Mechanical Injection Pump Problem & Fix

[ctaylor738]

I was having problems adjusting the idle mixture on the 230SL. This is done by pressing in and turning the adjusting knob on the back of the injection pump until you feel it engage and then turning it CW to richen, CCW to lean. You should feel a "click" as you turn it. It's specified in every manual that this must be done with the engine NOT running.

My problem was that the knob would not turn once engaged. Since the pump had a "rebuilt by Mercedes" tag on it, I initially thought that the adjustment had been set. I later learned from Gernold at SL-Tech that if an attempt is made to adjust the mixture with the engine running, the adjuster goes to full rich and jams. He described this as "very common" in the cars he sees.

My fix was to grasp the knob with a pair of small vise grips, push the knob in to engage, and turn it gently one "click" CCW. If this didn't work, the next step is to remove the plate and knob and try to move the adjuster with a screwdriver.

So with the adjuster freed up, I was able to go a couple of clicks CCW and get the CO to just over 4% - spec is 3.5 - 4.5%.

Getting the CO and idle speed right is interesting because you iterate between the idle air pump adjustments, both of which change the CO, until you get it right. The CO is very sensitive to changes in idle air. At least on my car, as the idle approached 850 RPM, which was my target, the slightest turn of the screw would drop the RPM to 750. I ended up settling for 900.