Bought both of these cars a couple years ago with high miles (180 &200k). Both immediately got new radiators, water pumps, and tstats, but still have the stock "vacuum hose nightmare under the manifold". On the red 91 318is, the temp needle would occasionally go to the red--symptoms similar to when my e32 735 had a blown head gasket. I swapped water pumps and tstats, and bled it many times, etc, etc., and finally had the engine rebuilt last year. It was OK on the temp for a while, but then the needle started creeping to the red, mainly on hot days this past summer. After horsing around with bleeding it for a week, again swapping water pumps and tstats, and eventually just trying to get the lower rad hose hot, I tried an experiment. I removed the upper radiator hose and substituted a piece of clear vinyl tubing in its place. I could see bubbles flowing. With the vinyl hose attached to the engine and disconnected from the radiator, and the disconnected end pointing up toward the sky, I poured coolant in. Then I pumped the lower radiator hose by squeezing it repeatedly. LOTS of bubbles came out of the upper hose, and then the coolant level dropped and I was able to pour another HALF GALLON of water into the vinyl hose. I reconnected the hose to the radiator, bled it, and it ran perfectly at the middle (12 O'clock) mark. Figuring the vinyl would fail, I re-installed the rubber upper radiator hose and bled it. In the following weeks the temp needle was mostly steady in the middle, but would occasionally creep toward the red. One day I gave the dash a smack, and VOILA! the needle jumped back to the middle.
Lessons: The temp gauge (or cluster circuitry) is definitely flaky (I've tightened the ground nut twice to no avail). Air bubbles keep the water pump from doing its job, and I had gotten a serious air bubble or pocket in the system when I swapped the water pump. And I might have also had a few bubbles left over from the engine rebuild--who knows? Maybe I need to let the coolant flow out of the bleed hole for a much longer time (I need a bigger drain pan and lots of 50/50 coolant on hand). And last, its hard to say if I wasted money on the engine rebuild.
Questions: I like the concept of rigging a tube to bleed coolant directly back into the reservoir--has anyone solved that puzzle? (iamcreepingdeath?) And why doesn't the little hole that spurts coolant into the top of the overflow reservoir serve to bleed the coolant? What is its purpose?
New problem: Fast forward 5 months. Last week, with cold weather forecast, I replaced the weak, dilute summer coolant with new bmw blue 50/50. It was a hurried job and maybe I didn’t bleed it enough. It is now in the teens and 20s outside and although the temp needle stays mostly in the middle, the heater doesn't stay warm--air coming out of the vents fluctuates from 50 to 100F, seemingly at random. However, the black 318is, which generally is slow to warm up but hasn't tried to overheat for over a year, barely got its temp gauge needle out of the blue last night on a three-mile drive, and yesterday its heater also blew air that fluctuated from cold to warm, all day long (100 miles), just like the red car was doing.
More questions: Maybe it is time for the old cardboard-in-front-of-the-radiator trick? Could that strange bypass valve many of us have between the heater hoses be part of the problem? Does removing the hose nightmare expedite bleeding? Will I ever learn how to bleed these engines?
