I’ve posted before but wanted to start a new thread with updates. I’m still fighting a bad idle and rich condition on my 1993 300SE and I cannot figure it out. I love this car but I’m so frustrated with it I’ve listed for sale and will lose quite a bit of money. If I can get it idling better I’ll keep it!

I’ve fixed the advance magnet and set the timing cam to the retarded position before I put it back together because someone had reinstalled it wrong. Confirmed timing. Replaced the fuel pump regulator because I had a hint of gas in the vacuum line. I replaced the fuel filter. I replaced the plugs, coil, wires, cap and rotor and O2 sensor, all with Bosch parts. I checked/cleaned the MAF sensor and it appears the ETA has been rebuilt, and the butterfly opens and closes very smoothly when the throttle cable is operated and is responsive. I checked the purge valve and it’s cycling. I put a smoke machine on it and found that I had a bad EGR valve so I deleted it temporarily by blocking the intake pipe where it enters the valve and plugged the vacuum line from the changeover switch. My wiring harness is not disintegrated but not new. My car does not have a separate idle solenoid. It’s all controlled through the ETA. I have NOT checked compression or fuel pressure but plan to check both next, however, the car drives great. It just has a horrible idle.

The car runs AMAZING once you pull out. It has great power (for this car anyway)But at idle it starts missing slightly and if the AC is on it will stall.

Most recently, it’s getting hard to start. So, I’m thinking maybe it could be the crank position sensor. Could this sensor be going bad and it still drive as good as it does?

I’ve checked the codes and got them all cleared except a code for the starter lockout/backup light switch. However, the backup lights work and I don’t see how this could affect idle. I have no other codes.

Everywhere I’ve posted this issue I don’t get much response. I know they only made this version of the car for a couple of years (M104 with a single distributor and coil…HM-SFI version) but surely someone can help me solve this.

I appreciate your time in advance!

Fuel pressure tested?

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Not yet but like I said that and the compression needs to be tested (it pulls hard so I would be surprised if it has low compression on a cylinder). What should the fuel pressure be?

Both pumped (engine spin w/starter) compression, and leak-down tests are important.
A leaky hydraulic lash compensator (or two or three) will affect valve timing at idle but will not be evident with increased RPM.
After compression and ignition are confirmed "perfect", an investigation of the fuel system will then be in order.

Usually a leaky lifter will make a tapping sound but not always. I’ll have to do the compression tests and report back.

55 psi fuel pressure is about normal

I know this might sound crazy but I just cured my 1991 560SEL by doing this: I bought ten gallons of 91 octane ETHYL FREE gasoline. you need to make sure the gasoline is, in fact, ETHYL FREE gasoline.

I put a bottle of Gum Out All In One System Cleaner into the tank followed by 10 gallons of ETHYL FREE 91 octane gasoline.

The difference was ASTOUNDING, the engine purrs like a kitten!

https://gumout.com/fuel-system-cleaner/all-in-one-fuel-system-cleaner-ethanol-treatment/

Tell us about your Mercedes!