My 05 MCS stumbles when starting in the morning and accelerating. If I rev the engine a few times before I start up the steep driveway, it will make it up but really only accelerates strongly after running several minutes.

I've replaced all the injectors and the fuel filter. I connected a pressure gauge to the fuel rail and it registered about 42-44 PSI. According to the shop manual, should be 50 +/-3.

When it's cold and I disconnected the fuel line from the rail, there is little if any pressure, despite the warnings in the manual.

I surmise that the fuel pressure is leaking off causing it to struggle on start up, and once the fuel pump restores pressure, it runs better (still not like when I first got it). The fuel line connection does not appear to be leaking, but it seems loose, it slides a bit.

So, should I replace the fuel pump, the fuel line/connector or look at something else completely?

 

It couldn't be leaking pressure without leaking actual fuel. Do you smell any leaking?
If not, I would suspect the fuel pump.

I replaced the short length of rubber fuel line before the injector manifold as preventative maintenance. It's not very expensive and is getting up there in age...

 

Thanks for the reply. No, I don't smell fuel. Is the rubber line the one you replaced the one with the fitting that connects to the fuel rail?

 

Yes. It has push on fittings sealed with orings. Comes as a single unit. You can get the disconnect tool from Amazon or use your fingers if you are strong or rig something. I pull the pump fuse and crank the engine a little to remove any pressure. It WILL spray fuel everywhere if you try to disconnect it under pressure.

 

Meant to add that the fuel pressure regulator is attached to the fuel rail and can be replaced on its own. Check the little vacuum hose that should be attached to it, too.

 

+1
I'd do this ...the tiny vac line gets brittle...
Next up...I would do the pump...you know you psi is low....not dramatically, but out if spec.

 

There may be another potential issue. The flexible fuel line inside that is connected to the fuel pump may have developed a split. Others have had the same issues (as do I) and found that was causing the fuel pressure loss. You would not smell gas as the fumes are contained within the fuel tank. I will be checking this out on mine shortly.