GEDaggett

So I just bought my E320 yesterday (great shape, good price, pictures to come later) Engine is very strong the transmission shifts flawlessly and engages from P or N without hesitation. The only issue with the car is that is has a very sporadic idle. At startup it goes to 1500 then drops to 500 and swings back and forth at variable idle speeds. On the highway if I am under more than 1/2 throttle above 65 there is no problem but If I back off to 1/4 throttle the idle starts getting screwy again and the car goes from surging to lag like I was pushing the gas pedal and then letting off over and over again. Do you guys have any ideas as to what to look at first. I was going to just slap plugs and wires on it and see if that fixes it but someone might have some insight into the issue to consider before I do anything I figured I would get some input.


Thanks

shdoug

It could be several things. First, I would check for codes at the on-board diagnostic. It is located by the battery. With the key on, engine not running, push the black button and hold for 2-3 seconds and then count the pulses. Here's the thread with more detail. https://mbworld.org/forums/e-class-w124/44010-need-help-94-e320.html

Then I would look for significant vacuum leaks such as broken or cracked hoses somewhere or EGR valve stuck open. If it has ASL (acceleration slip limiting- aka traction control), it could be bad wiring to the throttle actuator.

Spark plugs probably won't make a difference but it might not hurt. Be careful of the brittle petrified engine wiring harness to the coil packs if it is original. Do a search on "wiring harness" and pray yours has been replaced at some point. The same wiring was used on the throttle actuator.

I would definitely suspect a vacuum leak is causing the issue.

GEDaggett

Cool, Thanks for the code info. I will give it a shot and check for the vacuum issues.

1994e420

My 94 e420 had a similar issue.
Solution: replaced engine wiring harness....not cheap. Wires were brittle after 13+ years. Now, time to replace plugs and wires.

GEDaggett

Can you say MAS unit. Replaced the mass air sensor and the sporadic idle is gone. Now all I have is a slightly rough Idle.

Bigpete123

Was it the MAS Control unit or the mass air sensor/meter? those are two different things.

MAS Control unit definitely solved some issues on my car. But like you, it never made the rough idle go away.

tanktube67

check for any codes first, but it sounds loke the mas

GEDaggett

It was the unit that mounts onto the intake cross over. Did you ever resolve the rough Idle?

shdoug

That is the "hot film mass air flow sensor" that you replaced. I though the MAS unit was only on K-Jet systems, but I wouldn't swear to it.

GEDaggett

That is probably true. I am a still learning this car. Only had it since last Saturday. There is another sensor on the cross over just above the intake side of the engine. Do you know what that is? i looked at it and it looked fairly dirty on the filament contacts but I didn't want to screw cleaning it.