Cooling system questions - heater core & water pump

[juliea]

A few questions on the cooling system for '91 318is (attempting to troubleshoot an overheating problem):

1.) does the coolant flow from the engine to the heater core? (i.e. if there thermostat were stuck shut so that no coolant is flowing into the engine, would it also mean that I'd have no heat?) would seem to make sense - coolant goes through engine, gets hot, then goes into heater core when heat is turned on...

2.) how do I troubleshoot a waterpump? if car is overheating but coolant level seems to be stable, would seem to indicate either a problem with the thermostat or waterpump - is there any way to isolate & tell whether the problem is with one vs. the other?

For reference for those who are curious, I bled the system & there were no air bubbles.

[ti_tony]

There's a nipple on the driver-side rear of the head about 19mm that feeds the heater core.
After the car is started and is coming up to temp., reach down and feel the bottom hose on the passenger side, it should be getting warm/hot. If cold your thermostat is bad. Be careful of the fan! One of the best things I did was replace the thermo-housing bolts with studs, blue lock-tight on the studs. If the bottom hose stays cold it could be stopped-up radiator but not likely.

[inarticulate is]

I've replaced the water pump twice, both times the radiator fan got wobbly, check it for smooth operation.

[DesktopDave]

There's also one other component...BMW added it to e30's to prevent the heater core from popping under high pressure.  It's a small bypass located in the input hose from the head to the core.  The bypass connects to the output hose, thus preventing hot coolant from over-pressurizing the heater core.

I'm not sure if it's a pressure regulator, thermostat or both.  If it's bad, the heater core won't work but the car will otherwise function normally.

Did you run a pressure check on the system?  The caps do eventually fail, and IIRC they run at 2 bar, quite a bit higher than the more common 15psi caps on many other cars.  If that cap isn't maintaining specified pressure, the car will overheat.

Also be extremely careful when bleeding the radiator...that little plastic bleeder will snap in two if you look at it the wrong way.  I carry a few spares in my trunk toolkit, just in case.

[jrobie79]

I bought the brass bleed screw from bavauto...greatest 6 bux I've ever spent...no more bungling up the screw head for this guy