BMW E36 overheating weirdness

I drive a '97 328 that recently had a problem with the temperature gauge hit the red zone in traffic. The temperature didn't stay there long, though; I pulled into a parking lot to shut it off, but found the temperature would drop when it got moving, so I managed to limp it home.

Found out that sure enough, the auxiliary electric fan was dead. It wouldn't do anything when I fed it 12 volts by jumpering the fan relay. So I replaced that, and the new fan does switch on when I give it power. Should be an easy fix, right?

But I've found now that if I turn the engine off for a few minutes while hot and then have the car idle without moving for about two minutes or so, the fuel gauge can start climbing again. Goes back down shortly after the car starts moving. It's not losing coolant like I'd expect if it had a blown head gasket.

Any ideas on what to check? Could there be an air bubble in the system caused by that first overheating? The new fan not pulling enough air? Or what?

I would think the radiator is the likely suspect. Also isn't these one of the cars with the plastic water pump impeller that disintegrated? Might be a combination of problems.

Sticking thermostat? Water pump on its last legs? Is your clutch fan working?

FWIW, every BMW I buy gets an EMP Stewart racing water pump immediately after purchase. Yeah, it's a $180 water pump, but I see it as cheap insurance; the lifetime warranty means you never have to worry about it again, and the ~20% flow improvement takes up a lot of slack if something else in the system fails.

Do you still have the mechanical fan? How is the clutch on it? The mechanical fan is for engine cooling, the aux fan is for ac (simplified, but in general).

A quick tip for bleeding the cooling system too is to put the front up on blocks to get the rad and expansion tank as high as possible.

Yes, it's still got the mechanical fan. I checked the clutch by seeing if it freewheels with the engine hot - it doesn't, just spins a few degrees and stops.

A thermostat seems unlikely - I can't get the temperature to spike while moving, and these usually fail open.

Radiator is a Spectra Premium that I put in nearly 100,000 miles ago. If it were a stock unit, I'd be worried it might be a time bomb - no idea of Spectra copied BMW's mistake or not, but it doesn't look like it.

Is there a good quick water pump test?

MadScientistMatt wrote: Is there a good quick water pump test?

you can let the car cool completely, take the expansion tank cap off, start and rev the engine and look for coolant circulating. You can also probably feel it flow through the radiator hoses (once it heats up and the thermostat opens). Dunno if there is a test to tell if it is circulating enough volume though.

Reading your sitch, i'd try a re-bleed first, then look to the water pump. You may have boiled the fluid in a spot or two when it got hot and now you have air in the system. Seems plausible at least, and a bleed is easy and quick.

+1 on the introducing an air bubble from boiling during the overheat scenario. Sounds the most plausible to me.

Also, in regards to the BMW exploding radiators and expansion tanks: if you have a plastic radiator and/or tank, it is no better or worse than the original. There is no "mistake" in the design, just a side effect of the fact that BMW runs an extremely high cooling system pressure. Lots of guys swap to an E30 pressure cap to lower the pressure from 2 bar to 1.5 bar for less stress on the components, then run Water Wetter and the Stewart pump to pick up the slack. The only true fix is an aluminum radiator, expansion tank and thermostat conversion.

+2 on air in the system. Helps to have the nose on an incline when bleeding/filling

The E36 expansion tank is a bit of an oddity - I'm not sure it circulates coolant through it, and there doesn't seem to be a way to look at the end tanks. I'll try the air bleed first; thanks for the suggestion to point the nose up.

Full cold or full hot, stick a rolled up magazine against the fan blades and see if it stops. If it shreds the magazine, the fan clutch is good. If not, break out your 32mm spanner and replace that thing. Common problem. Then, go ahead and replace the rest of the cooling system.

I ran without the mechanical fan for a long time - years. Never overheated.

not sure on the regular e36s. but the 318ti's radiator/expansion tank, if you removed the cap cold and turned on the engine, you would see a fine stream of coolant from the top of the radiator down into tank

Thanks to all who responded! I put the front end on a lift last night, opened the air bleed valve, and warmed up the engine. It did end up "burping" out a few air bubbles, and I haven't been able to make the overheating problem re-occur yet.

pointofdeparture wrote: Sticking thermostat? Water pump on its last legs? Is your clutch fan working? FWIW, every BMW I buy gets an EMP Stewart racing water pump immediately after purchase. Yeah, it's a $180 water pump, but I see it as cheap insurance; the lifetime warranty means you never have to worry about it again, and the ~20% flow improvement takes up a lot of slack if something else in the system fails.

+100 on the Stewart pumps. My OBDII e36 M3 got a Stewart pump, aluminum upper flange/elbow, fresh hoses and a Zionsville AL race radiator + expansion tank + SPAL 16" puller fan w/ shrouding and 2-speed sender.

The temp sits at 180 after coming off track on a 105 degree, humid day or stuck in traffic w/ the AC on on a 10+ degree day.