1995 S500 Running Hot

[1995S500W140]

Hello -

Well I'm at my wits end, or close. I consider myself a pretty fair DIY mechanic but this one has me stumped. I have a 1995 S500 with 122k miles that is running hot. Hot being defined as rapidly climbs to just above 100 in any ambient temp. when standing still or moving at city traffic speeds. Overall the engine runs great, idles smooth, accelerates fine, but this rapid temp rise issue has just started to happen, never before as I have had this car for seven years.

Here is what I have observed:

Noticed back in July that during normal city driving the temp gage would hit 105 fairly quick WITHOUT the AC on. The temp would then go down to half way between the 80 and 100 mark when on the highway but rapidly go back up to the 100 mark once I stopped again. If in prolonged traffic at 85F or above outside I need to run the heater full blast to keep it at about 105 and stop the temp from climbing. No fun at all.

Here are the operating observations and history:

1) Oil looks good (no coolant contamination)
2) Coolant looks good (no oil contamination)
3) Replaced thermostat - No change
4) Recently while idling temp was half way between the 100 and 120 mark when the plastic four way heater hose plastic connector on the firewall blew apart and blew coolant in my driveway. I replaced the part (with many knuckle scratches).
5) Always had my radiator flushed regularly.
6) Checked the clutch fan and was able to stop it with a piece of heater hose but was told that this is normal if the temp is below the fans engagement point.
7) Water pump is original
8) Radiator is original
9) Overflow bottle radiator cap is new
10) Electric fans do click on.

I decided to take it to the Mercedes Dealer for some more diagnostics. They said it was a electric fan relay that was possibly bad and also the clutch fan was cutting out after 1,500 rpm independent of temperature. (This sounded unusual as I had a new clutch fan installed about 14 months ago.) I missed the $615 part warranty by 2 months. Not good.

OK - so I agreed with the MB dealer and I had them install a new fan relay and clutch fan. This was done two weeks ago. I drove it home and again no change in temp rise. It ran hot again. I checked the new clutch fan and I was able to stop it again with a heater hose at just below 100. I was convinced it was a defective new clutch fan that they just installed. The MB dealer agreed to replace the new clutch fan again with another new one. They said they could find no other problem and even checked the radiator zones with an infrared pyrometer to look for cool spots where flow might be blocked. They said no blockage was found.

I drove it home and again there was no change in the rapid temp rise. The car would get hot rapidly (above the 100 mark) at stop lights or low speed even when it was a cool 67F outside. I know that this is not normal because it never would get this hot in the past and I definately can not run the AC without excessive temp rise near the 120 as I'm afraid to blow the plastic four way conector again.

I'm not sure what to do next. I love driving this car but now it's parked.

I'm just at a loss and miss driving my car.

Any guidance would be greatly appreciated.

Thanks.

Hynek

[miloking]

Is your car realy that hot? i mean when your dash gauge is telling you 110C and you take one of those laser thermometers and point at engine and radiator etc. does it match with cluster gauge? Also if yes try to flush the system and fill it with destilled water and coolant 60/40 ratio should be fine....also while topping the sytem up make sure engines running to avoid having any air trapped in engine block.
And one more thing consider getting cool harness it reduced my overal running temperature by around 5C (used to be around 105 max now it never goes over 100C)

ohh and have a read here if something is not clear: K6JRF Auto Page

After reading your post carefuly i have one more comment, you say your replaced reservoir cap, do you have the correct one? Cooling systems need to have certain pressure in them and if you have cap that is not OEM spec to you car you will have overheating problems....

[Brett San Diego]

My first thoughts were also to independently measure the coolant temp through some means.

Also, you should read out the coolant temp values in the diagnostics available in the climate control module to see if it agrees with the gauge reading.

The coolant temp sensor probably isn't too expensive. You might try a new one just for kicks.

And, finally, what about the gauge itself? Sorry, no help from me with how to verify proper operation or calibrate it.

Brett

[1995S500W140]

Hello -

Thank you for the information so far. I'm fairly certain my coolant temp is high and that my instrument temp gage is correct. Also my reservoir cap is an original Mercedes replacement.

I'm now trying to determine if my water pump is contributing to my running hot problem and have a few questions. My water pump is original with 122k miles of service.

Questions:

If a water pump is not leaking from the weep hole does that mean it is working properly?

Does anyone know what material the impeller is made out of?

Can the impeller vanes crack or erode away?

Is there any way the water pump can become less efficient without showing signs of problems on the outside?


Thank you again for your help.

Hynek

[MBIOTCH]

Oh my gosh......
I went thru everything you are describing to a T............. I have a 1991 300 SL that had everything EXACTLY as you are describing! I also took it into the MB dealer and had all the work done as you did......SAME EXACT THINGS! Finally without any other resolutions the MB dealer decided to finally swap out the radiator............THAT WAS IT! Problem FIXED! Now I am happily driving it everywhere.
Also, which antifreeze are you using? I was told the orange is made specifically for aluminum core radiators. It's less corrosive, I was told.
Good luck and keep me posted. Now I'm curious!

[Brett San Diego]

Originally Posted by 1995S500W140

Hello -

Thank you for the information so far. I'm fairly certain my coolant temp is high and that my instrument temp gage is correct. Also my reservoir cap is an original Mercedes replacement.

I'm now trying to determine if my water pump is contributing to my running hot problem and have a few questions. My water pump is original with 122k miles of service.

Questions:

If a water pump is not leaking from the weep hole does that mean it is working properly?

