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I've been having the annoying issue where my 1986 528e doesn't want to reach operating temp, which means my heater doesn't work. Of course I discover this right when winter starts. The symptoms are the temp gauge doesn't reach the middle, and fluctuates a lot between about 1/5 position and 1/3 position depending on how fast I'm going, and the heater core doesn't warm up at all. According to the Bentley, this could be the thermostat, the auxiliary fan, or the fan clutch. I tested the thermostat in boiling water, and it opened at ~176 degrees as per design, and the I let the water cool and it closed at about 150 degrees. The auxiliary fan works as intended, and turns on with the A/C switch or when the low speed or high speed switch are closed, and the main fan spins freely with slight resistance with no signs of binding or leaking.

Seems like the most common issue for the car not reaching operating temp is the thermo, and the temperature fluctuation seems like another obvious symptom, but the thermo seemed to work fine in the boiling water, and never got stuck! I replaced the O-ring too, in case that was leaking, but that didn't work. Highly frustrating. I'm fixing to purchase a new thermo Anyway seeing as they aren't that expensive, but maybe there's another issue someone else knows of? I use green coolant (for now) instead of the blue stuff.... maybe that? Doesn't seem like it according to my research... Any help is appreciated!

This is also my first post, hopefully in the right spot.

It seems to me to be a problem with the heater water valve, a common problem in E28s. It is the valve next to the brake booster, easy to remove and open and eventually you will discover that the internal membrane is broken. This causes the stem to rust and stick.

That the temp gauge indicates 1/5 or 1/3 is normal in these cars (mine indicates the same), specially in winter time.

See this: https://www.mye28.com/viewtopic.php?t=150387

First off, welcome to the group, I see this is your first post!

You may have a couple of different issues going on. Thermostats tend to get weak with age, not always, but often, so the stat is suspect. The gauges aren't dead solid in the middle as many new cars, especially BMWs are, but depending on your location, and I don't know if it's LA or northern Minn, but in less than severe weather I've found them to be pretty much in the same place including in winter at just a little under the midpoint.

And as Aldo mentioned the heater valves themselves are something of a problem, particularly as the cars are now around 40.

Thanks for the inputs!

I've read much about the unreliable heater valves, and the poor quality rebuild kits. I've pulled mine out, and the rubber seal looking thing (I'm assuming this is the diaphragm) looks entirely intact. The entire assembly however was utterly disgusting, covered in dirt. The inside of the hoses also looks very dirty, so seems like the next course is to flush the coolant system. I've also tested the valve, and it works as intended. I can hear it operating when I turn the cabin temp knob to cold, and it opens and closes periodically as I move the knob towards warm.

Turns out it was a heater valve problem, the little circle clip that holds the plunger and diaphragm together was gone. Small detail that I only noticed because of the suspicious notch on the plunger, and I looked at a rebuild kit and saw they included circle clips. Used a small brass nut I found and some jb weld to secure the two parts together, and now the heater works great. Thanks for the help!

MacGyver to the rescue.

chaymac wrote: Dec 13, 2024 5:02 PM my heater doesn't work. Of course I discover this right when winter starts.

Check back at the start of summer when your a/c doesn't work.

welcome to mye28, enjoy your stay

Blue Shadow wrote: Dec 16, 2024 4:49 PM

chaymac wrote: Dec 13, 2024 5:02 PM my heater doesn't work. Of course I discover this right when winter starts.

Check back at the start of summer when your a/c doesn't work.

welcome to mye28, enjoy your stay

Yep, that's about how it goes. Although my a/c has gotten me through two hot florida summers so lord willing it'll pull through

Wasn't very confident in my jb weld job... I usually don't like using jb weld for a "permanent fix" on moving parts so I went ahead and deleted the diaphragm and sealed the top part of the valve with an o ring.