M103 coasting idle surge
 

My 88 300E automatic will surge from 1000 to 1400 rpm and back again a couple of times if I take my foot off the gas and coast under 40 mph. Only in gear, if I put it in neutral and coast, idle is normal, under 1000 rpm and stays there. A/C on or off makes no difference. IAC is not the issue as I have replaced it with a manual valve.

Check the small micro switch on the throttle linkage .Oil the linkage it at the same time .I use a brush and atf fliud ..

my m104.994 with 722.605 does that too.Been told its a servo,been doing it 6 years only when engine temp under 60c.

FI systems generally turn of fuel while decelerating to decrease emissions and increase MPG. This only occurs with closed throttle and above a certain RPM. As post 2 states, the linkage might not be working properly / on the edge of actuation causing the system to turn on and off.

It only surges after reaching operating temperature, 80c. I've checked the throttle micro-switch for continuity on/off and lubed it as well. I don't believe the CIS-E injection system ever shuts off fuel, it's constant unlike the LH hot wire system which does cut fuel. I'm thinking it may have something to do with the EHA valve as I turned it up just a tiny bit last year to improve drivability but this surge issue is new.

You could put a meter on the EHA and see if the surge tracks a particular reading. I'm thinking that the system goes full lean on coast and your adjustment pushed it slightly less lean and the engine is trying to run.

How about unplugging / jumpering the idle switch in the above idle state and seeing what happens?

The amount of time CIS spends in decel fuel cut is surprising.

I disconnected the micro-switch and got no surging. I don't know what driving around without it hooked up will do but I'll experiment.

As far as CIS-E goes, as long as the engine is running it's delivering fuel, even coasting. That's constant and the C in CIS. The only time it ever cuts fuel is when it hits redline.

New cap and rotor, seafoam and NGK non-resistor plugs installed yesterday. Running well.

The M103 does have deceleration fuel shutoff.

The C continuous in CIS-E differentiates the system from the timed pulsed injection of other systems but does not mean that fuel cannot be shut off. The EHA valve adjusts the control pressure in the fuel distributor and it can be made to cut the fuel.

I stand corrected. I did some research and the deceleration switch does indeed cut fuel. I have owned other CIS-E cars that didn't have the deceleration switch but MB does things differently. Thank you for the clarification.

I'll do some more tests on the switch and report back.

Disconnected throttle micro-switch, problem solved. So I don't have the fuel savings of cutting fuel, no big deal. I thought that switch had more than one function but now I know there is another switch at the throttle plate that sends info to the ECU for idle and WOT I have no worries.

I've replaced the IAC with a manual gate valve a few years ago and now I've disconnected the coasting fuel cut. A bit unconventional but the car runs very well.

Thanks for all the replies and good info gentlemen.

At idle , the mixture might be slightly different and there is a chance an idle to off idle switch transition may give a short term richer mixture ( like an accelerator pump on a carb ) . I don't know this for sure, just throwing possibilities out there.

I'd probably monitor the EHA with a meter and look for a change with the switch working / not working.

I'm pretty sure the throttle position switch on the throttle plate serves all other functions in regards to the ECU and EHA. As far as I know the switch I disconnected only controls fuel cut at coast.