Fuel Distributor/Air Flow Sensor Plate Help 300E - PeachParts Mercedes-Benz Forum

350SL4spd · #1 04-15-2011, 04:44 PM

Hey guys,

This is a bit of a tough one, but I hope someone here can help me. I've been chasing a rough idle/miss problem that happened suddenly on my 1991 300E. After much testing I decided to replace the fuel distributor with a warrantied used one. As soon as the used FD was in I still had an idle/stumbling problem, but it had changed. I no longer had some cylinders not firing as before, but the idle was still crap.

Then I touched the air flow sensor plate. Just ever-so-slightly pushing it down and the idle cleared right up.

Just a smidge more pressure & it stalls out.

Now, I've been reading up on adjusting the air flow sensor plate and it's damned scary, so I want to make sure that I'm not missing something else obvious first. Is there anything else that could be effecting airflow? It would seem that it would have to be the the sensor plate, as the only thing that changed was the FD. Unless being down 2cyl was masking a problem with the ICV??

Any help here?

Also, if I decide to go that route: any advice on adjusting the air flow plate?

Thanks!!

ps2cho · #2 04-15-2011, 05:47 PM

Used ones are an absolute crap-shoot to be honest...

The only true way to determine if a distributor is GOOD is by measuring fuel quantities at the injectors using plastic bottles...then measure fuel flow at the pump so you can determine where in the fuel lines the restriction is.

That said, my best advice would be to first center the throttle plate and see what happens.
Some info here:

350SL4spd · #3 04-15-2011, 06:04 PM

Just to be clear, the only thing replaced was the actual FD. The mixture assembly/air flow sensor plate etc are original to the car & untouched. From my testing thus far, the plate is centered and possible just the tiniest smidge high.

Maybe a more important question is: can a different FD change the height of the air flow plate?

JohnM. · #4 04-15-2011, 06:48 PM

Don't be scared to adjust the air flow plate. I think you just want to make sure that the "0" setting is correct. But that may not be the problem here.

Did you check the duty cycle with new FD? Also, if the old fuel distributor got junk in it, causing it to clog up...your injectors might be partially clogged as well.

350SL4spd · #5 04-15-2011, 09:28 PM

Haven't checked the duty cycle yet, but I plan to.

I did replace 2 injectors after replacing the FD because those cyls weren't firing. The rest seem to be firing ok right now (ie: idle gets worse when I pull any spark plug).

As far as adjusting the air flow plate, I just don't want to adjust it if it's not the problem & then have to remove the whole unit to tap that pin back out...

Ivanerrol · #6 04-16-2011, 06:39 AM

Quote:

Originally Posted by 350SL4spd

Just to be clear, the only thing replaced was the actual FD. The mixture assembly/air flow sensor plate etc are original to the car & untouched. From my testing thus far, the plate is centered and possible just the tiniest smidge high.

Maybe a more important question is: can a different FD change the height of the air flow plate?

Answer is yes. There is an adjustment between the FD plunger and the tab from the plate mechanism that actuates the it. To adjust it you need to remove the whole body of the air mass sensor.
Hope you put a brand new O ring between the FD and air mass body.

350SL4spd · #7 04-16-2011, 10:02 AM

Quote:

Originally Posted by Ivanerrol

Answer is yes. There is an adjustment between the FD plunger and the tab from the plate mechanism that actuates the it. To adjust it you need to remove the whole body of the air mass sensor.
Hope you put a brand new O ring between the FD and air mass body.

I did replace the O-ring.

Can you describe this adjustment a bit more clearly. The adjustment I was looking at was basically a drift pin in the mixture control unit between the air flow plate & the FD: Carefully knock it in a bit if the the plate is too high (w/o removing the unit) or remove the unit & knock it back out a bit if the plate iss too low.

Is there another adjustment I'm missing on the FD side?

Ivanerrol · #8 04-16-2011, 09:05 PM

With much thanks to another member:

Quote:

Two Adjustments on KE Airflow Sensor

Airflow sensor plate adjustment
Pressurize the fuel system.This can be done by removing the fuel pump relay and placing a jumper wire with a 16 amp fuse between terminals 7 and 8.

The airflow sensor plate should be centered on the narrowest part of the venturi.This usually means a gap all the way around the airflow sensor plate of about 0.002 inches.If the sensor plate is not centered,loosen the center bolt and adjust.The sensor plate should not bind when pressed down.

The rest position of the sensor plate should place the top of the sensor plate 0.008 inches below the top edge of the narrowest part of the venturi.

