1983 300CD rough idle - PeachParts Mercedes-Benz Forum

rocky raccoon · #1 08-09-2014, 10:39 PM

Looking for a sanity check please.

My 1983 300CD has given me three years and about 25k miles of largely trouble-free almost daily service. It has recently developed a very rough idle hot or cold, a little worse when cold of course.

It still starts within one engine revolution after a short glow and runs smooth, quiet and powerful above idle with no smoke. I have maintained it meticulously with 6k mile synthetic oil changes, valve adjustments and filter changes when mileage dictates.

I just completed a precise valve adjustment and changed fuel and air filters. After the filter change I gave it a Italian tune-up, all with no idle improvement. The fuel in the filter looks good and has no funky smell.
Blow-by is reasonable and has not changed during my ownership.

Although the idle is rough, it does not "lope" so I think I can rule out the rack damper. Based on starting performance, glow plugs are fine. I have not checked compression but I have no reason to suspect a problem based on overall performance.

I am thinking it is time for fuel injector service. Do you agree? Have I missed another potential rough idle source? This condition has worsened over a short time and now threatens to shake the fillings out of my teeth.

If injectors, Bosch, Bozio or Monark? Bozio seems to get good press. What is success rate in replacing nozzles only? If nozzles only is balancing required?

What say you-all?

toomany MBZ · #2 08-10-2014, 12:53 AM

If the issue is only with a hot engine, the rack damper would be the prime culprit.

As you state, it is always.

This would lead me to consider a leaking fuel line, allowing air in the system.

Not to rule out injector issues.

As far as new brands go, I have no clue.

Guest987 · #3 08-10-2014, 04:07 PM

You might consider giving it a diesel purge before going and doing the injectors.

ROLLGUY · #4 08-10-2014, 04:35 PM

I have the same problem with my '81 300D (turbo engine transplant recipient). The engine has a distinct miss at idle, but above idle and through the entire power band, it is smooth. The engine still provides quick starts and has lots of power, even though the idle is rough. I originally thought the injectors needed service, so I had them rebuilt with still the same rough idle. I adjusted and re-checked the valve adjustments to make sure, but still the same rough idle. I had thought it was a bad delivery valve seal, as I had another car with the same symptoms that was cured by just tightening the holders (obvious leak). On my 300D, there is no fuel leaks at any part of the fuel system, but still the rough idle. It looks like we have the exact same problem, so I will tag along with you on this one......Rich

rocky raccoon · #5 08-10-2014, 05:59 PM

I have an injector problem, specifically #1.

Today I loosened the fuel pipe nuts at the injectors, one at a time. When I loosened #1, the harsh clatter (nailing?) disappeared and the engine ran quietly although on 4 cylinders of course.

I have ordered five Bosio nozzles and the heat sink washers. When this stuff arrives, I'll pull my injectors and take them to my local diesel whiz for cleaning and balancing. I expect this to solve my problem.

I did however also order a new rack damper bolt and a new type lift pump. Hopefully, I can get all this done in time to make the PA GTG on the 23rd.

Diesel911 · #6 08-11-2014, 03:36 AM

I was going to say the Fuel Pressure Relief/Overflow Valve.

rocky raccoon · #7 08-14-2014, 05:48 PM

Took Bosio nozzles and injectors to my local diesel truck mechanic. He cleaned the injectors and popped to 135psi. Installed and now runs like a new car. Good to go for another 300k miles.

Still a slight rough idle. New style rack damper coming that should take care of that.

cho · #8 08-15-2014, 03:19 PM

Quote:

Originally Posted by ROLLGUY

I had thought it was a bad delivery valve seal, as I had another car with the same symptoms that was cured by just tightening the holders (obvious leak). On my 300D, there is no fuel leaks at any part of the fuel system, but still the rough idle.

you might have worn DV's,replacement is only solution (or JY IP...)

cheers

ROLLGUY · #9 08-15-2014, 06:52 PM

Quote:

Originally Posted by cho

you might have worn DV's,replacement is only solution (or JY IP...)

cheers

I had thought of that, but want to try every available option before going that far. I wish mine had the same problem as the O P, it would have been fixed by now.....Rich

eatont9999 · #10 08-18-2014, 07:22 PM

After much troubleshooting and replacing of parts, I ended up replacing the delivery valves to get rid of the shaking in my newer 300SD. I pulled a used set from the JY.

7033grip · #11 08-19-2014, 12:27 PM

I changed nozzles in my '78 300CD to Monarks and still had what I thought was rough idling. My mechanic suggested new motor and transmission mounts (250,000 miles) and BINGO !!!