Miss/Poor Idle, Then No Start M103: MAS?? - PeachParts Mercedes-Benz Forum

350SL4spd · #1 05-01-2011, 04:11 PM

Hey Guys,

The continuing stoooooory (remember the old Muppet Show Veterinarian's Hospital? No? lol) of my M103's decent into madness. Or mine anyway.

Brief History: I came of the highway one day with a shutter from a poor idle/miss. I've been chasing parts in the fuel system ever since & have gone from a miss to a no start in the process. To this point I have replaced 2 injectors that were suspect, the fuel filter (because I had one), and a rebuilt fuel distributor. I have 5.5bar pressure from the FD when I jumper the FPR.

When I put the new FD in, it started right up & idled well other than a miss on #1cyl. I shut it off, found the porcelain was cracked on that plug (not the problem most likely, but I replaced it), but it would not start & run again after that.

Currently the condition is thus: If I let the car sit overnight, it will crank 'til the cows come home & never start. However, if I pull the MAS, jumper #1 & #2 to cycle the fuel pumps, and then crank, it will run but only if I keep foot into the throttle halfway. If I let off, it dies immediately.

When I turn the key on I DO hear the pumps cycling up.

What does this indicate as a possible problem? Is the MAS not keeping pressure up? Or are the fuel pumps failing? Or is something else entirely the problem?

Feel free to ask questions: I've done more than stated here for simplicity. And please toss out ideas. I'm stuck & I need my car back someday...

mbdoc · #2 05-03-2011, 08:26 AM

The MAS needs the RPM signal from the EZL to keep the fuel pumps running.

When you jump the MAS and make the pumps run, the engine should STAY running unless something else is BAD.

Can you check the basic fuel pressure (85psi)?

350SL4spd · #3 05-03-2011, 09:57 AM

Not sure what you mean by 'basic' FP, but I have 5.5bar @ the FD.

I did jumper the FPs on the MAS with the MAS installed and the car would not run that way. So I'm thinking I ruled out the MAS.

A member on here is lending me an EZL to test & see if mine is bad.

I did replace the Crank Sensor about 2yrs ago, So I doubt that's the problem either...

Any ideas out there?

S-Class Guru · #4 05-03-2011, 11:22 AM

A non-functioning/disconnected EHA has those exact symptoms.
Haven't done any real troubleshooting on it, just had it disconnected one day to measure current and forgot to reconnect it. Wouldn't start or run unless I gave it half throttle. Probably not it, but worth a check.

DG

350SL4spd · #5 05-03-2011, 11:27 AM

I have 2 EHAs that should be good (one of which was new 2yrs ago). I've swapped both in & nothing changed.

Which doesn't say that they both could be bad I guess, but those are long odds...

I supposed I should look at the wiring for the EHA for a short though...

#6 05-06-2011, 07:34 PM

What kind of readings are you getting on your CPS?

350SL4spd · #7 05-06-2011, 07:46 PM

CPS is new about 2yrs ago.

Just found that the hard rubber "L" hose btw the valve cover & the head was cracked causing a large vacuum leak. Replaced it & the car will run again, just not that great. Runs a lot better if I depress the airflow plate just a little bit. Time for tuning I guess...