Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help

Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Tech Help

LinkBack Thread Tools Display Modes
Old 12-09-2010, 08:07 PM
Registered User
Join Date: Dec 2010
Location: Austin, TX and LU, CH
Posts: 58
CIS-E Tuning (W124 - 103.983)


As helpful as the net has been I would like to give back a bit. My car is a 1992 300E (w124, 103.983) with 200k miles with only about 35k on a new tranny. There is lots of research between some of the steps below, so it took me much longer than it will seem.
In addition to standard tools I also used the AFX Wideband Air-Fuel Ratio Monitor (<$300), an old Fluke multimeter, a propane torch, a fuel pressure gauge and a digital scale. I also have an ASNU injector bench tester, but you really don’t need one. There is an injector cleaning / testing procedure on this site that only requires some carb cleaner and compressed air.

If you have already read up on the CIS-E system you will know that fuel pressures are critical items. For me those came first. The car had been sitting for a couple years before I bought it, so I cleaned the tank and found that it needed a new tank filter and new pumps (note on pumps below). I also blew the old gas out the filter, in the reverse direction (low pressure), and through the entire system from the pump input out the return line at the tank. With it all hooked up and the injectors cleaned (I used the ultrasonic tank on the ASNU), flowed (three injectors flowed about 20% less than the other three) and put back in their original locations, I flow tested the installed injectors.

To do this I pulled the fuel distributor (no wires connected) off the base (three Torx bolts-watch out for the o-ring around the plunger when you pull the distributor off the base) and put plastic water bottles over each injector. I weighed the bottles empty since I did not have seven (cold start injector-CSI) that matched and wrote the empty weights on the bottles. I then ran wires directly from a battery near the rear wheel to both pumps. I ran the pumps for about a minute as I pushed the plunger all the way up into the distributor (WOT). Wide Open Throttle

After removing the bottles and getting the fuel weights, the first run gave the following results.
60 – 65 – 68 – 62 – 64 – 72 with nothing from the CSI - 0

I then removed the cap screw on the bottom of the fuel distributor corresponding to injector 6 and turned the exposed set screw IN one turn and then recapped. Performing nearly the same flow test as above, but this time varying the amount that the plunger was pushed in (simulating varying throttle position), I got the following results.
70 – 75 – 79 – 72 – 73 – 66 (0 CSI) That told me that varied or fully depressed, the flows stayed about the same (relative).
Three runs later and a ¼ turn here and an 1/8 turn there (NEVER touching #4) I ended up with the following results after a longer run (plunger fully depressed since WOT was a bigger issue for me to match than idle).
123 – 122 – 123 – 121 – 123 – 121

Happy, I now replaced all the bad rubber vacuum lines and put it all back together (after lots of cleaning including the old bad gas under the throttle butterfly), and fired it up with the AFX display in the car and the sensor in the tailpipe. Started right up running just over 9 to 1 A/F (way rich). Once it warmed up the idle started to hunt a bit and the A/F moved back and forth from about 11 to 9. Assuming an air leak, I replaced the rubber tubes to the Idle Control Unit (mounted to #2 intake rail). They were hard. Restart, same problem. Pulled the battery and the CIS-E control unit behind it and checked the ohms between pins 5 and 14 (Intake Air Temp Sensor – IAT). Open circuit, so I pulled the IAT sensor (just in front of the air filter. It disconnects without any wires, though it appears it wants to come out with wires attached) and sure enough the wires were good but the sensor was not, so I replaced the small sensor. I also used a propane torch to check the O2 sensor (connector right under the carpet just in front of the passenger seat, 7/8” or 22 mm wrench for sensor). Tip in flame voltage about 1, tip out of flame voltage should drop to near 0 in a few seconds. Mine was bad, so I got a new one.

I also used the X11 (on driver’s fender) 2, 3 and 6 pins to attempt to troubleshoot, but I never really found the information useful. I did get a funny “no TD signal” fault signal at one point, so I removed the MAS relay/control unit (also behind the battery) and checked more of the harness. On attempted restart, nothing. After chasing my tail and lots of time on the phone with Dallas Dave “I fix ‘em” Poole (thank you!) trying to figure out what I could have done to the MAS relay (which replaced the fuel, AC, and air pump relay along with being the go between on other items the CIS-E controls / watches, I discovered I had no gas in the tank. Since it was just running and “all of a sudden” there was no fuel pressure I assumed the MAS relay had gone bad. I removed the MAS relay and jumpered from 2 to 10 to 20 to run the fuel pumps and the O2 heater and saw that there was still no fuel pressure. Four gallons later I was running again.

