Solved Stumbling Idle on 1994 E320 M104
 

Wanted to thank the members of this forum and let you all know I overcame my too cautious, don't risk making it worse, mentality and, I'm pretty darn sure, fixed the stumbling idle on our 1994 E320 wagon's M104.

The problem started last Fall (just after an oil/oil filter change) and ranged from barely noticeable to really annoying. The engine never stalled, but it was sometimes uncomfortable to have it in gear while idling the engine. Basically, the engine speed would bounce up and down just enough to, on occasion, make the car fee like it was about to lurch out from the brakes.

Our trusty indy recently did a very complete 105k/15k mile engine service, and, frankly, could have gotten my blessing to throw parts at the problem. The techs and I left it (thank you, guys) that until it got worse and more diagnosable, we would leave it alone.

So I started scratching my head trying to figure out what the problem was. I'm almost embarrassed to say how much time I've spent researching on this site. (Keep in mind that this engine, under Starmark warranty, had extensive “catch up” work done to it about three years ago. In addition to the wiring harness, head, water pump, radiator, throttle actuator, (and a bunch of other major parts I can't recall) just about every electrical component was replaced). Except for this recent idle problem, the engine has performed flawlessly.

I read here that these types of problems are often vacuum related and tricky to properly diagnose. This weekend, while pulling the oil filter wrench off the newly installed filter, I noticed I knocked a fairly large black hose down in that area around pretty good. I put my hand on the tube and it wiggled ever so slightly. I thought no that can't be it but what the heck. I used my longest flat head screwdriver and tightened the screw (about an eight of one turn) of the clamp that holds that black tube in place against whatever that big part of the engine is down there.

Well I'll be damned. This engine has never idled better. Talk about silky smooth and solid. The problem cleared up immediately and has not come back for almost one week.

Hope this helps someone else.

Congratulations and a job well done!

Thanks for the info
 

Hi there Michael...I was just wondering where exactly is the hose that you're referring to. I have a very similar problem. Can you post a picture? I would really appreciate it. Thanks.

I got at it with the hood in the fully raised position with a very long, thin screw driver. You know where the oil filter is? Look for the black hose lower than the oil filter area between the filter and the engine at the very back of the engine. If I remember correctly, the tube was connected to the engine itself. Black hose with a silvery clamp holding it tight to the engine. The hose was snug but, when I put my hand on it, it wiggled just a tad. One quarter turn on the already snug clamp screw tightend it up properly.

I'm curious to know what it's called and what it does. I could be crazy, but I swear this solved the problem. It's been a month or two now and the idle remains silky smooth.

Good luck and best regards,

Good job and well done,

A picture will be very informative.

I'm way too lazy for digital photography, but here's a M104 picture from the Internet. Under where the arrow is. Who knows what that hose connects to?

Thanks
 

Thank you so much for your reply, I'll look into that in the morning, I will give you an update. Thanks again.

I dont have a E320 but its sounds like the idle air valve vacuum which goes to under the throttle body.

Mike.