Hello,
I have a 1975 240d(w115) that ran when parked in a barn in 1996 and now I am trying to get some signs of life. I've gone through the "venting injection system" protocol, but the IP seems to be dead. Some posts I've read elude to the fact that it may take a lot of turning over to purge air and what not.
I have also read about replacing a diaphragm, but I don't know what problem requires that remedy. The motor has around 80,000 miles. Are there some troubleshooting procedures out there? Thanks.

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1975 240d

Somthing is missing here. The "venting injection system" protocol would also have you loosening the Injector Hard Line nuts up by the Injectors and cranking the Engine until all the air is out of the Injector Hard Lines and you see Fuel comming out from under the nuts; before you tighten them and attempt to start.

If you were getting Fuel up to the Injectors during the venting process that would be a good indication that the IP is working.

Are you getting Fuel up to the Injectors?

Think about the IP Diaphragm last as it cost around $280-$300 US + Shipping.
The Diaphragm operates on the Manifold Vacuum created by the Butterfly Valve that is in the Manifold. The Valve and the Vacuum Tubing going to the IP need to be intact and working.

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84 300D, 82 Volvo 244Gl Diesel

You're right, we were not getting fuel to the injector, so we removed a line from the the IP and there was no movement or bubbling.

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1975 240d

I would preemptively replace the lift pump attached to the side of the injection pump. Those are a common source for air to enter the fuel system. You'll also want to check the fuel tank strainer, and see if it's clogged. Don't mess with the diaphragm until you've fixed the fuel issues.

Other comments: Some people insist on reviving old Engines with the old Fuel Still in the Fuel Tank and the Old Oil in the Engine. I do not think either is a good policy.

The Fuel Supply Pump could be removed, taken apart and cleaned and re-assembled. However, if the Hand Primer is working; it would be easy enough during a starting attempt to have someone pump on the hand primer to creat pressure.
If the Engine starts and runs while someone is pumping but dies when they stop that is a good indication there could be a Supply Pump Problem.

Also, no comment has been made as to why the Car was "Barned" in 1996 to begin with.

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84 300D, 82 Volvo 244Gl Diesel

Yeah, the handpump was what I was referring to earlier. I no longer trust the old style with the screw top plunger. They all get changed nowadays.

The hand pump seems to be fine, but I honestly haven't changed the fuel out or the oil... I was told it was parked because they didn't need an extra car. According to the stack of paper work I have, It had a Mercedes Dealer engine replacement in 1985 and the only reason given was a "lower engine problem" ? Anyway, I've emailed the previous owner and asked him again if there were any problems at all when it was parked. In the mean time I'll be draining the tank and replacing the oil.

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1975 240d

You do not have to change the Fuel in the tank yet. For test starting you can just run a longer fuel hose (might cost $5 or so to do so but you can save the hose for later use) to a suitable container with several liters of clean diesel fuel in it.
Since you would be bypassing the Fuel Tank you would also be eliminating the Tank as a possible source of the problem.

Unfortunately Diesel Fuel absorbs water from the air.
Not, makint 13+ year old Fuel a good choice when you are troubleshooting.

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84 300D, 82 Volvo 244Gl Diesel

OK, I'll do that! So, when I'm turning the engine with the fresh supply of fuel, do I need to have someone pumping the manual pump with all injector lines closed?

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1975 240d

After putting the hose into the Container with clean Fuel I would pump it with the Hand Primer to get some of the new clean Fuel into the IP housing and bleed any air out of the what I am going to call the low pressure area of the IP housing.

When I crank the Engine I would want all of the Injector Hard Line Nuts open. So, I could see if I was getting Fuel up to the Injectors.

If I as getting no Fuel Up to the Injectors; I would do everthing the same again except that I would now be pumping on the Hand Primer. If I get Fuel up to the Injectors doing this it would take that as an indication that I have a problem inside of the Fuel Fupply Pump.

If you are getting Fuel up to the Injectors; Tighten the Injector Hard Line nuts; continue to do what you did to get the Fuel up to the Injectors and attempt to start the Engine.

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84 300D, 82 Volvo 244Gl Diesel