Everyone, I and my shop need some help. I've searched and read every thread here and on other forums. We have the MB service manuals.

Car is a 1984 US 300D. 32K miles (yes, that's not a mistype) The symptom is a hot rough shaking idle which becomes smooth as soon as RPMs get >1K. Cold idle is smooth. And, there was a "stutter" around 1500-2000 rpm; this was fixed with injector pump rebuild/recalibration (see below). Cracking the line on the #1 injector would smooth out the hot idle as well.

What has been done (roughly in order, and after thousands of $$$$ spent):

Diesel purge multiple times
New fuel filters (after every purge)
New motor mounts and shocks
New rack damper bolt
New injectors (tested to ensure equal pressures)
New delivery valve seals
Valve adjustment
Compression test, and compression is good and equal across all cylinders
Injector pump rebuild and recalibration

(The injector pump was clearly messed with by someone before us, because it did not test correctly, so it's good that it was rebuilt and calibrated properly. Cracking the line on #1 injector doesn't have any effect anymore; that clearly was a fuel delivery calibration in the pump.)

Now, after all of this, especially injection pump rebuild, hot idle is better, but still not smooth. I have other W123s, and I know what it should be.

What are we missing here? Any help is greatly appreciated.

Could just be air insursion on the suction side of the lift pump.
Check that the fuel hoses are tight and can not be rotated.

Possibly worn out pressure relief valve, primer pump, and/or worn out fuel lift pump?
Springs can get fatigued over time, also the o-ring for the lift pump shaft may need to be replaced just due to age.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

"New injectors (tested to ensure equal pressures)"

Did the Injectors get new nozzles and then have the pop/opening pressures set so they are balanced?

Are Bosh Nozzles in them or an other make of nozzles?

I have in the past had some shaking caused by an air leak on the fuel inlet side.

Just curious is it a 84 Federal or is it an 84 California Car?

I have not had this happen but if the EGR was opening when it was not supposed to that would also cause a shaking issue and the EGR has a temp sensor. How about up on the valve cover where the little valves with the plungers and levers and usually the levers wear out and allow the plungers to come out more then they should?

To test it is simply easier to disconnect the Vacuum line at the EGR. In theory that should take the EGR out of the picture.

If yous till have another W123 that is not shaking abnormally try swapping the Injectors from the non-shaking W123 and see if that cures it. Just for trouble shooting.

When you have the Injectors out look inside of the Pre-combustion Chambers and get some hook type tool and see if the Ball Pins are in good shape and if they are loose or not.

Is the idles speed within the normal range.

You could also try backing off the rack damper bolt and adjusting the idle speed with the normal idle speed adjustment to the high end of normal idle speed range and adjust the rack damper bolt.
In theory you are supposed to be able to run in the rack damper bolt in and remove as much shaking as possible with out raising the idle speed.

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84 300D, 82 Volvo 244Gl Diesel

From the Hand Primer?

I don't know what is claimed to have been done on in the rebuild of the Fuel Injection Pump but when they do a complete rebuild they also rebuild the Fuel Supply/Lift Pump.

The original poster said that the Delivery Valve Seals were replaced. This makes me think that his Fuel Injection Pump was not entirely rebuilt but re-calibrated.

Also there is that Fuel Pressure Relief/Overflow Valve which may or many not have gone with the Fuel Injection Pump when what ever work was done on it.

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84 300D, 82 Volvo 244Gl Diesel

Yes, air incursion on the suction side of the lift pump would include the hand primer pump, primary (clear plastic) fuel filter, metal fuel lines, and fuel hoses all the way back to the fuel tank.

Hoses were listed a first checkpoint as they are not always replaced, and can leak air when re-used. BTDT.

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

You have done quite alot. Next steps are to:

1) Rebuild the lift pump (not the hand primer pump but the lift pump it screws into)
2) Replace the OFV spring

Greazzer aka Dieselfuelinjector guru does this quite effectively.

