Help with overheating

[bignslow]

Looking for assistance with overheating with my 91' 318is. A little back story. I got a car a few years ago for dirt cheap as a non runner. It needed a fuel pump to get going. I did a complete overhaul of the whole car. All engine gaskets are new, thermostat is new. When I started driving it I noticed the car looked like it was overheating. I drive 50 km to work every day and on the highway the gauge would creep into red. The faster I drove the hotter it got. I would pull over and pop the hood and it didn't feel very hot. No steam or anything coming from the engine bay so I assumed the gauge was faulty. What I also noticed is when I would get off the highway into the city the temperature gauge would begin to drop. This seems very strange to me. I would think it would run cooler on the highway than it would in the city. I got through the summer driving the car on and off during the nicer days. I don't drive the car very often as it's more of a toy than anything.

This winter I decided to ditch the mechanical fan and do E36 electric fan conversion. I got the fan and E36 fan switch installed. While doing some tests I got the car to normal temp idling in the garage and gauge went into the red but electric fan never kicked on. If I turn the ac on low speed fan will activate as it should. Jumping the pins on the temp switch connector also activated both fans as it should so it seems like the temp in the rad doesn't get hot enough to activate the fan.

I replaced the thermostat yet again with 80 degree unit. Boiled both old and new in hot water and they both open as they should. Pulled out the water pump to inspect - there is no damage and it spins as it should.

I thought maybe the rad is clogged and it's not able to circulate the coolant properly so I pulled it out and filled with Evaporust for a week. Some crud came out. Put the rad back in and filled with coolant, bled the system of air and same thing is happening. Electric fan won't turn on.

I am scratching my head as I can't figure it out. Any thoughts?

[DesktopDave]

I've had trouble with the aux fanstat wiring connector, but you've already checked that out. I've also had problems with both the aux fan wiring in the fuse box. My thought is that the contacts overheat and slightly melt the supporting plastic, then you get a weak connection inside the fuse box. IIRC low speed is relay K1/fuse 3, high speed is relay K6/fuse 18. Wiggle the fuses, jump the relay socket to see if the wiring is OK.

I'd also double-check the thermostat bleed passage. The M42 doesn't have a bleeder hole in the thermostat, it's cast into the housing. Some have cut the paper gasket in order to open up the bleeder passage, others have drilled a small hole to eliminate that air pocket.

Is your water pump the newer model? The early plastic bladed unit impeller could fracture.

I'd also strongly recommend a pressure test. I had some similar problems on my E30, and it ended up being multiple pressure leaks. I found leaks in the heater core, and o-ring seals in the expansion tank. Oddly, the infamous cracking expansion tank was in excellent shape. Once I got all that done, the car held temperature perfectly. I only ran the engine-driven fan in the summer, removed it for the other three seasons. The car really didn't need a fan at all.

I've also found that mixing a little more water into the coolant really helped with bleeding. I run a 66:33 ratio of water to coolant with a splash of surfactant (RedLine WaterWetter or the like).