Water pump -- does this make sense?

I have a 1984 318i and I was stuck in traffic recently and noticed that the temperature was going up. I figured that if I ran the heater it might help cool down the engine. But when I turned it on, it blew
cold air.
Once out of traffic and back up to speed, and the car cooled down almost immediately. And then later I tried the heater again and it worked.
A mechanic said that when the water pump starts to go, it will still work at speed, but won't pump at idle. I would think that a water pump would just go, all or nothing.
Also, I did notice that the coolant was down, and have added half a jug. I haven't tried to test this in anyway since then.
Is there some test I should do?
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Don't know if it affected your model, but the plastic impellers were a problem on some.
Al.
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Okay, so you know hot water wasn't getting to the heater core. And you know it's not a permanent plugging.
Either you have a bad water pump, or air in the system. Try and bleed air out. Almost certainly you'll find there isn't any in there, but it will only take a minute to try.

No, this is one of the common failure modes. I believe you are fortunate not to have a car with one of the notorious BMW water pump issues, though.

Find out why the coolant was down. Maybe it was down from a bad cap or a small leak, and air bubbles got in when it got low. Maybe it was down because it is leaking out the the water pump.

Fill it up, bleed the system. Buy a spare water pump and carry it around because if you don't need it now, you'll need it in the future. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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I suspect you have not been checking coolant level regularly and there was just not enough in to fill the block, radiator and heater core.
Fill the radiator up properly, run the engine briefly (to fill up empty bits) and top it again.
Check the level [at least; esp if no sensor] weekly as you may have a leak.
If you have an auxiliary electric fan check that it is working.
Don't rev the engine or run the AC when stuck in traffic, until you diagnose this completely, but my guess is that the main reason is just low coolant.
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R. Mark Clayton wrote:

Coolant isn't something that should need topping up. A leak free system with an overflow tank shouldn't EVER need topping up.

I'd suggest he take it to a mechanic and have it looked at before he warps the head on his 318... which is known for a "profile gasket" problem, which can result in loosing ALL the coolant very suddenly.
There are people who shouldn't be mucking about under the hood relying on internet advice.
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admin wrote:

of engineering with no profile gasket problem.
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Ok, just to clarify.
There's no overflow tank for the coolant on this car. I added about 1qt / 1 litre of coolant, not the 1/2 jug I originally stated, so I don't know if being down that much would be enough to make a difference.
It seems like a good idea to get myself a water pump as they don't cost much. But it'd be good if there was an effective way to test it. I suppose I could sit with the car idling for a while.
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wrote:

Ford V8 which would run out of coolant while under load but refused to act up at the shop. We finally discovered a very small crack in one of the heads (but it took almost 3 months to find this item).
I would get the coolant system pressure tested. It might be that such a test would reveal a small leak somewhere.
Jim
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wrote:

Hell of a difference. How far since you last checked it?
Have you checked the oil?

If it ain't broke; don't fix it.
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Had a similar problem with my old E38 V8 - twas the plastic thermostat housing cracked. No problem when running but only leaked when cooling down - a small jet of water dipping on the fan belt which was quickly dispersed so really no trace until I shone a torch down there at night and saw the water glinting.
Cheap to fix.
No idea if your car has similar but it worth a look.
Hugh
--

Sir Hugh of Bognor

The difference between men and boys is the price of their toys.
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