Thanks Mark, this explanation fits with all the facts of my overheating car. I think I will have the mechanic replace the water pump, and then if necessary, replace the radiator.
Chuck
84300sd
--- In diesel_mercedes@yahoogroups.com, "Mark in Lakewood, CO" <beeser750@...> wrote:
>
> Hmm.... Just because a water pump is turning and not leaking doesn't necessarily mean it's good. The first diesel I ever drove was a '74 240D that belonged to a friend of mine in Arcardia, CA. When I came out to visit him, and asked him why he wasn't driving the 'Benz, he said it kept running too hot. He'd replaced the thermostat, had the radiator rodded out, and flushed the block. Still, it ran hot. I noticed a small leak coming from the water pump area, though not from the weep hole, so, I removed the water pump, and found that the aluminum pump housing had so much internal corrosion, that the machined part that the pump impeller rides up against (not quite touching, but, almost) was completely gone. The pump wasn 't pushing the coolant around, but merely stirring it in the housing. In fact, the corrosion had made the housing wall so thin there was a pinhole leak. Pump itself was fine, but we replaced the housing, and the hot running was gone.
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> (This was a Southern California car, and the corrosion was the result of too many years of previous owners not using any sort of anti-freeze/anti-corrosion in the cooling system. Just water.... I've since encountered former SoCal cars in which the pump impeller was completely rusted away, too!)
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> As to the thermostats: It must be pointed out that Mercedes thermostats are double-acting; that is, not only does the thermostat allow and block coolant flow through the radiator, but it also blocks/opens flow through the by-pass hose at the bottom of the thermostat housing. Looking at the thermostat one will notice a lower disc BELOW and mounted on the same shaft as the main part of the assembly. When the thermostat opens, this lower disc closes off the bypass circuit. Running a Mercedes engine without a thermostat will have the coolant directionless, not knowing whether it should go through the radiator, or the bypass. So, taking the pass of least resistance, it will go through the bypass hose. Running a Mercedes engine without a thermostat creates a condition where it is just as likely to over heat as running the engine with a thermostat that is stuck closed.
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> Mark in Lakewood, CO
> ----- Original Message -----
> From: "n61cm" <jim@...>
> To: "diesel mercedes" <diesel_mercedes@yahoogroups.com>
> Sent: Sunday, July 8, 2012 8:27:59 AM
> Subject: [diesel_mercedes] Re: water pump w126
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> I'd say as a general rule, you leave the water pump alone until it starts leaking. If it has a belt connected to it and it's turning and not leaking, it's pumping. If you've taken the thermostat out and you're still overheating, you probably need a new radiator. Heating and cooling a radiator bends the vanes where they are attached to the tubes over and over like bending a coat hanger back and forth...eventually they break loose, and that leads to loss of cooling power. They can also get clogged up inside. If you have a radiator shop nearby they might be able to "boil it out" and repair it for you, but it's hard for them to compete against overseas labor, so you might find that a new radiator is your best option.
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> While you have it off, you might want to clean out the cooling channels inside the block.
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> Could be something else, but that's my guess.
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> Jim
>
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