I recently had a leaking waterpump replaced. The car immediately overheated as I drove away from the shop, nearly into the red where the temp seemed to sabilize. My mechanic thinks(maybe) the headgasket blew but there is no sign of this he can find. I had driven the car with a low pressure radiator cap for two days before the waterpump was replaced during which time it ran at normal temp so I think replacing the waterpump is the cause of the overheating? He took the new pump out to check it but the problem is exactly the same. He also says the new pump is exactly like the original. Any ideas?

Was it ever running hot prior to the water pump replacement or was the pump replaced only because it was dripping from the weep hole?

If it wasn't running hot before the replacement then what changed? Did enough 50/50 (or 60/40) mix get put back in? The 4.5 should take about 3 1/2 gallons of H2O/coolant mix. It is possible you have air trapped in the water jacket somewhere, has your mechanic (or you) tried burping the system?

Can you see the coolant moving in the upper tank when the t-stat opens?

If the head gasket is blown there should be some sign - water vapor out exhuast pipe, oil in the water, water in the oil and if none of those are present there will still be hydrocarbons in the coolant (there is a sniffer to detect).

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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

This might sound stupid, but can the thermostat be upside down?

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Darryl, Hill
2005 SL55 AMG Kleemanized
1984 500 SEC
1967 W113 California Coupe
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This sounds to me like air trapped in the system...

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Bill Streep
San Antonio
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)

Is the car actually overheating, or is the guage just going into the red? From your original post, it did not sound like it ran long enough to overheat.

Chuck

As a matter of fact I cannot see the coolant move when the thermostat opens. Is this an obvious effect? As I look into the radiator filler the coolant just sits there. I have tried burping the system by parking the car on a steep incline and adding fluid as needed. Is there another way to burp it?

I don't know about your particular model. Many cars this can be done by simply letting the engine come up to temperature and add coolant as it drops. Other engines may require you to remove a hose in order to purge the system of air.

You should see the coolant "move" when the thermostat opens...

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Bill Streep
San Antonio
'57 190SL (toy)
'08 S5500 (mine)
'09 CLK550 (wife's)
'06 SLK350 (daughter's)
'11 GLK350 (daughter's)
'03 CLK310 (spare)

First of all, thanks guys for all the advice. Just one more question. Is it possible to get a "bad" waterpump from an aftermarket supplier that could make such a profound difference to the flow of coolant? The reason I ask is because it's the only thing that was changed when the overheating began. Before the waterpump was changed the car did not overheat eventhough the original pump was leaking thru the weep hole.

Was the thermostat also replaced? I think that some mercedes models have an air bleed hole in the thermostat which needs to be oriented with the hole to the top so that any air trapped in the system will escape. Others have a bleed screw on the thermostat housing .... just two more things to check out.

I had a 250S which weeped at the water pump. I replaced the pump and the car ran about 10 degrees hotter than before. I put a second pump in there and got the same thing. The only conclusion I came to was that the original pump were either made better or the fit on install was better.

Here's the trick:

The 4.5 is notorious for getting air pockets if the cooling system is opened. So, what you do when refilling the coolant is to disconnect the upper radiator hose from the radiator and pour the 50/50 mix through the hose. Try to hold the hose as high as possible (you'll hear it burping) and gently rock the car to "shake" it a little to further purge it. Just keep adding coolant until it flows back out of the upper hose. Reattach hose and top up, start car and put the defrost on. You'll probably see the auxiliary fan kick on if there is still an air pocket. Just keep an eye on the temperature gauge and feel the air coming out of the vents to make sure the t-stat has opened. One other note, the older sixes are also known for air pockets, the thing to do with them is to fill the coolant in, run the car and if and when the temp gauge begins to rise above the normal mark, shut it off and let it bleed itself for 15-20 minutes. Check the level in the radiator and top up if necessary. Start it back up and make sure the thermostat opens. Repeat again if still no heat.

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Regards,
Aaron