Hi! OK - a little bummed, here. It was just Halloween (my birthday), and I get stuck with the greatest treat of all - car trouble.
So, the problem was confirmed yesterday... it actually began last week. My '95 525i and I are driving on a rural road, going about 65, and my cluster chimes at me that coolant temperature is high. I'm usually pretty good about looking down at my gauges every minute or so, so this happened quickly. As soon as I saw that my temp needle was near red I killed the engine and coasted off on a side road.
My first impression was that I must have busted a radiator hose (upper/lower) or sprung a leak somewhere. Not the case. As I examined the car everything was orderly, and no leaks anywhere.
I then thought it may be a stuck thermostat, but upon feeling the upper radiator hose, it was HOT. So I'm thinking that isn't it, unless it opened up and allowed the coolant to pass (or is stuck partially closed). I had the car sit for about 6-7 minutes, then turned the key to pre-ignition. The temperature had returned to normal! Fast, huh? That's what I thought.
So I started the car back up, ran smooth as usual, and for the remaining 80 miles I drove that day (highway and city) it was perfect.
This made me think that perhaps the coolant system was not bled properly... I followed a DIY on this (cold engine, set heat to high, run for 30 seconds, lower heat, make sure it's topped off - then run up to normal operating temp, elevate front wheels, use bleed screw to release air until bubbles stop and liquid comes out). I didn't see a whole lot of bubbles, so I'm assuming that when I had the shop replace the thermostat a couple of months ago, they did it correctly (plus I probably would have had problems before now, especially when the weather was warmer).
OK - so I thought this may have been a sensor anomaly, so I drove it two days ago. A-OK until the highway, then it spiked to halfway between normal and hot. Lowered my speed, and it was good (temp gauge back to normal). And in the city/idling conditions, coolant temperature holds perfect - not hot, gauge right in the middle where it should be.
Re-bled yesterday, just in case I missed something. Drove it yesterday afternoon. Same thing - works perfectly driving here and there around town. And during a 25 minute idle while my wife waited in the car with our baby during my haircut (it was cold out, and he was sleeping), no elevation in temperature at all, as she checked it every minute or so. But once on the highway to visit my parents, the temp needle (sure enough) starts to climb. It reached about 3/4 way to the red, and I slowed and we went back home (switched cars to our uber-reliable '95 MB C280... starting to wish we had two of those).
ANYBODY - any ideas? I've heard that if it indeed has the original water pump, the impeller may be suspect, as they were plastic (all the replacements have a metal impeller). Or does it sound like this might still be the thermostat, stuck closed (or at least partially closed)?
AND Ive read that when the auxiliary coolant pump (electric motor that helps circulate the coolant for the heating system) starts to go, it can keep up with city driving, but can't keep up during highway jaunts. BUT, I don't think this is the issue, as we have heat (at least on the passenger side - driver's side tends to be a bit cool with dual climate control). I would think that if this were the case there would be little/no heat.
SO - let me know if you BMW guys/ladies have any thoughts or similar experiences. I REALLY appreciate it!!!