MFI Thermostat adjustment

BSNMOE · 07-30-2009, 11:41 AM

I removed and cleaned out my thermostat on my 2.4 ltr S. I read posts about removing spacers to lean out and speed up the leaning out process. I live in South Fla. and rarely start up below 50 deg. I would like to eliminate some of the "over rich" mixture and also speed up the duration. One problem with this method is the shaft moves forward into the pump mechanism and as the bi-metal disc's expand will that put too much pressure on the shaft and the pump mechanism?

matt demaria · 07-30-2009, 01:26 PM

From my memory there was information in the literature about doing this. I believe that Porsche said to start reversing some of the washers. Not to remove them. Normally the washers are stacked concave face to concave face in pairs along the stack. The new arraignment would have some of the washers arraigned convex face to concave face only for a few washers. The amount is found by trial and error.
As you know when fully warm the stack has expanded. (I think the movement was about 10 mm) The pump has to be set up in this condition. If the stack has been altered, the warm protrusion is usually less. That means the pump will be exceedingly rich at this setting. Whenever the stack length is altered in the warm mode the pump has to be re calibrated. This is not a five minute adjustment.

356RS · 356RS's Garage

BSNMOE, Removing spacers will not speed up the expansion movement and will also add more free play to the shaft and that will just add more time for the disc's to expand and take up the space before the shaft starts to move. Don't worry about the shaft having to much pressure moving froward. The lever inside the pump has a very strong stop mechanism. I would think the best method for fast warm up is to make sure your hose is the correct OEM type and has no leaks and provides good flow through the thermostat.
Matt, I must have missed that concave to convex disc arrangement information. Can't see how that would speed things up. I have a NOS thermostat and can try that method on the bench and see what happens.

BSNMOE · 08-01-2009, 03:10 AM

Matt and Mark, I think the problem that I have is when I start up I have to back out of my garage go down sort of an alley with 2 stop signs, then a few right turns, with stop signs. If I could just drive off at 3,000 RPM the plugs would stay nice and clean. I'm running MSD and #7 NGK's. If I warm up the car, pull a plug, clean it, it will stay clean all day. After warm up I removed the top cover( the one with 2 screws and can see that the thermostat is fully opened. I'll play with the metal disc's to see if that helps.

304065 · 08-01-2009, 04:23 AM

I wouldn't monkey with the discs, I've never heard or seen advice from the factory about reversing the discs (not saying it doesn't exist, I would love to see it if you can post it).

Are you absolutely certain that the hose from the heat exchanger to the engine tin, and the two hoses from the engine tin to the thermostat housing, are in perfect condition?

Once the thermostat gets up around 150 degrees that's when it starts closing. If you start from a higher ambient it DOES close faster. I would verify that you are getting plenty of hot air from the exit from the thermostat before the next step.

matt demaria · 08-01-2009, 12:22 PM

Please refer to Check Measure Adjust of the mechanical fuel injection publication, the first printing (4532.20). On page 37 Porsche notes “Don’t slip the expansion elements off the shaft. If the expansion elements or discs are mixed up, the warm up characteristics of the thermostat will be changed.”

Also refer to Up-fixin der Porsche Vol 4, p38. Roger Channey states “ It is also possible that some thermostats were incorrectly calibrated at Bosch (this should probably read most )”

This means that if the bellview springs are not facing each other the cones cannot expand against each other and change (or increase) their distance. Channey talks about adding shims 1/32 at a time to move the shaft to the front of the car to lean the cold start. Reversing one of the pair of belleview springs would move the shaft to the rear of the car but the net travel of the shaft due to temperature (all in at 140 degrees) would be less. If the net travel is less, It would start in a leaner cold start mode provided that the pump was re-calibrated in the warm mode.

PS: This is what I could dig up at a moments notice. Been at this stuff for forty years. I can't remember all the sources.

BSNMOE · 08-05-2009, 02:56 AM

I shimmed the thermostat using washers.The combination of washers that I used pushed the rod .030, almost 1 mm , to lean running. The car started at 70 deg. and ran fine, however, I could feel it was a bit lean for the first few shifts. (no backfire or bucking) I did notice after 3 runs that my plugs and tail pipe are much cleaner. This may be somewhat of a problem at 50 deg. start up, however, the trade off is well worth it.