intermittant rough idle and long cranking fixed
 

I have had an intermittant rough idle for a couple of years and just recently started to have a problem starting the car (long cranking time) after it sat for a few days. If I turn the key on but did not start it for 20-30 seconds it would start right away. In searching the archives I found the answer for the hard starting to be the Fuel Pump Checkvalves. I replaced them and the hard starting is gone but to my suprise so is the rough idle. Another easy and inexpensive fix do to the information here on the shop forum. Thanks to all who share their experiences and especially to
Steve B. and MBDoc for all of their time spent helping and explaining to those of us that do not work on MB cars for a living. If it weren't for this forum I would not have purchased a second MB. PS. Fastlane is fast and afordable. :D :D

Dave,

Can you be more specific about what it takes to replace the fuel pump check valves? My SE has had a minor rough idle for a year and the starts are getting more difficult. I'm about ready to replace the fuel filter. I sure hope the check valves are in the same area.

Thanks,
Sixto
91 300SE
81 300SD

intermittant rough idle and long cranking fixed
 

On the 300 E both of the fuel pumps the fuel filter and the accumulater are mounted on rubberbands under the car at the right rear, forward of the axle. There is a plastic cover over them held on with four 11mm nuts. I removed the fuel line to the first pump and to the accumulater (clamp them off so fuel won't flow), then remove the wires to the pumps and the whole assembly can be taken off of the rubber mounts. It was much easier to work on on the bench. The checkvalve is at the output end of each pump a 17mm flarenut wrench,a 17mm open end I believe a 14mm open end were required. Try not to round off any corners
on the pump housing. You have to get the new valve on very tight (use the new copper washers) to prevent leaks. Reassemble in the reverse and
test for leaks. Keep in mind you will be working with gasoline and don't cause any sparks. You could disconnect the battery for an extra margin of safety.

Dave,

Did anything seem not right with the old check valves? Is a physical inspection possible?

Thanks,
Sixto
91 300SE
81 300SD

I did not look too closely at the old valve yet. It is a 1.5" long piece with a ball and spring inside it and threaded on both ends with the output end sized for the banjo fitting and a cap nut. I will try to dissect it this weekend to see if it is obvious what went wrong. I don't think they would be repairable in any event. $11.20 ea in fast lane, too cheap to fool with in my opinion

Thanks, Dave. I didn't mean to suggest that the check valves were repairable or that it might be worth doing. I'm just wondering if there are distinct symptoms of bad check valves. Hard starting doesn't seem distinct enough as it could also mean a bad accumulator or leaky injectors or other things. Since you say you found info in the archives I'll look there.

Sixto
91 300SE
81 300SD

DMarble,

Did you have to depressurize the fuel system before removing the fuel line? Nothing more annoying than a being sprayed in the face with gasoline!

Since my checkvalve was leaking there was very little pressure on the system as the car had been sitting overnight since it was started. I do not know the correct procedure for venting a pressurized fuel system but I would wrap a rag around one of the clamped hoses and loosen it a little to relieve the pressure and catch the fuel.

Sixto The way I determined it to be the checkvalve was to turn the key without starting the engine and wait for the fuel pumps to pressureize the system. When I did this the car would start in 1-2 seconds if I did not do that it would take 5-8 seconds (overnight) and 15-20 after several days of sitting. It would also run very rough for a few seconds after it started until the pressure came up. A fuel system pressure check would be the proper way to diagnose this for sure.

Depressurizing...
 

Just remove the gas cap. YOu should be able to work on your fuel pump and related components then.:)

The segment to be depressurized in this case is between the pump check valve/s and the injectors, not the tank. Though ironically this whole thread stems from the system's inability to hold pressure.

Sixto
91 300SE
81 300SD