Does anyone know what material the impeller is made out of?

Can the impeller vanes crack or erode away?

Is there any way the water pump can become less efficient without showing signs of problems on the outside?


Thank you again for your help.

Hynek

The water pump impeller is cast metal. I replaced mine myself, and even after holding the pump in my hand and fiddling with the impeller, I can't tell you whether it was aluminum or steel. I really should have been able to tell, but I can't say definitively. I seriously doubt that the impeller can break or "erode" in a manner that impairs its pumping ability without other major complications.

If you're sure that your temps read from your gauge are real, I would start with MBIOTCH's suggestion, new radiator. That's where the heat exhange occurs. If the car is running hot, it means that it simply isn't shedding the heat to the environment, which means that the radiator isn't radiating as well as it should. I'm assuming of course that everything else (water pump, thermostat) is working fine. I'm sure it doesn't take much of a build up of corrosion inside to seriously reduce the heat transfer efficiency. There is also corrosion build up on the outside that will reduce efficiency over time.

What about the overall cleanliness of the radiator? Are there any major blockages between the fins of the radiator? Clear of leaves and other debris?

What about your coolant? Is it a 50/50 mix? Too much antifreeze and not enough water will redue cooling efficiency.

At 120,000 miles, maybe it's time for some preventative maintenance anyway. The water pump is probably on borrowed time. I'd be inclind to replace it along with the radiator. It's easy spending someone else's money.

Brett

[SimonClaringbold]

1995S500W140, I'd agree with Brett on this one. We've both had water pumps seize with NO loss of coolant.
My car had run MB coolant for it's 11 years - there were absolutely NO signs on any corrosion on the water pump impeller or housing.

If you can do without the car for a while you should be able to find a specialist radiator shop that can replace the core of your radiator for cheaper than the cost of a new radiator. Also look around on the internet for a replacement waterpump. Sometimes you can find them for 1/4 of the dealer price.

As Miloking says check Jim Forgione's page:
K6JRF Auto Page

His 'Cool Harness' might be worth a try.

[1995S500W140]

MBIOTCH,Brett,Simon -

Thank you for the information. I have always used MB factory coolant at a 50/50 mix but not always with distilled water, usually tap water. The explanation of a clogged radiator makes sense in terms of the fact that my heat transfer is not as efficient as before. I will take the plunge and replace both the waterpump and radiator.

I am surprised that using MB coolant can result in clogged radiator even with regular flushing but it is 12 years old and does sit a lot.

Curious as to what miles/years of service a radiator would require replacement? If you have any stats please let me know.

I will update on what happens after the replacement of these components.

Thank you.

Hynek

[Mercedes Mechanix]

Originally Posted by 1995S500W140

MBIOTCH,Brett,Simon -

Thank you for the information. I have always used MB factory coolant at a 50/50 mix but not always with distilled water, usually tap water. The explanation of a clogged radiator makes sense in terms of the fact that my heat transfer is not as efficient as before. I will take the plunge and replace both the waterpump and radiator.

I am surprised that using MB coolant can result in clogged radiator even with regular flushing but it is 12 years old and does sit a lot.

Curious as to what miles/years of service a radiator would require replacement? If you have any stats please let me know.

I will update on what happens after the replacement of these components.

Thank you.

Hynek

If I could chime in, I was hesitant being new to BenzWorld... but here goes. I would suggest rechecking the Temperature Sensor with a digital ohm-meter:
COOLANT TEMPERATURE SENSOR (B13) RESISTANCE

At 60 °C 110 Ohm
At 80 °C 67 Ohm
At 100 °C 38 Ohm
At 120 °C 20 Ohm

then take a look at attached pictures of a Mercedes water pump impellar (cast) and look at the outer circumferance. PLEASE REMEMBER, the lower exterior cover openings are much smaller and you will NOT be removing the Timing chain cover, etc!This picture depicts behind the water-pump and within the timing-chain cover. The lower left radiator hose to upper engine block connection is above the timing idler sprocket and the two openings beneath the sprocket are for the water pumps lower rear housing, etc. (And of course the upper radiator hose and Thermostat are on the right exterior of water pump, etc) I have not seen an impellar fail as you mentioned, yet I have seen blades chip and fill with debris and many failures with shaft/seal bearing issues. Might I suggest removing upper and lower radiator hoses and recheck radiator flow and then if satisfied, think about removing the water pump and put a water hose into your upper engine hose block connection and verify un-obstructed flow thru your block. Also, put that thermostat into warm water and reverify it is opening and staying open, I like to see things for myself. I am sure you will have put a real gut eye on your water pump by this time, make sure to wiggle that shaft good too! (by the way.... the weep hole es just that..... a drain for water to evacuate if seal burps, shaft bearing starts having problems and causes seal to start leaking: it is pending shaft bearing failure, but will not verify if the impellar is fully operational. Be systamatic, set yourself a plan to verify that water flow is staying open during noemal engine temperature and that you are seeing valid temperature data as well. I hope this is of assistance.... regards, Dutch

[AlexanderZwo]

If you shouldn't have an Ohm-meter: Like Brett said before, check sensor itself: Press and hold REST on AC until display changes. Scroll with AUTO-buttons to value 6 (if I remember well) and you see coolant temperature measured by a different sensor mounted next to the one taking the information for the cluster gauge. But should be high as fans are coming up, which is due to the same sensor.

Curious to know what the final result is.

Best regards
Hubert