Special Cirumstances rough idle WTF?

[aieeegrunt]

I've had an issue recently surface with my 1978 300D. Occasionally I get a pretty intense WUMP WUMP WUMP that shakes the car a bit, but if and only if the engine is fully warmed up (temp just above 80 and oil pressure at 2 bar), it's in Drive, and it's not moving (i.e. stopped at a red light).

It only happens every so often, and if it's happening and I put the car in neutral or reverse or park it stops. If I take my foot off the brake and start moving it stops. It sounds and feels like a bowling ball in a cement mixer under the hood. Otherwise the idle is pretty smooth.

The car drives and shifts normally. Oil level and trans fluid is normal.

My brother in law is a diesel mechanic. We drove around all day Saturday in the car and of course the car behaved itself the bugger. My brother in law hazarded a guess that maybe air is getting in the fuel feed somehow but without seeing the problem first hand couldn't really offer an diagnosis. I have one slightly loose injector return line (the braided hose) that seems a little damp, other than that I see no fuel leaks.

It's a non turbo 617 engine. I'm not sure what the deal here is or how to solve it, any advice would be greatly appreciated.

[Diesel911]

Due to the about 14.5 psi of Air Pressure small cracks or other leaks may not leak fuel out. It is cheap to change the to Fuel Inet Hoses. 5/16" id Hose from the local Auto Parts store is rated for Diesel Fuel.
If when you pump on Hand Primer it leaks Fuel it could also be allowing Air to come in

Unfortunately there is a list of things besides Air.

Need of a Valve Adjustment

Injectors need rebuilding or cleaning

Idle speed too low

Governor idle/Rack Damper Pin/Bolt needs adjusting or a new one

Motor and or Transmission mounts.

If your Crankcase Breather is restricted the pressure pushes on You Shutoff.

Got a Tank of bad Fuel

On mine doing the Valve Adjustment and rebuilding the Injectors took away the bulk of the Shaking.
Followd by replacing the Motor and Transmission Mounts.

The first thing I had done was to buy a New Gold Rack Damper Pin but it did not help until all the other stuff had been done first.

Try driving without your Fuel Fill Cap on and see if things improve. If so your Fuel Tank Vent is restricted.
If when you have a Full Tank of Fuel and are on level ground the trouble goes away I could suspect a Fuel Supply problem in the Fuel Supply Pump or the Fuel Pressure Relief Valve.

[aieeegrunt]

As an update I'm replacing the various fuel lines, and I've decided to pay somebody to do a valve adjustment on it. I know I could probably do the latter myself and at some point I will, but it's been at least a dozen years since the adjustment was done, and there will probably be some shenanigans because of that like the valve cover gasket being melted on or what not. Probably should let somebody with better tools and experience do it.

[barry12345]

Should do it yourself. It may need done several times after the first time if many miles have been driven. Can become pretty expensive. In otherwords once may not be enough.

Any carbon that has built up on the valve seat/valve face area would get pounded off changing the adjustment. Actually making it tighter again somewhat.

Depends on several variables but the adjustment should be checked again perhaps in a week or two after the initial adjustment. Just to make sure this has not ocurred. Milage and usage pattern have a large influence on this.

[Diesel911]

What Barry said.

[Diesel911]

X2.
After driving sever 100 miles after the the Valve Adjustment the crud built up on the Valve Faces and seats is going to wear off and the clearences will get tighter again.

At least that is what they say on the various MB Forums.

[Zacharias]

Originally Posted by Diesel911

Idle speed too low

Governor idle/Rack Damper Pin/Bolt needs adjusting or a new one

Three notes:
- FWIW, I agree with the idle speed comment
- If it happens after a highway run that can be a symptom of a governor wear, I had one car that did that
- the OP stated it's a non-turbo so there is no rack damper bolt

[aieeegrunt]

Originally Posted by barry12345

Should do it yourself. It may need done several times after the first time if many miles have been driven. Can become pretty expensive. In otherwords once may not be enough.

Any carbon that has built up on the valve seat/valve face area would get pounded off changing the adjustment. Actually making it tighter again somewhat.

Depends on several variables but the adjustment should be checked again perhaps in a week or two after the initial adjustment. Just to make sure this has not ocurred. Milage and usage pattern have a large influence on this.

I'm letting the mechanic do the first one. Since it hasn't been done in a dozen years there could be various shenanigans with getting the valve cover and gasket off, not to mention the various stuff connected to the top of it. Once that has been done and stuff is "freed up" I'm more confident about doing them myself without breaking a bolt or linkage or something that hasn't been touched in a decade.

Replacing a bunch of fuel lines has quieted the motor down quite a bit all on it's own, but after a while driving at operating temp the WUMP WUMP WUMP does reassert itself.

Once this is done I'm going to replace the old loop plugs with pencils and probably put new injectors (or injector nozzles) in as well. That job I'm more confident doing myself since it's more of a question of having the right sockets and torque wrenching and whatnot. Now that I've said that I'll probably shear off a glow plug or something.

[aieeegrunt]

...and I completely forgot to update this thread!

The valve adjustment and alternator belt were done a while ago. The improvement in starting and running after the adjustment was incredible, not that there is any doubt but this should always be the number one thing to do or make sure has been done on these cars!

Recently replaced both motor mounts. Another huge improvement in reducing noise and rough idling.

I've also replaced most of the passenger side floor. The driver side will get done when I get a guaranteed undisturbed weekend where I know I won't have to drive the car anywhere.

[Whiskeydan]

Wow! Valves out of adjustment caused the floor to fall out?

It must have been really shaking.

[aieeegrunt]

Heh heh heh let that be a lesson to the rest of you!