The distance between the airflow sensor lever and the bottom of the control plunger (free play) should be 0.040 to 0.080 inches.If gently lifting the sensor plate does not reveal this tolerance,then adjust free play by driving the guide pin up or down as required.

Airflow Sensor Potentiometer Adjustment
There should be between 3,200 and 4,800 ohms of resistance between terminals 14 and 18 of the airflow potentiometer when the airflow sensor is at rest.There should be between 560 to 1,060 ohms from terminals 14 and 17.The reading should increase to between 3,760 and 5,640 when the airflow sensor plate is to it's
lowest point.Adjust as necessary.

350SL4spd · #9 04-17-2011, 10:18 AM

That was the adjustment I was eyeing. I was just trying to see if there was an easier one that I was missing before I tried that.

Now, if the plate checks out to be w/in spec today, should I just adj the idle @ the IAC? Wait...I can't adj the idle there on this car...I was thinking of the SL... Is there anywhere to adj the idle other than at the Lambda tower?

KJZ78701 · #10 04-17-2011, 06:05 PM

Search for "CSI-E Tuning" if you have not already read it. There is no easy way to get the system tuned and there is probably no need for parts, just time.

EDIT: The protrusion of the plunger from the FD is critical and can be adjusted. Remove the outer lock nut and washer and then turn the cylinder around the plunger. Special tool or a couple of flatheads will get it done.

350SL4spd · #11 04-17-2011, 07:41 PM

Quote:

Originally Posted by KJZ78701

Search for "CSI-E Tuning" if you have not already read it. There is no easy way to get the system tuned and there is probably no need for parts, just time.

EDIT: The protrusion of the plunger from the FD is critical and can be adjusted. Remove the outer lock nut and washer and then turn the cylinder around the plunger. Special tool or a couple of flatheads will get it done.

I've tuned this car before but it was over 5yrs ago. Was a PITA then.

Did not know that the plunger on the FD was adjustable. Any idea what spec length is? Or is it just an "adjust until it runs right" kinda thing?

KJZ78701 · #12 04-17-2011, 09:45 PM

that's all in the CSI-E Tuning Topic

Unlikely the flapper needs to be adjusted. Understand what happens when you slightly push down on the flapper. The value on the potentiometer may change a bit and FD plunger may be pushed up, delivering a bit more fuel. Note the "may"s. You need to figure out what is happening. Could be one or the other, could be both.

Also note that turning the idle/CO screw (3 mm Allen) CW will do the same thing as pushing the flapper down. That may be your "quick fix". Try 1/2 a turn at a time and give the motor a flick of throttle between adjustments.

HTH!

EDIT: Quick answer. There is no spec, just a recommended gap when the car is running well. Yes...a little tail chasing.

350SL4spd · #13 04-17-2011, 11:19 PM

I did finally find a mention in the M103 manual for the FD plunger under the heading of "Replacing the Sealing Ring for Control Piston". It said to VERY carefully measure the length of the pin and then re-set it as such with a big warning that if you f&*$ it up, there's no going back.

Interestingly, I swapped my old FD back in as an experiment & the car would run much better, just with a wandering miss/burble/stumble/whatever. The take-away there is that the FD is actually the problem, just that my original one & the used one I got are both not working properly with my set-up.

What I'll probably do now is just get a re-man FD from Phil. The car has been down for over 2mo while I slowly chip away at the problem & I have no time left for mucking about. That and I could spend another couple weeks fooling with adjustments & settings, only to find out that the used FD that I got has a fatal flaw...

KJZ78701 · #14 04-18-2011, 09:57 AM

Read this:
CIS-E Tuning (W124 - 103.983)

You don't need another FD. You can adjust your original one. Understand that the FD and the injectors need to be matched as a unit. Matching old "seasoned" parts should give you longer lasting consistency than new parts.

If you have the tools, the link above will help you get your car running in a weekend.

350SL4spd · #15 04-18-2011, 11:28 AM

Do you really think I can tune the old FD back in? Just for background:

Car was running great until I came off the highway from a 400mi trip 2mo ago. As soon as I hit the first stop-light off the highway I noticed that I had developed a miss. Tested fuel pressure etc and ruled out the pumps and the filter. Check each cyl. & found I wasn't getting fuel to #6. Replaced that injector and then had fuel to that cyl, but another cyl started missing, so I replaced that injector, got fuel to that cyl (now all cyl), but still had a ****ty idle/breaking up under throttle. So I bought a used FD, hoping to alleviate the problem.

Any additional/different advice based on this info?

Thanks a ton! I really want to get this thing back on her feet...