KE Jetronic Fuel Mixture Adjustment

In the mid 1970s, Robert Bosch GmbH developed a mechanical fuel injection system called K (konstant) Jetronic.  It was a very clever system, which could compensate for variables like engine temperature and air density without the use of electronics.  As time marched on and emissions requirements became more strict, the folks at Bosch added electronic controls to the system to extend its life.  This was called KE Jetronic.

My daily driver (a W124) is equipped with KE-Jet.  Recently I began having problems with the spark plugs fouling.  The car doesn't burn oil, and various sensors used by the fuel system tested okay.  Some research lead me to discover that as these cars age, they end up running too rich.  The fuel distributor can be adjusted to correct the problem.  The problem with this is that federal regulations required anti-tamper equipment be put in place to keep the fuel mixture from being trivially adjusted.  I suppose there was a belief that some shithead hammer mechanic would think they could get more power out of their car by dumping more gasoline into the engine.  Fortunately, it is pretty easy to get around the anti tamper stuff.

Here we see the throttle body and fuel distributor.  The little metal tower at the base of the fuel distributor covers the adjustment screw.  The cap at the top of it is actually an extremely thin piece of metal.  If you drill through it slowly and carefully, you'll find a small metal disk and a couple of pieces of felt.  Under that is the mixture screw.

After you drill through this, reassemble the air cleaner box.  Start the car, allow it to warm up, and connect a multimeter in duty cycle mode to the X11 diagnostic connector.  The negative lead goes to pin 2, and the positive to pin 3.  There is a small hole in the top of the air cleaner box.  Carefully insert a long #3 Allen bit into this hole.  Pressing down and turning will adjust the mixture (counter clockwise is lean, clockwise is rich).  Make your adjustments slowly, no more than a quarter turn at a time.  You will want to wait a little while between each adjustment.  The initial article I read said you wanted to wait at least 10 seconds.  I found it to be more like 30.  When the duty cycle is alternating between 45% and 55% (it will move), it is set properly.

Since making this adjustment, I have found that my car idles much better.  I expect to see improved fuel economy (since I'm not sending unburned fuel out the exhaust pipe) too, but time will tell on that one.

This is yet another example why I consider attempts to restrict access to a person's own property to be hostile.  I have a right to repair my car.  The anti-tampering stuff was put in place with the assumption that I am some kind of moron who would dump more gasoline than can be burned into the engine.  Bypassing this  created a small but real risk of drilling through something other than what I intended to.  The risk paid off for me, but what if it didn't?