Once again, rich idle, but it drove. I ohmed the two pins to the EHA, and got 20. Good. (The EHA mounts to the back of the fuel distributor and controls fuel flow within the distributor.) NOTE: You can put the car in constant open loop (runs, but rich) by pulling the EHA and O2 sensor plugs. (Thanks PS2CHO!) This helped me when I could not start the car warm. So, no warm start, pull the EHA and O2 sensors out of the equation and it started right up. This is where the tuning really started for me.

There is a 2 mm Allen adjusting screw behind a cap screw on the EHA which becomes clear after you remove the EHA. I purged the gas pressure each time before I removed the EGA to make sure I didn’t blow the little o-rings out. FP gauge purge button (or crack the inlet line) and push the flapper down all the way. Since I was rich at idle I turned the EHA Allen ½ a turn CCW and turned the CO/Idle adjusting screw (3 mm in the “tower” between the fuel distributor and the air inlet / flapper) ½ turn CW. Oddly, the idle stayed rich 9-1 and high RPM went lean, then it died at idle. Still experimenting, I went ¾ turn CW on the EHA (so ¼ turn CW net) and 1 turn CCW on the CO/Idle start even with gas pedal on the floor. HMMMM….still 72 psi on the lower port and 78 psi on the upper port of the fuel distributor (upper port faces the valve cover, lower port faces the driver’s headlight). Continuing to turn the CO screw CCW I finally got it to start 1 ½ turns CCW later with a mild hunt (idle going up and down). Going half a turn CW eliminated the hunt, but then it wouldn’t start again.

Then I discovered something very interesting. If I put a small piece of vacuum line between the idle microswitch (right behind the fuel distributor on the throttle linkage) and its actuation arm so that it would not close at idle EVERYTHING changed. AF at idle now went up to 11+-. I then drove observing no throttle AF (coasting in gear) and checked AF under throttle. After many runs and adjustments I found that the EHA setting will determine whether or not the car will start and really establishes overall AF even though the upper and lower fuel pressures really did not move around much. Something internal of which I am still ignorant. Also note that my idle was around 2000 in P and N with the microswitch overridden and the EHA and CO adjustments not where they should have been; Makes sense since the microswitch tells the CIS-E to control the idle via the Idle Control Valve. What I found odd was that in gear the idle came right down to 1000. I also found that once I got the EHA and CO settings where the car was running at 14.5-7 to 1, the idle in P or N dropped to about 1700.

Reply With Quote
Old 12-09-2010, 08:09 PM
Registered User
Join Date: Dec 2010
Location: Austin, TX and LU, CH
Posts: 58
Part 2

Part 2

I still have more work to do, but I hope I have given some of you enough info to have the confidence to dig in and check part by part and system by system so you don’t keep throwing new parts at it until it runs again.

If anyone can tell me where the idle microswitch wires go I would be grateful. I really should have put the switch back in the game once I got the car in the 14.x to 1 AF range but I wanted to see if the car would cold start and I had to clean up. It did cold start right away this am, but I had no real desire to get back in there just yet in case the warm start problems and the rich idle came back with the switch back in line.

Don’t be scared to adjust the fuel distributor or the EHA. It’s really just like setting the CO/Idle..with a few more steps. And save your money on injectors. Just because they are “only” $20 is no reason not to clean and test them yourself first. You can put a few drops of oil in them and test the “crack open” point with an air compressor and a rubber tipped nozzle. Just keep cranking the regulator up until they open. A few drops of carb cleaner and 70 psi should clean and show you pattern. I always run oil through my injectors when I’m done cleaning and flowing them if I’m not going to use them right away.

Hope that helps.
Note on pumps. Mine were frozen by varnished gas, so I put them in a cup of fuel for a couple hours and then fed them 12 volts. They slowly freed up and started spinning but flow was horrible so I took one apart. There is a small disc deep down in there with some sort of lining on the inner surface. The one I took apart had worn through the lining / coating and had also worn the metal. I didn’t dig anymore and assumed the pump had lost clearance in that area allowing the pressure to drop off, so I just bought new pumps. Based on the rubber boots on the connections, it looked like the pumps were original. The boots fell apart when I tried to pull them back to remove the wires.
Reply With Quote
Old 12-12-2010, 10:46 PM
mak mak is offline
Join Date: Jul 2006
Location: Westfeld .
Posts: 687
Thanks for sharing the information with us . Gives a very good insight to the CIS system .Please do update as you progress to a silky smooth idle of a 103 engine ,an elusive goal for many of us.

