As per the title I have a 2000 C280 (M112) that has a significant vibration at idle. It can be felt at the engine in neutral and really felt in gear. Mileage is 212.000. There are no codes. The car runs fine above idle and gets excellent fuel economy. The following items have been done in the recent past:

1) Motor and transmission mounts (dealer sourced) 4 1/2 years/70,000 miles ago.
2) Spark plugs (dealer sourced) 3 years/35,000 miles ago.
3) MAF cleaned recently.

I have tried the "torque test" for the motor mounts (put in drive, step on gas - same in reverse) and the mounts seem good. There is no sign of leakage of the fluid from the mounts.

Ideas? I would think if I have a coil or wire problem I would get a code, but as I said, there are none.

Usually an miss fire will trigger a cylinder specific or "general" miss fire code.

If might be a mechanical vibration where something is out of balance. Did the start vibe suddenly or gradually?

I bought the car as a Mercedes-Benz CPO car with 60K miles in 2006. It has always been serviced according (and mostly exceeding) to M-B schedule including fluids, spark plugs, brake flushes, etc. It has never had a problem like this and most replacement items failed after a long service life.

The vibration is recent and happened over a short period.

Today I pulled all the spark plug wires and measured the resistance. I found one at 2830 ohms. I changed that one and three others that exceeded more than 10% tolerance; i.e. any with readings more than 2200 ohms. No real difference.

I FEELS like a miss, but without any codes it makes me scratch my head. And the car runs strong and is very smooth just off idle.

Can you unplug the primary wire from each coil and see if the roughness changes? ( The 12V wire not the high voltage spark plug wire side. ) This way you can narrow it down to a cylinder. Or unplug an injector wire. When you do this, the computer will likely shut down the injector for this cylinder to prevent car overheating. My car does this. To reset, shut engine off then restart.

If you do find a cyl that does not change when a cyl is disabled, do a compression test, if low do a leak down test.

I disconnected the 12v feed to each coil. I got the expected response; a noticeable miss and check engine light with the corresponding code. No cylinder indicates and obvious problem. But I still have the roughness at idle.

While poking around checking for a possible vacuum leak I noticed a faint hissing sound from the right rear of the engine. I can't nail down exactly where it is coming from but I will remove the intake tube and inspect the area. The EGR valve is in that area.

I assume you are referring to a 202.089 chassis vehicle. Of course it could be many things but it could be the EGR valve is stuck open or the EGR control valve is permitting vacuum to reach the EGR valve when it shouldn't. I had this occur on an ML500 (same basic part design as yours) where the control valve stuck open with poor idle quality/misfire. Above idle it ran fine. To diagnose remove the vacuum hose that connects to the EGR valve to check if the situation is corrected. If it is, the control valve is defective or the control signal to the control valve is defective. Reconnect the vacuum hose and disconnect the electrical connection to the control valve---if the situation is corrected the signal to the control valve is at fault. On the other hand, if you take a length of hose and attach it directly to the EGR valve and apply vacuum you can determine if the EGR valve itself is stuck open and causing the problem. BTW, it takes very little vacuum to activate the EGR valve and destroy the idle quality. If it is the EGR valve or control valve, Mercedes sells it as a unit and it is wise to buy all the ancillary stainless tubing, ferrule, nut and gaskets that are associated with it. The flexible corrugated tubing is very easy to damage and you will not be able to remove the ferrule from the tubing if you purchase only the stainless tubing and gaskets which will delay the repair until all of the parts are obtained from the dealer. Good luck.

Mark

I recently changed my motor mounts. Experienced a dramatic difference before and after amount of vibration at idle...

__________________

1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
1957 Ponton 220S
2001 S600 Daily Driver
The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com

This is a 202.029 chassis car.

The EGR valve is functioning correctly. However, the packing on the valve stem was leaking. Keep in mind that there is vacuum (via the pipe connecting to the manifold) above the valve disc when the valve is shut. There was a fairly significant vacuum leak as a result. After changing out the EGR valve and locating one other small vacuum leak the engine runs very smoothly.

However, too much vibration can still be felt in the seat. My tests indicate the left side motor mount may have collapsed. If you noticed in my first post, the mounts are fairly recent, so I am disappointed to say the least. The original mounts went nine years and 142K and were still decent. These appear to have only lasted 4 1/2 years and 70K.

The problem was the motor mounts. I spent a lot of effort troubleshooting because I have never had a set of these mounts fail so early.

I hesitated to suggest that Motor Mounts could make a difference in vibration at idle, except that mine went away after changing the Motor Mounts... Glad my wild guess was helpful.

__________________

1962 220Sb ~ The Emerald Bullet http://www.youtube.com/watch?v=hx6tN1W48_o
1957 Ponton 220S
2001 S600 Daily Driver
The Universe is Abundant ~ Life is GOOD!http://www.classiccarclock.com