Hello everyone,

My car is a 1990 300ce 24 valve.

The topic of rough idling, especially on w124's, seems to be covered in great detail on this forum. Lots of good info.

My car has been suffering from a rough idle and bad hesitation when accelerating from a stop for almost my entire 3.5 years of ownership. The rough idle is more like a "bump" or "cough". It's not a constant thing- it's just a single, erratic shake every 2 seconds or so (sometimes more, sometimes less). The condition is present regardless of whether the car is hot or cold. The hesitation issue continues through to about 2,500 RPM, after which the car accellerates smoothly and normally... when it hits about 2,500 RPM, it's literally like somebody flipped a switch- my head snaps back from the increase in power.

Another symptom with the hesitation is that the single "bump" occurs also as I accelerate, but it increases perfectly with the increase in RPM's. It's like one cylinder isn't firing or something.

Oh and another thing. I miserably failed emissions last year. I got it through by having the mechanic lean out the mixture, but the car ran like absolute CRAP. It would die/etc. So I had to bring it back and have put the mixture back. I feel guilty because I know my car's emissions are probably terrible right now. The exhaust smells very rich.

Mechanics have had a really hard time coming up with a permanent solution, and i've spent a fortune on dead-end diagnostics.

My check engine light has been coming on for some time now, and I get code 17, which is "Oxygen sensor signal wire shorted to positive or ground". A mechanic had the same code over a year ago and I suspect the same code has been coming up for a few years.

My 02 sensor is fairly new (less than 10k miles/2 years on it). From what I've read on the forum, this code can come up even if there isn't really a short and even if the 02 sensor is fine. I've read that it can come up because the mixture is too lean, causing the 02 sensor to get "pegged". And the lean condition can be caused by vacuum leaks.

The problem is, I don't think the mixture is actually lean... I say this because I know my mechanic is made the mixture very rich, and like I said above, the exhaust is really rich-smelling.

One mechanic did say he was getting strange electronic readings from the Idle Control Valve. I have since cleaned the ICV (it looked really new and wasn't too dirty), but didn't inspect the connection. But I'm not sure if this could be an upstream problem- i.e. the 02 sensor is pegged, which is effing with the ICV...

I have been keeping a list of all of the potential/common causes for this problem, and I think the first one I want to try is simply looking for vacuum leaks. Most people say to check the hoses in/out of the ICV. Mine looked pretty new... flexible, etc. But one thing I did notice is that there aren't any hose clamps on the ICV and the hoses could pull off with a minimum of effort. So I might just put some clamps on it and see if that helps. I'm skeptical, but it's worth a try since it's cheap.

But on the same thread of vacuum leaks, where else should I look? What other hoses/connections should I look for? I'm a newbie to working on cars, so please go easy on the acronyms!

If vacuum leak searches don't yield any results, my research has shown another potential culprit... The Exhuast Gas Recirculation (EGR) valve... To me, this actually makes some sense because my exhaust could be rich due to too much being recirculated back into the intake manifold, but has the same effect as leaning out the mixture. Again, I'm not a mechanic so my understanding of the EGR valve is just based on internet searches.

Here is the relevant work that's been done in the last few years (not all were done solely to solve this problem):

Ignition coil (apparently my car has only 1)
Plugs, cap, rotor, plug wires
All injectors
02 Sensor
OVP Relay
Cleaned ICV
Crank Position sensor
Replaced leaky EHA valve
Fuel Distributor replaced
Many mixture adjustments, which help for maybe a week, only for the problem to return.

I only use Chevron and Shell Premium 92 Octane. I occasionally run Techron through, and had a BG44k treatment maybe a year and a half ago.

Sorry for the long post, but I would really appreciate help!!!