1987 300sdl won't start after sitting overnight - Page 2 - PeachParts Mercedes-Benz Forum

Diseasel300 · #16 12-29-2016, 11:37 PM

If I'm not mistaken, the 60x engines don't use a rebuildable lift pump. They are different to the 61x series.

JustJim · #17 04-07-2017, 06:40 AM

Sorry for the delay in the follow-up. I do appreciate all of the insight you all have given, and I hope to be back to working on the 300SDL this weekend. Had a bad accident right after the last post and haven't been able to work on the car since, but now the cast is off and I'm starting to walk a little better. I'll let you know what I find with the relief valve.

Thanks again to all.

dkr · #18 04-07-2017, 11:23 AM

I just wanted to add, I had a similar incident with the plastic pre-filter. I had put a brand new one on and the old one had a hairline crack that was practically invisible. Once I put a replacement in, all of the symptoms you mentioned went away.

Dkr.

JustJim · #19 04-10-2017, 09:41 AM

I removed the 3 banjo fittings and changed the filter housing hoping this would eliminate the pressure relief valve question. Hooked up the electric fuel pump and purged the injector lines again. Removed the electric pump and reconnected the fuel line to the lift pump. Started the car and let it run for about 15 minutes, when it started to run a little rough. Increased the rpms to smooth out the engine and it died and wouldn't start again until I hooked up the electric pump and purged the injector lines again.

Also, I already removed the plastic in-line filter upstream of the lift pump. That's where I am plumbing the electric pump. I wish it had solved my problems!

Diseasel300 · #20 04-10-2017, 10:30 AM

Connect the fuel supply directly to the lift pump and try again. A common source of air ingress is the fuel heater. The electric pump will largely avoid that problem since it has a constant flow. The lift pump is a piston action and will suck quite a lot more air than the electric pump will. Additionally, if you have the electric pump where the plastic inline filter usually goes, you are supplying pressure to the usual air leak points, which leak under a negative pressure.

JustJim · #21 04-10-2017, 11:07 AM

Fuel supply already goes directly to the lift pump since I removed the plastic in-line filter. The heater was bypassed when I bought the car.

dieselbenz1 · #22 04-10-2017, 11:20 AM

Did you replace the seal rings on your large fuel filter? I had your problem and that was my source of air overnight. Some one else also had a hairline crack on the same fuel filter housing.

Diseasel300 · #23 04-10-2017, 11:36 AM

Since the fuel heater is bypassed, it's time to look upstream of the lift pump. The canister filter can cause the fuel to leak down, but it won't cause an airlock situation during normal operation since it is under pressure, not vacuum.

When was the last time the rubber fuel lines were replaced? It might be a good time to do that now. If they're old and hardened, they will leak air. Crawled under the car and checked the rubber lines coming out of the fuel tank? Same thing as above - if they're old they can leak. Usually if you have an issue here, they'll be wet or "sweating". You could also have a pinhole in one of the hard fuel lines under the car. It doesn't take much.

The bypass/overflow valve in the M pumps that the SDL's use operates differently than the 617's did. The orifice in the valve is very small in comparison to the earlier valve and serves as a restriction against the pump pressure to maintain a positive pressure in the fuel rack. The spring is very weak in these valves and serves mainly to act as a check valve to prevent fuel drainback. While this can cause an issue when the car sits, it won't be an issue when running.

Get a strong light and shine it in the clear (probably now brown) line going from the lift pump to the canister filter housing and watch it when the engine is running. If you see bubbles, its time to find the air leak in your fuel lines.