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!!!

help!

My problem turned out to be the crank vent tubes. Pretty big tubes under the intake manifold.

__________________
95 SL500 Smoke Silver, Parchment 64K
07 E350 4matic Station Wagon White 34K
02 E320 4Matic Silver/grey 80K
05 F150 Silver 44K

Thanks for your reply. Were you having the same symptoms as I am? Hesitation, rough idle, poor emissions, code 17?

Thanks again!

Bigpete:
The crank vent tube assembly is two tubes with a center connector. The tubes are rubber/rubber-like and get hard as a rock over time. Also, the assembly is very hard to get to - I replaced mine when I replaced the head gasket and was still difficult. I got new ones from the dealer (I don't think there is any choice here - special order) and it took two visits because the dealer's schematic/drawing of parts that included the tube assmebly was incorrect - it didn't show two pieces and I first though my old one had previously broken and be repaired with a connector. I have the part numbers if you need them.
Cary

Thanks very much for the reply. A vacuum leak like that sounds like it very well could be the problem... How were you able to determine that this was the cause, especially since they are so buried behind/under everything?

Did you have the same code come up in your car, and did you have similar symptoms to mine?

How many hours would you estimate for a job like that?

I have decided to put my faith in another indy mechanic... I'm bascially going to tell him that all I want him to do is check for vacuum leaks... that I don't want him to try and diagnose the problem outside of the potential for such leaks. I think mechanics in the past have been lazy on this, I want him to definitively tell me that he is will to bet his life that I don't have a vacuum leak. If we get to that point, maybe I'll let him try to figure out the problem using his own creative thinking... but at $90 - $95/hr, mechanics in the past sure have spent a lot of time "thinking" for me and not enough time solving problems...

lol can you tell I'm bitter?!?

If I were you I would put some effort into finding another 90-92 300CE with which to swap EZLs (ignition control module) for a few minutes. I say this because they are expensive new and hard to come by used. The symptoms of a bad one can vary to some degree but it is clearly a known problem on your car. Try a search - seems to me there's several good threads on the subject. There are several companies who test and/or rebuild them, e.g.:

http://www.4mercedes.com/ezl.htm

wow... I truly hope that's not what's wrong with my car... cuz if that's the case, I'm sending it out to the pasture. There comes a point where the bleeding must stop!

Well at least now I have a subject that I can spend another few hours on this site researching!

There's actually a wrecking yard in the area that has a lot of old w124's, so I might give them a call. Chances are they will be picked over.

Thanks again for your help, dean.

That particular part is especially picked over - it's rarely available as a used part, and of course when available the salvage yards know what they have and price accordingly.

yuck. Well it goes into the mechanic first thing tomorrow morning. We'll see what they have to say.

BigPete:

If you find a fault with the ignition module, your best bet might be to get it rebuilt (about $975). I had to replace my module, but I got very lucky and found someone who had a used one and did not know what they had.

Regarding the vent tube, I replaced mine while doing the head gasket because it was in bad shape and had been "repaired" before. I was not having any problems.

If you get discouraged and really want to get rid of your CE, let me know please.

Cary

90 300CE
92 300TE 4Matic
76 Jag XJ12C
49 MGTC

ha ha you already have one!!!

The car is in the shop right now. I should hear back from the mechanic any minute, although they're going to need it for at least one more day (which isn't a good sign that they know what the problem is yet).

Okay, just got off the phone with the mechanic. He's scratching his head, just like every other mechanic who has touched this car. I'm about the throw a temper tantrum.

He did extensive checks for vacuum leaks and he said he's willing to bet his life that there's no vacuum leak. Used a smoke machine and everything.

He tested the spark, and found that cylinder #4 is misfiring and that the signal is inconsistent. He swapped out the wires and still said that fixed the spark problem, but the car is misfiring all the same.

He seems baffled. He thinks it COULD be the fuel distributor, which was replaced not too long ago with a rebuilt unit... I think he said it was rebuilt by some outfit named viper or cobra or something like that... I should've written it down. But he said he's tried two of these units on the same car once before and they didn't perform well at all... and that he eventually went to a wrecking yard, picked up a rebuilt unit, and the car was smooth as silk... so maybe my rebuilt fuel distributor is effed.

I actually am really lucky, because he has another 1990 300ce in his shop right now. So he's going to swap out the EZL units just to test. But he is skeptical that it will be the source of the problem.

In general, he said he's having a tough time zeroing in on the problem because whenever he tinkers with one thing, it changes everything... that the problem is so inconsistent...

I'm nervous about what the repair bill is going to look like...

a couple of things to add, come to think of it. I think the distributor was rebuilt by "python". I knew it was some snake name or something.

He also said that if he sprayed carb cleaner or something like that into cylinder #4, it would smooth the idle right out. He said he was careful only to use a tiny bit.

He also mentioned that a backed up catalytic converter could be causing it, so that's one of the things he'll look at.

S'up Bigpete123

I have a 1991 300CE myself(bought a few weeks ago at 140k miles) and am experiencing the same problem - a rough shake (bump/cough) during idle. I'll keep my eye on the oil pressure during a stop light and will see it drop slightly for a second and get the "bump" that you described. The problem doesn't come up until after driving around for a while.

Recently, my mechanic has swapped out the Ignition coil and it has helped tremendously, but the problem still comes up here and there. I'm gonna give him a call tomorrow and bring it in to him during the weekend.

Keep me posted and I'll do the same.