Fuel starvation at start up ?
 

About a month ago I went out to start sons 240D. It would start run ruff and die. After the third attempt it finally stayed running and started idling normal. I changed the fuel filters and this I thought had taken care of it. My son informed me the other day it’s done it to him a few times the last couple of weeks. Other than the lift pump what else should I be looking at? Also how can I check the lift pump?

Eyeball the lines for weeping from front to back. Rubber deteriorates, steel lines rust.

Strainer in fuel tank?

You might want to check the tank for negative pressure after running the engine a bit. I've heard those lift pumps will collapse a tank if the vent get plugged.

If it does it mainly from a cold start-up, I would suspect the glow plugs first. Bosch and Beru are the recommended brands.

Yes it is at start up. Glow plugs are probably 5 years old but we’ve only put maybe 20,000 miles on if that many since then. I hadn’t thought of the fuel strainer in the tank. Why is it this stuff always happens when it’s cold. Can’t get it in the garage, even if I could it’s not heated. I’m pretty sure the lift pump is good, no problem priming after changing fuel filters.

I've had glow plugs fail with much less than that. Sometimes that area of the pre-chamber gets a build up of carbon and a reamer is needed. You can usually get a general idea of their condition by using a multi-meter set on Ohms. A typical reading from the connection to ground is about 1.0 ohm. Just two failed ones will give you problems.

I will check the glow plugs this weekend. How do I check the gas tank for negative pressure?

Run until the engine stalls. Then open the fuel cap slowly. If it's under vacuum, you'll hear it. It the weather unusually cold where you are? A tip from up north: I used to glow my '79 123 for two or even three cycles before starting when the weather was cold.

So will just one. Especially on a 240D.

If this car has serial plugs, it wouldn't start with one bad plug.

I’m pretty sure that it’s not a series circuit. I think I checked that out when I bought it.
It has been cold here in Southeast Missouri. Temps have been anywhere from below 0 degrees Fahrenheit to above 60 degrees all in the same week. But mainly in the around 20-30 degrees range the last month & a half.

At zero you can have problems with fuel gelling, especially if you still have last summer's fuel in the tank. And you would definitely need at least two glow cycles to get it warm enough to start reliably.

you can easily check the tank prefilter by swapping supply and return lines in the engine compartment. Both hit the bottom of the tank, and the car can run just fine drawing from the return line.

If the problem changes, then consider changing your prefilter.

It takes 1 min to check resistance of my glow plugs in my 300D's, so should take 4/5 min for the OP's 240D. I do it every time I change the oil, or my son brings one home. I found one bad GP last time, which improved cold starts. Please report back since others are reading and using this info.

An air suction leak would be there all the time, or perhaps worse when hot. A clogged strainer would affect flow immediately since the fuel is incompressible. Since it idled fine once warm, it sounds like a bad GP and/or low compression is most likely.