Have a new to me 99 that runs great, but it’s a real hard starter. New glow plugs, relay is fine, o rings were replaced on plastic lines. Injectors rebuilt, new filter. The prevailing theory is either air in the lines, which I can’t see, or a weak lift pump. My idea was to place an electric helper pump around 5 psi on the supply line just as it enters the engine compartment. Would this help me out?

Did you replace the shut off valve O ring as well? Does your car leak fuel at all?
A leaky shut off valve is known to do this.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Yep be sure the o-ring between the shutoff and the pump is good. Also don't forget the o-ring on the 'pool strainer' pre filter. If any air gets in the system on this engine, you are going to have a hard time. Get an LED flashlight and shine it behind the clear hoses and look for bubbles after it has sat. Park it nose up to enhance the process.

You may want to think about replacing the SOV due to age, mine had a crack in it, and it was leaking fuel out the connector.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

How about doing the obvious first and check compression. If it’s good there is no need to jerry rig some home made solution.

Compression isn't the issue. My SDL has crap compression on two cylinders and it fires right up. OP claims it runs fine.

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Now it’s missing on cylinder 5. Cracked lines to see which one and 5 makes no difference. Made a odd mechanical clatter and started missing after that. Is something ruined or just the injector?

Definitely fix all the fuel leaks first, Did you use the proper size and material o-rings on the fuel lines and change them all? Changing the delivery valve seals and shut off valve o-ring, fuel filters and the rubber line from the hard lines to the engine would help too. Get as much of it from the dealer as you can, trying to save a few dollars by not using quite the right part is basically installing an "unreliable beater" conversion kit.

Hold up OP. We need the WHOLE STORY. First you said it was running fine, now you say the engine is missing.

So the car all of a sudden developed miss?

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1998 Ford Escort ZX2 5 speed - 279,000 miles My Daily

1992 Mercedes 300D 2.5 202,000 - Pure junk
2000 Mercedes E320 Black - 136,000 miles - Needs repair

Don't forget to grease the screw and threads on the spring compressor.

Well people are telling me I don’t have compression on that cylinder based upon how it sounds on startup and to check valve clearances. Is that even possible on a om606?

Whoever said check valve clearances... ask them how you check the clearances on an engine with hydraulic lifters!

How can someone also tell if it has poor compression without physically doing a compression test?

Take a video and let’s see what’s going on

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k