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Cooling System Dilemma - '92 900i

Advice needed...I have been having overheating problems. This part:

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which sits right over the alternator, started leaking. I pulled it out, and it seems there was some kind of passive valve in there that ain't there any more! I assume it blew forward, in the direction of the straight hole. This goes into a metal pipe that takes you to to the back of the engine (front of the car) and goes into the motor. I checked the pipe, it is clear. Is it stuck somewhere in the block, or did it blow through all the way to the radiator? The radiator seems to warm evenly, so maybe that is a good sign - maybe it is right at the entrance. Or maybe I have it all wrong. I don't have any idea which in direction the water flows.

The car is cool as long as I don't take it on the highway. That makes me think there is a partial blockage somewhere, but enough water gets through for low speed driving.

Where should I look? Also, I sealed up the part in question with blue RTV and plan to drive it a few days until I can replace it. It the missing valve the whole problem, as it prevents proper flow? If so, what about the lost valve? It had to go somewhere!

All suggestions appreciated!

Robobass

Reply to
robanzellotti
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Hi,

The part you're talking about is the heater bypass valve, which is only present on 1992 and up 900s. It's supposed to improve flow to the heater core when the car is at an idle, though I'm not exactly sure how it works.

As I understand it, coolant flows from the cylinder head through the elbow hose, then through the metal pipe, and ultimately into the heater core. So it seems unlikely that any part of this valve could have ended up anywhere near the radiator.

Were you having the overheating problems before this valve started leaking? It could be that the leaking valve is unrelated to the overheating, or that the leaking valve is preventing the system from maintaining pressure.

I suggest you get yourself a new bypass valve (which you'll need anyway) and compare it to your failed valve to figure out what parts may have escaped. Also, this is probably a good time to drain/flush your cooling system, replace the top hose (thermostat housing to radiator), check the function of the radiator cooling fans, and replace the thermostat. Depending on the milage, you might also consider preemptively replacing the radiator. Doing all these things should fix just about any cooling system problem, I think.

John

Reply to
John B

The system is supposed to be under pressure when the engine is hot. Excessive pressure is released by exploding hoses :) Running only water in the system could conceivably have caused the leak, as it boils at a lower temperature than coolant and could have caused excess pressure to escape through the bypass valve. So I think you should probably drain the water, put in a new thermostat (and maybe radiator), replace the failed bypass valve, and fill up with properly mixed coolant. With a new thermostat and radiator, I think overheating is unlikely to happen.

John

Reply to
John B

Thanks John, I just put in a new water pump last week. The car seemed okay until then, except that the old pump was leaking like crazy. For the last few weeks I had only water in the system. Pretty much everything except the radiator is relatively new now. The radiator is pretty corroded, but it does heat evenly. The fan clicks on like it should. The bypass valve only started leaking after I changed the water pump. Could it be that I somehow have too much pressure in the system, and that caused the valve to fail? How is excessive pressure released? As I said, I only am overheating at highway speeds.

Robobass

John B wrote:

Reply to
robanzellotti

Reply to
robanzellotti

Well, once you have the system drained, the radiator job isn't hard at all.

John

Reply to
John B

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