Reply With Quote
Old 12-14-2010, 12:24 AM
Mike Murrell's Avatar
Registered User
Join Date: Feb 2000
Posts: 2,574
Outstanding work.

Thanks for sharing.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
Reply With Quote
Old 04-05-2011, 06:09 PM
Registered User
Join Date: Dec 2010
Location: Austin, TX and LU, CH
Posts: 58
Final? Update

I finally have the car running really well. If I had to do it again I would do it a little differently.

My rich idle was the result of the fuel distributor’s plunger extending too far down onto the roller on the flapper. Adjusting the position of the roller with the 3 mm CO screw did not work for me. I could get the idle mixture correct but then something else went wrong: bad throttle response, hard restarts, surging idle, high RPM idle, etc. The rich idle could have been the result of the flapper spring fatiguing over time as well. Who knows? Bottom line, adjusting the plunger (moving it up into the distributor) about half a turn allowed the other parts of the fuel system to work as designed.

If I had to do it again on a running car I would:

-Make notes of fuel economy, idle mixture and fuel pressures at the top and bottom of the distributor (there are removable plugs on the top and bottom where you can attach your FP gauge) before I took anything apart. I would also remove the air filter and make a note of the position of the flapper when it just contacts the fuel distributor’s (FD's) plunger with the motor just turned off (residual fuel pressure in the distributor). This is a feel thing and a magnet helps to lift the flapper so you can let it drop just until you feel the point of greater resistance. Now mark that point on the aluminum housing around the flapper. Start the car and see if the flapper has moved down past the mark. If so, measure the drop. The ratio of flapper movement to plunger movement is 7 to 1. If I read the specs correctly, there should be from 0.1 to 2 mm of free play at the flapper at idle. That means 0.014 to 0.29 mm at the roller/plunger which you really can’t measure, but you can “feel”. Point is that the flapper should NOT push the roller into the plunger at idle. I do not know the thread pitch of the screw on the plunger but I assume it is a 1.0 mm pitch. If the flapper moves down 7 mm at idle from the resistance point you felt with the car off then you need to turn the plunger screw IN just over one turn. More likely you will need less. While you have it all taken apart, check the injectors for flow with plunger all the way out (idle) and all the way in and then adjust flow of up to five of the six injectors until all are matched. The min spec is 4-6 cc/min but suggested is 6.0 – 6.6 cc/min. A cc of water is a gram. Fuel is about 75% less massive, so 4.5 – 5 grams of fuel per minute is your target if you use a scale and plastic water bottles as I did. I flowed mine for three minutes at idle and was over the spec. Also check flow rates with the plunger pressed all the way in. There are two specs here: 100 – 109 cc/min (75 – 82 grams/min) and 140 cc/min. The 140 is a max. I just got the idle numbers in spec and then used the high flow to more precisely match the injectors. Since I removed the fuel distributor from the flapper assembly while flow matching I had no way to know how far the flapper was depressing the plunger so I could not check the “full” numbers. I saw mid 80 grams per minute with the plunger completely pressed into the distributor, so I concluded the fuel distributor was okay.

-Check the resistances of the air temp and water temp sensors at the computer’s connector. If out of spec (or more likely completely open or closed circuits), fix. Check the O2 sensor on the bench with a volt meter and torch.

-Put it all back together and go back to your notes. The differential pressure should be 3 – 4.5 Bar (difference between pressures at the top and bottom of the fuel distributor). If you are out of spec, remove the EHA and adjust the small Allen behind the cap screw. Be careful here. The lower pressure changes as the car warms up, so make notes of the ranges you see. If you can’t start the car after the EHA adjustment, you will need to adjust the idle/CO screw. If you turned the EHA CW a ¼ turn you will need to turn the idle/CO screw about ½ turn CCW, or the other way and twice as much (roughly). Careful again. If you had to go CCW on the EHA and CW on the idle/CO too much you will/might have allowed the flapper roller to contact the FD’s plunger at idle….meaning another adjustment to the plunger screw, sorry it all comes apart again.

-Once you get here you should be able to give the idle/CO screw small adjustments to get the car to start and run well. If you are not a hot-rodder, you can adjust it all on the lean side and improve your fuel mileage.

Reply With Quote
Old 11-26-2013, 02:37 AM
Registered User
Join Date: Nov 2013
Location: Eugene OR
Posts: 52
This is good info it was a big help in tuning every last bit of HP out of my M103.
Thanks for sharing!

Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

All times are GMT -4. The time now is 02:48 PM.

Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page