Hi all,
I decided to replace the air box thermostat on my 92 wagon, not knowing if it ever had been replaced and as preventative maintenance before winter. A few weeks ago, I had replaced the flexible metal pipe directing warm air to the air box from the right engine side (it was falling apart and full of gaping holes so even if the thermostat worked, it may not have done the job.

Anyway, got the airbox out, and figured out how to pop out the thermostat. The flap was closed at room/garage temperature (about 15C). I noticed that the thermostat in there had a short stem coming out of it, maybe 5mm. Whereas the brand new thermostat I ordered for my car has a stem about 1.7cm at room temperature. That would leave the flap wide open at engine cold and ambient temperature 15C. I tested the new thermostat by immersing the metal base in very warm water (80C?) and came out slightly, to about 2.1cm. I tested it then by putting it in the freezer for a couple of minutes and it retracted to about 1.6cm (from 1.7 at room temperature).

1. Why is the new one so much longer than the original/old one? Was I sent an improper thermostat?

2. Assuming the new one is correct, what do I do to make it close the flap at cold temperature (cut the stem)? Will 5mm travel from frozen to very warm be enough to effect a full travel of the flap in operation?

First time I've taken these suckers apart, so I'm a little baffled and will welcome words of advice. The Bentley manual is not very helpful, and the Haynes is even worse.

Thanks.

Lose the thermostat and secure the flap so that it remains in the position to bring in outside air. The thermostat serves no usefull purpose except to slighty reduce emissions during the short warm up period. This helps keeps the Global Warming Nazis happy. Leave the hose in place as the emission police may fail your car at inspection time. In my state they do not open the airbox, they only look to see that the metal hose from exhaust is in place.

Remove the useless thing and fix the flap to fresh air only for ever,

Stick the new one in the freezer for awhile and be ready to move fast along with a creative language session. Try not to snap the plastic pin off the flapper or you'll be off to the boneyard for another ( don't ask how I know this :-)
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale

Thanks for the tip about putting the new thermostat in the freezer before installing it. Anyway, just popped it in and when I put the contraption back into the air box I could see the flap was quite closed. As I put my fingers on the thermostat body to warm it up, it responded quite quickly and moved the flap the other way, to block off the hole coming from the warm air pipe from the engine. Kind of neat to see it work.

God knows how long the old one has been faulty! And what harm that may have done to my AMM since I replaced the rusted-full-of-holes flex pipe a couple of months ago, getting only warm air through. Hopefully I've given the AMM a new lease on life, we'll see. But that will accelerate my trying to get a good spare (anybody got one ending in "...016" (for a '92)? I'll trade you for 2 AMM's ending in "...007" that I got off other cars, until I realized that they changed them around '88 on.

How long do these thermostats usually last anyway? At the cost, probably not a bad idea to replace every couple of years just to be safe!

Next job: flame trap replacement (another first time...) Hopefully that will set my car up nicely for winter. Oh I want to get an inline cooling line heater (installing a block heater would probably be a little costly).

I'm going to be replacing those things every couple of years from now on after a recent incident in west texas when the one on my 940t folded. after recirculating hot engine compartment air at 110deg+ outside it melted down a number of sensors including the amm . it got so hot the plastic for the clips on the air box melted, and the engine temp spiked pretty good too ( had evans waterless coolant in the thing so no engine damage).
definately schedualing replacement on the air box thermostats from now on.

I think block heaters are only about $15 at the parts bin, but a coolant line tank type heater you don't have to worry aboutleaks where the heater goes into a freeze plug
--
-------Robert, '93 940t, '90 240 wagon, '84 240 diesel (she's sick) , '80 245 diesel, '86 740 GLE turbo diesel, '82 Mercedes 300SD for sale

1. Why is the new one so much longer than the original/old one? Was I sent an improper thermostat?

Answer: Because the old one was shot. You may have rescued your AMM from an early death. Failed air box t-stat = always hot air to AMM = toast.

Regards,

Bob

:>)

I was worried about that! Indeed, with the old one shot, it was probably keeping the flap closed all the time, which means that since I replaced the flexible metal pipe feeding warm engine air, that's all that's been going through the AMM!! And I noticed the car getting a little sluggish one day, idling fluctuating etc... So hopefully I'm getting at the thermostat in time to save the AMM from total failure.

I like the other poster who said: get rid of the thing and leave the flap open all the time (sounds like a simpler solution, because how do I know for sure that the flap will work as desired with the new thermostat?)

The spring on the flap mechanism is strong and puts a lot of force on the "pin" that extends from the stat body, so once it's in place, I bet it will retract when cold, and open the flap to let hot air in.

How to check function?...when I replaced one, I had the flap mechanism removed from the airbox. I put it and the new stat in the fridge, and a while later it had moved the flap to the "hot air" position. I then put it on the workbench on a warm day, and the pin soon began to extend, making little "sproingggg" noises as it went, until it had the flap in the cold position. The pin moves slowly - ie: it doesn't react quickly to temp changes.
--
Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, 94-944 B230FD; hobbycar 77 MGB, and a few old motorcycles)