1991 300D - intermittent rough idle

[Chris_]

Hello.

My 1991 300D 2.5 is providing an intermittent rough idle...here is the detail of symptom.
1) Just before coming to a complete stop ( on braking gently) there is a discernible vibration which feels like a cylinder miss.

2) at a complete stop in drive... the car will vibrate which also feels like a miss however if while holding the brake I lightly spike the throttle and allow the engine to drop right back to idle the "miss" will either dissipate or completely disappear until the next time I come to a stop

3) To clarify ..the "miss" is not always present and varies in intensity and is only discernible at idle

4) furthermore the car runs very well down the highway and at intermediate speeds with as much or possibly more power than normal..in addition when accelerating from a dead stop with or without the miss present it accelerates very acceptably ( fuel mileage is good )

* the car has 188K on it and has had a headgasket at 150K and a timing chain at 174K both of which I have done myself

A) Is this possibly an injector ?

B) Could this be a fuel quality issue or could it be related to the injection system/pump

any thoughts appreciated...Thanks
Chris

[Chris_]

Anyone have any ideas ? It is understandably a concern for me as I have driven this car 100K virtually trouble free until this issue has surfaced recently. It is used by my wife primarily so I need to get to the bottom of this before it creates a possibly avoidable inconvenience for her.

thankyou

UPDATE: The problem seems to manifest more noticeably or possibly exclusively when the engine is up to full operating temperature

[bd1308]

To be up front, I own a W123, so this may not apply to you at all (I'm sorry if it doesnt --- im just trying to help out )

On my car, the poker chip primer pump started leaking, and would cause air bubbles to be in my fuel system. When idling and first going, it felt like a miss that would go away.

That may help, or it could have just wasted some bits of internet traffic

[Chris_]

Originally Posted by bd1308

To be up front, I own a W123, so this may not apply to you at all (I'm sorry if it doesnt --- im just trying to help out )

On my car, the poker chip primer pump started leaking, and would cause air bubbles to be in my fuel system. When idling and first going, it felt like a miss that would go away.

That may help, or it could have just wasted some bits of internet traffic

Thankyou very much - and yes this may be something similar as it is getting very slowly but steadily worse. I wonder where the primer pump is on the W124
or what other components could introduce air into the system. I may have to start another thread to research this. Thanks again for your reply

[bd1308]

It should be on the lift pump somewhere. On mine, its right smack dab next to the IP on the driver side of the engine

[compress ignite]

(At the expense of your Battery and Starter.)

The earlier W124 OM603 s had a "Fix" for "Shuddering" under " DeCeleration" that's mentioned in the FSM.

How're the Motor Mounts? Do they still stand more than 13MM proud of the
supporting member? Or do they need replacing? Don't forget the Tranny Mount.

[TimFreeh]

When the car is 'missing' at idle does the RPM drop-off to 500-600 RPM?

Idle speed in this car is controlled by computer, if the idle speed controller isn't working the car will default to 500-600 rpm and it will shake and shudder a bit at idle but run normally at all other times.

[gsxr]

There is no primer pump, as mentioned above. It's not air in the system. Idle speed should be ~650rpm, if not, fix that first. Possible causes:

1) Engine mounts (have you measured them via FSM procedure?)
2) Delivery valve O-rings
3) Injector(s)
4) Bad injection pump
5) Head gasket (compression test needed)

I've personally experienced #4 and the cure was a replacement pump. Never had the old one diagnosed though. On that car the symptom was a clear misfire at idle on one or two cylinders, but it would smooth out and run pretty well at higher RPM's (i.e., freeway cruising). But idle and off-idle was extremely rough.

You could try isolating the affected cylinder(s), then moving those injector(s) to other positions and see if the problem follows. If so, the injectors are likely the problem. If not, you can probably rule them out.