Thermostat Modification to help in Bleeding

[ClayW]

I saw this modification on a blog for an M20. The idea is to drill a 1/8" hole in the thermostat to help in bleeding the air out of the system. What do you guys think?

[monty23psk]

People with m42s do it also. I dont as BMW didnt plan it that way, my opinion. Also if you look at the thermostat housing, there is a little slot that does not touch the paper gasket. I people this serves the same purpose but it gets gunked up if too much sealant is used with the paper gasket. The m42 is a pain to bleed, not enough coolant and the radiator itself has all those passages ways. If you do it right, and a few times, bleeding an m42 is easy. The key from doing it so many times, is make sure the engine is dead cold when you do a bleet. I sometimes, just walk away and  come back the next day to do it.

[DesktopDave]

It'd work, but like Alex pointed out there is a passage there already.  The M20 also has a bleeder right in the thermostat housing to make life easier.

If BMW had designed it right the system would practically bleed itself.  I'd understand if the e30 M42 had this problem (being a low-cost stopgap until the e36's came out), but the problem wasn't addressed for whatever reason.

I'll bet BMWs with the expansion tank on the firewall (like the e34) are far easier to bleed.  Putting that at the high point of the system is good practice, unfortunately the M42 is too close to the firewall.

[ClayW]

Hmmm......I noticed that passage in the M42 thermostat housing when installing the new thermostat. I'm now certain that it's clogged with sealant. Will that make it impossible to bleed?

Here's my plan (tell me if I'm flawed): The car is on an incline in the hotel parking lot (I'm 40 miles from home).The engine is dead cold and I have been checking an filling the coolant to the top with the bleeder valve open every ten minutes or so. The level is staying at the top now.

So now I should close the bleeder valve and use my turkey baster to suck coolant out of the overflow tank until the level is at the "cold" level, then start it, right? Anything else?

[monty23psk]

Your plan seems ok. Everybody always has different ways to bleed but one way, which I havent tried yet, from the e36 m42 Bentley, is remove bleeder screw, and then fill up the main tank until you have coolant coming out of the bleeder screw.

I think you are ok, then you can turn the car on and bleed it with the engine on to make sure you get coolant past the thermostat and into the other side of the radiator.

[cristimm]

The hole in the thermostat is always a good idea. The hole can be smaller than in the picture above.

[bmwman91]

I think that the main problem people run into is having the thermostat passage plugged up. It makes it very hard to bleed since the thermostat has to be open to get air through. I would say that the "right" way to bleed the system is to pull the thermostat cover & clear the passage. I always cut the gasket too. There is no reason to have any paper over the gap (they do it so that the gasket is sturdier in handling). I use wire snippers to make a corresponding channel through the gasket so it won't cover the channel in the housing. The gasket is just a catch-point for gunk otherwise.

Find some non-curing gasket sealer (Permatex makes a variety of products). Paint a THIN layer onto the housing, set the gasket on, paint a THIN layer onto the gasket & then stick it on the motor. I usually use a finger to wipe the stuff to minimum thickness by the bleeding channel so it won't squeeze out into there.

I just bled the system last night & it seems to be fine. When the car is DEAD cold, pour coolant in with the bleeder screw loose/removed. Make sure to have the heater on max heat. Keep adding coolant over the course of the hour until the level no longer drops. You should be able to add close to a gallon before it stops dropping, if the thermostat housing is clear like it needs to be.

My procedure:
Install the bleeder screw, and have it loose enough to be bleeding coolant, but not gushing. Start the car and watch the coolant level. When running, maintain it maybe 1" above the "max" line. Rev the motor to 2500-3000RPM and watch the coolant level. You need to get the car warmed up, past the 1/4 mark for the thermostat to open. When it opens, chances are that the coolant level will drop fast, and nothing but air will come out of the bleeder screw for 5-10 seconds. Keep it revved up until it pisses coolant for 6-10 seconds without bubbling. Tighten the bleed screw & make sure that the coolant level is at the "max" mark or a little above. In a day or two, after driving maybe 50 or so miles check the level again. It may need a little topping off as the last remaining air has worked its way into the expansion tank. Then drive & check it on the regular interval recommended by BMW.