauge went dead shortly after I first got it. Problem was that the temp sender was not screwed in tightly and was leaking tiny amount of coolant. The water was wicking up the wire and eventually corroded it to the point where it broke free from the terminal. The point here is, if that part of the circuit goes open, the temp gauge goes to zero, therefore it follows that if that point of the circuit develops intermittant resistance such as from corrosion, the gauge would likely tend to twitch downwards, not upwards.

Stevo · #12 05-16-2005, 10:55 AM

There is a ground attachment in back of the instrument cluster that sometimes needs cleaning and re tightening. With the cluster out you cant miss it.

I agree with Brian. If your cars not running at the proper temp its a sign somethings not right and you need to find out what.

LJ DERBY · #13 06-01-2005, 08:04 PM

Well, I finally got my infrared thermometer and checked temps. The thermostat housing seems to be a good reference place as it is easy to get to and it appears to read almost the same if not the same as other places on the block and head. When my temp gauge was at 87c the housing was at 86c, this is after a run of 30 or so minutes. Then the temp gauge went up to about 100c the infrared thermometer read 86c. So I don't have a temp problem just an indicator problem. Lest I forget I think I also had a "burping" problem. I had to add about 2 qts of liquid at various times during test runs. It now seems to be stable. When you have two problems at once the results can be extremely confusing. If I rap the dash sharply with the heel of my hand the gauge usually reads normal. I say sometimes because other times it takes a few blows. Next job is to remove the instrument cluster and look for bad connections. I've made the removal tools already. The outside air temp and humidity also seem to enter into how the gauge reacts. Thanks to everyone for thier help. Lee

By the way, that infrared thermometer really does make temp problems a lot easier to troubleshoot.

84300DT · #14 06-01-2005, 08:54 PM

Quote:

Originally Posted by t walgamuth

i agree with the last post... it is not clear to me that you actually are overheating, just running hotter sometimes than others.

that said,

i would probably change the water pump since that is the only thing you have not. i am not sure exactly how they tend to fail but only that they only tend to last about 40,000 and when they do the car runs progressively hotter. a clear sign is leaking from the relief hole in the pump casting.

my car has always done this. the gauge is erratic and a not so gentle
tap on the dash will bring the needle down from over 100 to 85 deg C.
under a heavy load/up steep hills it will run hotter than normal.

when my water pump failed the temp gauge shot up to 120 deg in a matter of seconds.

t walgamuth · #15 06-02-2005, 12:44 AM

yours may have failed that way but they dont always do so. in my experience i have seen a lot fail and they have never done it suddenly as you have described. usually a dribble and some modest warm running.

also an earlier post asked about broken fins on the pump. i dont think that is very likely. in my experience the mb pumps fail by losing seal and then losing bearings and losing water. other cars i have seen shear off the connection between the impeller and the shaft. i personally havent seen this type failure on a benz.

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