'89 525, check engine light when heat is on

[delussional77]

Girlfriend drives an '89 525 (think it's an i ). Anyway, now that's in the dead of winter at good ol' purdue she constantly has the heater and defroster on all the time. At random points throughout turning the car on and driving the check engine light will come on and go off. When you push the sliders on the heat all the way over it will go away, like wise when the defroster is turned off the same will happen (it will go away), and sometimes when just the heat level is turned from 4 to 1 (or off) it will go off as well. I'm kinda perplexed by this problem. Thank god the M5 does not do this. Any ideas for troubleshooting or straight fixing the problem? The engine has about 120,000 miles on it (not bad for a 15 year old bimmer!!!)

 

[Eb39]

engine light issue

Check the thermostat. If it is bad, the engine light may come on. Maybe if it is stuck in an odd position, a certain heater setting warms or cools the engine to a point of triggering the light.

 

[epj3]

I agree with Eb39, but I would try adding some antifreeze. If it is too low, when the heater is off it might be high enough, but if the heater is on and the heater core is allowed to draw in coolant, the temp sensor might not be reading anything.