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Why I will never do business with "DieselKraut" again

http://www.peachparts.com/shopforum/mercedes-benz-used-parts-sale-wanted/378935-why-i-will-never-do-business-dieselkraut-again.html

Thank you for all of the help and suggestions! Wow, I just posted this last night. What a community.

Injection pump was completely rebuilt AND recalibrated by a Bosch diesel center. This included the lift pump.

All fuel hoses are tight, no leaks, and there shouldn't be any air incursion. But probably need to double-check all of that again.

And I forgot to mention in my original post: valve clearances were checked multiple times as well, and all valves are adjusted properly.

Idle is at 750 rpm.

Remember, it's only a hot idle problem. Cold it's smooth as silk. I would think that if there was an issue with fuel delivery, it would occur both hot and cold.

I like the idea of checking the EGR system, because that would be something that would change with engine temperature. We will do that next.

Why was the injection pump in need of a rebuild before 32k. Seems like there is more to the story?

The injection pump was rebuilt/recalibrated because it was clear someone had "been in there" before. The yellow paint marks were broken on some of the bolts. And, frankly, we felt like we had tried everything else.

Turns out that it was good that it was rebuilt/recalibrated; it HAD been messed with and was not setup right.

The car is a true 32K mile car. Not sure why someone was in there, but they were.

(It's weird; I had posted something else earlier but it was held in moderation and hasn't appeared.)

First of all, I'd like to thank everyone for their comments. Wow, I just posted this last night; this is a great community!

I updated my first post to reflect that I had forgotten that we have checked valve adjustment multiple times, and it's exactly to spec.

The interesting thing about this is that it's only a _hot_ rough idle. Cold it's very smooth. So something is changing with engine temperature.

Fuel hoses are all tight and there are no leaks. But we will go back and verify that again, although I don't think there is any air incursion. And, if it really was a fuel (or air) bleed issue, that would have an effect cold or hot, right?

I like the idea of taking the EGR out of the circuit. That is definitely something that would work differently cold and hot. Something that we definitely need to investigate.

Thank you again for all of the thoughts, and I will update the thread as we go forward. Needless to say this is very frustrating. :-)

If the valves were out of adjustment when you did them it may be worth another adjustment. It's not uncommon to have to do them twice.

Have you gotten the engine fully warmed up and run it very hard, even uphills at long full throttle stints if you can?

32K, is it a gem?

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Valves were never out of adjustment. All lash was right on spec.

I've driven the car a few hundred miles, and have driven it _very_ hard. I know the car was sitting for a long time (obvious with the mileage), and I wanted to make sure that it was thoroughly "blown out". Hasn't made a difference, unfortunately.

And, yes, it's a gem. There are many "tells" for the low mileage, and I believe it to be original. I recently got another lower mileage car, but my heart is in this one for some reason.

Just checking that you have seen my post above? I experienced similar issues as you did, and having tried everything, was able to narrow it down to the two items I shared.

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Why I will never do business with "DieselKraut" again

http://www.peachparts.com/shopforum/mercedes-benz-used-parts-sale-wanted/378935-why-i-will-never-do-business-dieselkraut-again.html

Sorry; lift pump and OFV were addressed in injection pump rebuild. The pump was done by a Bosch diesel service center and tested and calibrated on the Bosch test equipment.

Wow you have done a lot. I went through all of this a few months back.

Crazy idea here, but maybe you’re putting too much faith in the Bosch test. They test cold not hot. Maybe something is going out of spec in the pump when hot. Maybe something was missed during their pressure test. The car is basically new. Maybe there is some crud or swarf in the pump that binds as it thermally expands.

The other tell is that somebody went in the pump before you…at 32k or earlier. This has probably been an ongoing thing.

I would be very tempted to go down to the JY and pull a pump and swap it in just to see what happens. It doesn’t cost much compared to what you’ve already done.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles