I have a 91 500sl which had a leaking EHA valve. I ordered a Bosch EHA and installed it. When I tried starting the car it ran roughly so I "Googled" adjustments to it and found several videos. Well, I adjusted it and tried several positions and gave up due to time constraints about six months ago. The car's been sitting all of that time and now I'm finally getting back to her.

The car will not start at all now. It does start with a squirt of starter fluid (spray). I understand that there needs to be a differential pressure of .4 bar between the upper and lower FD chambers, but the car needs to be running to test that. I've also tried measuring the distance the EHA adjustment screw is in and I'm 6.8 mm, roughly one turn from the bottom, per yet another video. Still no start. The will occasionally stumble when cranking like its about to catch (start) but no go.

Yes, I'm a dumb-ass and messed with the EHA screw. Any advice on how to undo my dumb-ass-ness?

More info...

Ok, I checked the diagnostic signals for the following:

• MAS Controller, no error, one flash on pin 14
• Distributor Ignition, no error, one flash on pin 8
• Continuous Fuel Injection System, got a 17, O2 sensor shorted (possibly) on

pin 3 (I can't physically trace the O2 wire right now due to my physical limitations but will look as soon as I'm able.)

I also did a duty cycle check on my X11, pin 3 ground, pin 2, signal (I think) and got a 70% duty cycle per my scope with the ignition on, in the running position. Apparently, this is ok too.

To the best of my limited knowledge, the O2 short probably wouldn't stop the car from running, it would just run rich.

Oops, my above note about says I had my scope's ground on pin 3 and lambda connection on pin 2, this is actually backwards. So my 70% is actually 30%. I've read that the way that MB represents duty-cycles is the inverse of the traditional way. So am I right in saying that getting a 30% is actually a 70%, so I'm still "good?"

So, circling back around... Can anyone please give me some advice on where the mixture screw should be set roughly for a 91 500? I'll do the fine pressure differential adjustments once the car is actually running.

Thanks in advance.

I just reinstalled my old, untouched leaky, EHA and the car still doesn't start...

I removed the cold start injector and attempted to see if it was squirting fuel. I removed it and placed it so it would spray into a jar. No spray... I did a voltage test and got 12v when cranking. I then did a bench test of my cold start injector/valve and it works fine. I can blow though it when it's energized and not when it's disconnected. I did get only a trickle of fuel when I originally removed the cold start injector fuel feed line, so I attempted to do a quick pressure test of the bottom of the FDist vs the top injector outlet. The bottom showed 2.4 bar when cranking. My hose going to the top leaked so I couldn't get a good reading, but it did show that there's pressurized fuel going to the CS injector.

I have no idea why it wasn't squirting...

The low fuel pressure suggests that one of the pumps is not operating. KE-jet system pressure should be 5.4-5.8 bar.

Thanks for your reply. Is that 5.8 bar when cranking the engine?

System pressure will be somewhat lower when cranking; pump output varies with applied voltage.
Pressure checks are best performed by removing the fuel pump relay and bridging the pin sockets.

Thank you again. I need to find the fuel pump relay and then I'll give it a try. Do you have a pic of where it is and what the pins to bridge are? From what I can find, I remove the MAS module, then bridge pins 1 and 2. This should make the pumps run. I'll let you know what I find tomorrow as I'm running out of daylight right now.

I'm "assuming" that each of the pumps will either be working or not... Ideally it would be great if I could run pump #1 and then pump #2 separately, so I can determine if either is defective, without getting under the car but I'm not seeing any separation of the pumps.


Many thanks!

Good morning all,

I removed the MAS and shorted pins one and two together, put the key in "run" position and got a reading of about 2.8 bar at the control (bottom) of the FD, no where near the suggested 5.8 bar.

At this point unless someone has a different opinion, I'm ordering two new fuel pumps and replacing the pair.

I sure hope this is it!

There is the possibility of a defective fuel pressure regulator.

I hope not, kinda pricey, but we'll see.

In the recent past I had an issue with the car not doing a warm start well, or running like crap after it started. It could barely accelerate out of its own way. Hoping that might of been a symptom of failing fuel pump as well... Would kinda make sense, the one pump was intermittent and barely pumped enough fuel when warm. Then again, I've been known to overthink things...

Thanks again for your reply!