Mechanical Injection Pump Problem & Fix - PeachParts Mercedes-Benz Forum

ctaylor738 · #1 03-13-2009, 10:25 AM

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.

Arthur Dalton · #2 03-13-2009, 11:58 AM

It is best to set the air bleed on the rich side on R11 pumps at idle. That gives you a better transition into the mid range. Bosch richened the lower part of the load range on the next pump series [ R18] to solve that problem, but you can get the same results by setting the air bleed with a 50 RPM drop or as close to 4.5 as you can get it. Also make sure the throttle plate is completely closed before any adjustments ..they are never right b/c previous owners try to change the idle by adjusting the plate stop.. ALL intake air for idle should have to come thru the air bleed valve once the coolant is up to temp.
A simple test I use is to block off the intake throat with a piece of cardboard when the car is at idle..if the RPMS change, then the plate is allowing unmetered air into the plenum, bypassing the idle bleed screw..which is what you do not want , but is commonly found.

bwostosh · #3 03-13-2009, 01:10 PM

Arthur , great idea to check throttle plate, Years ago a hack smog shop did that to Dad's 280SE, and failed to pass, he was in 70's then, the "fix" was to go to a nearby reputable shop (no names but) pay $2,300 for the MFI rebuild. Well, Dad called, and next weekend I went and undid the idle mixture screw, adjusted by ear and experience, and passed smog.
I will now use the cardboard test as a routine.

ctaylor738 · #4 03-13-2009, 02:09 PM

I did check the throttle plate and it's not allowing any air past. In fact the stop is not even touching the adjusting screw. The screw and nut the absolutely refuse to budge so I have left well enough alone.

Arthur Dalton · #5 03-13-2009, 04:28 PM

Last think you want is for the throttle stop to not be adjusted as a Stop...I have seen the T-plates wear the housing and plate right out....that is it's purpose/design funtion..

Arthur Dalton · #6 03-13-2009, 04:35 PM

Yes , that is just a quicky test I came up with years ago..I have a few tricks on these cuz I used to be a Tweaker on these systems. Aother is to take off the Booster hose when tweaking the A/F mixture , just to eliminate any other possible unmetered intake air.

The best test is the Factory rack position access port for using a 4x.8mm test screw [ 10/32 works] on the early pumps... That tells the tale w/o any linkage removal and is very accuate and simple to do. Couple that with a vac gauge and a good ear and you are set to go.
I use that test bolt on any pump adjustments. Too bad the later [ R22>0n] had that cool feature eliminated.

ctaylor738 · #7 03-13-2009, 05:00 PM

I am on it.

Arthur Dalton · #8 03-13-2009, 05:49 PM

Aside from BP Blaster and a heat gun, the trick to a stuck throttle adjuster screw w/o breaking it off is to place a screwdriver blade into the adjuster slot and hold that rigid as you turn the locking hex w/a 6 point box end...by holding the screw with the blade , you are less apt to have it snap off flush with the housing [ which many do] b/c it is can not turn with the nut....once you get the lock nut to release.. you will find the screw will turn freely...

ctaylor738 · #9 03-13-2009, 08:01 PM

Thanks, Arthur et al.

Crisis averted, all it took was a little PB and a little determined effort.

I have the engine pretty well sorted out and am driving the car regularly. Here's a picture from last weekend when it was Spring here.

justincook · #10 09-26-2009, 07:55 PM

Chuck did you have to remove to FI pump to get a wrench on the idle adjusting screw?

I think I have the same problem. Car idles really rich (you can smell it) and I can't get the screw to budge by hand.

Car looks great BTW!

ctaylor738 · #11 09-26-2009, 09:05 PM

Thanks.

What worked for me was to push the adjuster in until it engaged, then put a small vise grip on the knob. I was then able to turn the screw one notch, then adjust it normally.

Adjust the vise grip to the right size first.