My A++ mechanic can't figure it out, can you?
 

1991 420 SEL with 200,002 miles does the following:
There is the sound of something engaging near the passenger side of the engine bay at which time the RPM's drop to almost the point of stalling, the economy gauge rises to mid-way and a little puff of smoke comes out. At this time the idle recovers and settles until it happens again. Hot/cold, it does not matternor is it effected by A/C. I have changed or checked O2 sensor, idle valve, temp sensor, OVP.
Obviously something is triggering an overly rich mixture for a few moments but what is malfunctioning?

Does it have an EGR system/valve?

Duke

Fuel pump around that area on that car?

How long since you've
 

changed or at least checked your evaporative emissions control canister, and related hardware. On some other models and brands, if the canister is full of liquid fuel, it will produce such behavior, but I don't know any models of Mercedes as new as yours first hand.

On the other hand, are you sure it's not just the AC compressor cycling on and off?

Thank you for the replys;

The fuel pump is back by the tank, so it isn't that.

I will ask him about the canister being the cause

Anybody else?

I am going to drop off the car next week but I was hoping for some ideas because I have never seen this mechanic at a loss.

Can you define "engaging"? Meaning something electrical or perhaps a driven accesory?

As far as driven accessories, the only thing I can think of is the AIR pump which is moounted on the right side (at least on the 5.6, not sure on the 4.2). If the pump is malfunctioning, possibly binding, it might load the engine to the point it tries to stall, but the belt would give out quickly I think.

You still haven't told us if it has an EGR system or not. Failed EGR valves/actuators that cause the EGR valve to open at idle can cause rough idle an other problems.

Duke

Thank you gentlemen,

Mike,
It was indeed the air pump engaging but I have since disconnected it with no change in the symptoms(other than the fact that the engaging sound has ceased)

Duke,
I did ask my mechanic and he said it was not an EGR issue but I'm not sure if he meant there was not one involved or it was functioning properly.

He seems to feel it is electrical in nature. It is now occuring more often and now stalls at about every third stop.

I am asuming you have already checked the OVP and 10amp fuse on top (or two 10amp fuses on the newer version OVP). The stalling at stop makes me want to look there.

Yes, I changed the OVP first thing because it was the orig. part and I concluded that even if it was not the current problem it soon could be one. The first code the mech. read suggested O2 sensor but he tried a new part with out any change. He said he would check each and every wire for a short but I was hoping to find someone who experienced similar problems on this board.

Find the source of the A/P activating. When the A/P comes on, it also opens some valves to let the air into the exhaust system. Any air leaks into the exhaust will cause extra oxygen to get in and fool the O2 sensor. I have seen poor running on a 420 that turned out to be one of the hoses hooked to the air pump leaking causing air to get into the exhaust. I think in your case, when the A/P is activated, the switch over valve (i think that is what it is called) is also activated causing your problem.
There is a temperature sensor somewhere (I'm not sure which one it is right now) that allows the A/P relay to turn on to activate the A/P.\

Look into this and let us know what you find.

The mechanic has a new idea, he thinks it may be the "green cable" that runs from the ignition module to the bell housing. The purpose is to idicate TDC.

Any thoughts???

Anybody know about this "green cable"?

The green cable goes to the crankshaft position sensor. If it were bad, the car wouldn't run. Also, it doesn't make any noise.
It doesn't enguage and disenguage. It either works or it doesn't.

Does your "mechanic" actually have any experience with Mercs? He seems to be just grabbing at straws.

With your symptoms the first thing I would do is an EGR functional check if the engine has an EGR system.

Duke

The air pump has a test done on it by the ECU , where it gets energized to see if the o2 sensor reads a lean condition..
You have unplugged the pump, but the Pump SOV is also energized at the same time [ from the same power source] , opening the control valve between the pump and the engine.
BC you say it happens when the pump would come on, I would look for a vac leak at this main hose [ around the control valve], as the valve is still energized even though the pump is not...
I would also see if the check valve is gummed up and stuck open,, this would open the exhaust to pump plumbing when the SOV gets a signal....

I like Paul's idea about the air pump. If it was engaging while warm, it was also probably activating the check valve. If you have since disconnected the pump clutch you still may have the check valve opening. Paul is right that the exhaust will aspirate air in through the pump if that valve goes open. This will fool the O2 sensor into thinking you have a lean mixture and the thing will go full rich trying to compensate.

smog pump rebuilder
 

Fred,

I recently worked on a 1990 190E with a bad smog air pump. It was making a terrible noise whenever it kicked in. It was defective and here in CA we can't pass smog if it's not working. I searched around for a new or rebuilt one and the best price i got was for a rebuilt one from CVJ Axles in Denver. Cost was $295 and has a 3yr warranty.

I'm giving you this info just-in-case someone tries to sell you a $600 pump and you have to put it in for smog purposes.

Thank you to every one for the replys, I love this board!!!!!

The car is now in the hands of Enrique, I'll let you know what the outcome is.