I have an on going problem with my 1974 240D that the engine will not start. The engine had been running without any problems and was running OK when I left it parked at the airport for 5 days. When I got back it would not start. I had it towed to the shop and was told it had a leaky fuel line which they repaired and the car now starts fine. I picked the car up and drove it around some for the rest of the day. It started fine each time I started it. The next morning again it would not start. Back to the shop it went. Now the problem is supposed to be a bad lift pump. Replace the lift pump. Side note, just try and find a lift pump for a 1974 240D!! The car is running fine "again". I picked it up and again drove around some for the rest of the day, starting and stopping without any problems. This morning it will not start again.

The battery is charged, the fuel tank is full, and the starter and glow plugs are OK. When I use the hand primer pump I see some bubbles in the clear plastic fuel filter. I loosened the connection on the main filter and pumped but did not see any air bubbles come out. If I continue pumping the primer pump until the injection pump value starts to click, the bubbles in the clear filter seem to be gone. The car still will not start. Any ideas or suggestions from anyone? Is more information necessary to make a diagnosis?

These machines are relatively simple. Is yours a manual or automatic? If it is a manual and you can push start it (get the glow plugs glowing, and roll the car at a decent speed, then put it in third and drop the clutch), it might be worth knowing how much trouble push starting is - in my experience if the normal starting procedure doesn't work once ambient temperatures begin to drop, and the car can be easily push/pop started (manual transmission only), the issue lies with one or a combination of the following:
1. Weak battery-affects glow plug performance and starter speed
2. Weak glow plug or two, but it really only takes one
3. Weak starter-lower starter speed leads to lower compression heating of air charge, thus poor firing
4. Oil viscosity to high - creates too much drag on starter, so you get the effects of 3. above


If the issue is that you suddenly have a problem with no signs of degradation and pop starting is equally unsuccessful, the problem is possibly more complex. Air leaks into the fuel system are definitely a problem as you will run the battery down and possibly overheat the starter trying to use the injection pump to clear any accumulated air.

Normally when air gets into the system, Diesel also gets out, and you should see or smell Diesel fuel. I would check from the tank to the injectors. Also, there is a possiblility your tank vent, which is located at the right rear of the car and the vent line vents under or behind the right rear bumper corner, is clogged. This will cause running the car to create a vacuum in the tank, and make getting air in the system much more probable. You can check this by taking the fuel filler cap off. If you hear air rushing in, you may have a clogged vent line. The parts that clog are the rubber connectors (the vent line is made of the same stuff as the vacuum lines with rubber connectors that are dissolved by Diesel fuel and get gummy. Overfilling the tank is the primary cause for these items to get wet and then collapse and cause a problem.), which are a PITA to find and get to, but they are generally cheap and easy to replace once you find them.

Good luck, and I hope some of this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)