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Old 03-25-2008, 03:11 PM
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azurite300E azurite300E is offline
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300E Idle Mix ok, but when revved goes off the scale to rich

This is a 1986 300E, 117KMi. I've had this car for over 4 years now, and this is my first serious attempt to fully adjust and fine tune the fuel mixture since the missing o2 sensor was reinstalled.

I was able to successfully adjust the idle mixture for close to 0 mA (+/- ~2mA) at the EHA, and very close to 50% duty cycle. But when I rev up the engine to check if it maintains the mixture at 2000 rpm, the EHA current pegs to -10mA in a matter of about 8-10 seconds (the controller is obviously kicking in full-tilt trying to lean the mixture). At that point the o2 sensor reads .94V (very rich), which also confirms the overly rich condition. Also, duty cycle goes down to 20%. Once the engine is allowed to go back to idle, after a few minutes mixture will go back to normal, and the system will get back to closed-loop mode (o2 sensor reads between 0.3 and 0.7V), Duty-cycle goes to 55-60%, EHA current stays within +/- 2mA. BTW, if I disconnect the EHA with engine at 2000 rpm's there is no appreciable change in the mixture. Nor if disconnected at idle because its already set close to 0mA, so basically its having minimum intervention at that point.

Adjustment was done with engine at operating temp (~82 C), at idle, no A/C. Water temp sensor measured 355-ohm at said temp, so it is within the target zone, and both sides to ground (its a 2-pin unit) reading the same resistance.

Performed the usual static tests before commencing the mixture adjust (engine off, ignition on):

- Controller: Initial duty-cycle indicates 70% (standard 49 state controller, status ok).
- Iddle contact: Slightly deflected the airflow plate, duty-cycle dropped to 10%, as it should.
- Full load contact: Open throttle completely, duty cycle went to 20%, as expected.
- EHA coil measures 19-ohm (within spec).
- EHA current with engine off: 20mA.

Also, in preparation for today's work, last night I stayed up until 4am reading treads in this and other forums regarding the mixture adjustment procedure, and also the magical "1/4 turn CW at the EHA".

The o2 sensor was just installed a few weeks back, and seems to be functioning correctly as I've monitored the output voltage, responds quickly to changes in mixute, and voltage readings coincide with what the duty cycle is also reporting, and what the EHA is trying to accomplish (aprox. voltage range between 0.3 and 0.7V when system is operating closed-loop, but only at idle). When engine is revved to about 2000 rpm o2 sensor reading pegs to 0.94~95V (full rich mixture).

I don't have a fuel pressure measuring gauge, so no idea what the upper and lower chambers are running at. When I took off the EHA there was obvious tampering on the brass cover, so previous owner was obviously at it. Tried rotating the 2mm hex base mixture screw, first both ways a 1/4 turn. CCW rotation would cause engine not starting, CW rotation by the same amount past the original position made no difference to the initial condition, besides that I had to slightly readjust the 3mm mixture screw on the air intake tower to lean out the mixture in order to compensate and get back to 0mA on the EHA and about 50-60% duty-cycle. Further CW turns, some made in 1/8 and some of 1/4 turn increases did nothing to change the situation. On every occasion I just readjusted the mixture screw and got back the proper idle mixture ratio and closed loop mode. But as soon as the engine was revved up to 2000rpm the EHA would again peg to -10mA in a matter of about 10 seconds, and then return back down close to 0mA after a few minutes.

Something is letting a lot of fuel into the system past idle. Since I disconnected the EHA and it made no difference, I would think that the controller and its sensors could be excluded. I checked bellow the airflow plate after engine shut-off for any signs of fuel leaks, and it was bone dry, so this is probably not a leaking o-ring problem.

My main suspect now would be either that the chamber pressure differential of the fuel distributor (FD) is way off, and might be causing the rich condition only when engine rpm's are brought up. Not sure if this scenario is possible, I'm only familiar with the innards of the FD in a general way. My other suspect would be the FD itself is malfunctioning, or maybe its just the EHA that is bad? Wish I had a spare to perform that test. Probably will order one next week, hopefully someone on ebay will have one at a decent price. Will also try to borrow a fuel press gauge from a mechanic and check the differential pressure setting.

Any comments and suggestions, or additional tests anyone would like me to perform, will be very welcomed. Thanks for reading all this.
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300E 1986 - 117KMi
Sikkens Metallic Azurite Blue
Member #120 of MB Club VE
Caracas, Venezuela
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