'76 W115 240D, 75K miles. The car sat garaged and little used--10K miles since 1986. I've done extensive work on it to get it refreshed and show ready. I've serviced the engine pretty thoroughly, including:

Remove and reseal fuel tank
New fuel filters
New fuel lines
New fuel strainer
New glow plugs
Injectors rebuilt
New primer pump
New antifreeze, all hoses and belts, radiator cap, thermostat, water pump
Oil bath filter service
Vacuum pump diaphragm and check valves
Motor mounts
Valve adjustment
Three oil and filter changes
Two IP oil changes

I have been noticing what I thought was fuel/oil dripping from my cross member, particularly after highway driving. It took a trip underneath to realize where the leak was. I was losing the contents of my injection pump, hence the clean oil/fuel mix. It was leaking from the diaphragm and the seal behind it, and pouring from the bottom of the governor housing. Fuel/oil was also being sucked through the vacuum hose into the intake. I contacted the local Bosch shop and got a diaphragm and gasket, and went to work.

Upon removal and inspection, I see that the diaphragm is completely saturated with fuel. I extracted the contents of the pump and it was the fuel and oil I would have expected to find. When I removed the diaphragm I couldn't find any rips or tears. I can only guess that with such limited use over the years, the diaphragm dried out, became porous, and failed.

I thought the car ran well before, but wow! It's quieter, much more responsive (ha!), and the nailing is gone. Except.........

Now it smokes like crazy on startup, idles at a higher RPM, and shakes while doing it.

I'm not sure that it wasn't idling much too low before. In fact, applying the brakes at idle, hot or cold, seemed to strain the engine similar to turning on the AC. That has stopped, and the oil pressure at hot idle is at a much more likable level (above 30, finally).

But the smoke on startup and the shaking is new. What else should I look for? I haven't done a compression test yet, but it starts up immediately with minimal glow, even with a saturated IP diaphragm, plus low mileage verified by receipts from new, a compression test hasn't seemed like a priority.

I can't comment on the Diaphragm issue excet to say that there is shims and springs that need to go back where they were on the old diaphragm.
Even then there is a tool that you are supposed to use to do the above.

I am going to comment on the Fuel Oil inside of the Fuel Injection Pump.

There is 2 areas where it can come from. One is there is a little O-ring that seals the shart/push rod on the Fuel Supply Lift Pump. There is a rebuild kit for the Fuel Supply/Lift Pump that is somewhere in the $30-$40 range (I have never priced one so I am not sure).
I copied this from someones thread: "Bosch Fuel Supply lift pump kits
Bosch kit# DGK301 ($14.60 CDN).
rebuilt kit for the IP for the W115 300D (75-76) is also still available: DGK128 ($37.25 CDN)"
I don't know if the part numbers are correct for you specific Lift Pump. There should be a number plate on the Pump itself.

More Fuel Supply Lift Pump rebuild kit stuff
Refreshing the fuel pump on an OM617 (European spec – non turbo)

The other source of Fuel getting inside of the housing is from the Elements (plungers and barrels) being worn and or scratched. Even in a new Fuel Injection Pump there is some Fuel leak by because that is what luricates the Elements.

As far as overfueling goes if there is a vacuum leak the Governor wants to give you more fuel.
I believe there is also somthing on the Fuel Injection Pump that hookes that Gorilla knob that is there to give you excess fuel for starting. I can't comment on how it works. Someone who owns one will need to chime in on that.

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84 300D, 82 Volvo 244Gl Diesel

Thanks, I knew about the shims and springs, and was very careful to reassemble them correctly. I saw the gauge in the FSM, but obviously don't have one. I'm not unhappy (I don't think ) about the current idle, although I might soon attempt to test it. I think I saw a remotely operated tachometer online somewhere??

This one works well and you can get it for less with one of their 20% off coupons (or when on sale).

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

The shaft that is on that Governor Housing can also be the sourc of a vacuum Leak.

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84 300D, 82 Volvo 244Gl Diesel

I had seen that shaft as a possibility also, but didn't attempt anything other than cleaning and lubrication.

I drove it today on a 130 mile trip, mostly highway speeds. It really starts to beg for an overdrive at about 60mph--measured by a smartphone app as the speedo is 5-8 mph optimistic at that speed. Here's the skinny on how it's acting:

On first start, glow until the element starts to glow, 10-15 seconds. Glow plugs are new, not upgraded, but I've never timed it. Engine fires immediately and runs. Belch of smoke, likely a little oil, but mostly fuel. Once cleared, smoke is minimal, but still there.

Idle is a bit high, and the idle knob on the dash has little effect on it. Shaky, enough to shake the whole car when the car is stopped.

Drive 35 or 40 miles, and it runs like a champ. No smoke to be seen in the mirrors, acceleration and shifting is at an all time high, no complaints. I stop and check the fluids. All is good, and no leaks I didn't already know about. Idle is still high, and the dash knob has zero effect. Shaky at idle.

In the town portion of the trip, same story. Two out of three times at idle it shakes, one out of three times it is smooth as silk. Idle speed never changes.

When starting hot, I still have to heat up the glow plugs about half time, if I don't, there's black smoke, proportional to the amount of time the starter spins.

Drive it home, stopping to fill up with fuel. It got about 23MPG. Two weeks ago, before the diaphragm change, it got 25.8 on a 106 mile trip. There's a pretty large margin of error there, using a mix of optimistic odometer readings and google maps.

Pull it into the garage, and shoot this video. Mild shaking and high idle.

https://www.youtube.com/watch?v=PePXVZUpZaI

And here it is this past February.

https://www.youtube.com/watch?v=cJ7Vy6qI4Ow

I need to find the happy medium, and get rid of that shaking.