1993 300 CE Fuel Problem

[rdooley]

1993 300CE - Car ran fine - parked it for a year. Will not start - cranks fine - have spark on all plugs - no fuel in fuel rain - no electricity at fuel pumps. Replaced fuel pump relay - still no power to pumps. Have power at relay connector on two of the 4 sockets ( battery voltage on one, < 2 volts on another ) Reading the repair info I have it looks like the fuel relay is controlled by the N3 fuel injection computer. Need help on how to proceed - where to look.

[Arthur Dalton]

Jumper terminal 30/87 at FP relay and see if rail gets pressure...[ with KO]

[rdooley]

Thanks for your response - I was not testing the fuel pump electrical supply correctly. It only works for 1 second with the ignition on and engine not running. After I learned how to test - I have power to the pumps and did get fuel to the rail. Car still did not start - I will check the ignition again tomorrow. I also suspect bad gas - it smelled " funny " when I changed the fuel filter. Again, thanks for your reply to my problem.

[Arthur Dalton]

Check the fuse on top of the OVP relay..

[rdooley]

I checked it - its ok. I am thinking I might have a fuel pressure problem - how do I check the pressure on the fuel rail ? Just hook a gage to it ?

[Arthur Dalton]

Yes ..right at the test port on the fuel rail

Car at idle with vac hose to reg connected:
3.2-3.6 Bar
At idle, vac hose disconnected :
3.7-4.2 Bar
System pressure after at least 30 min wait :
No less than 2.5 Bar.

[Krautmister]

I just went through this... Mr Dalton is the man...OVP fixed me... From the outside it looked fine, fuse was good, but the inside was a different story. OVP and be done!
Again Thank You sir!

[Arthur Dalton]

You're Welcome.............

[rdooley]

Mr. Dalton

It has been awhile, but I had a problem with the cars starter and just got it fixed. I am now back to fixing the lack of fuel problem.

I did what you advised and jumped terminal 30 and 87 on the fuel pump relay and had voltage at the pumps. ( I also have voltage on terminal 30 with the relay pulled. ) The pumps make no noise and deliver no fuel.

I dont understand how both pumps fail at the same time, or are they somehow " tied " together?

Can I test them or do anything to repair or is it best just to replace ?

Any advice is appreciated - thanks for your time.

[Arthur Dalton]

If you have 12 V power to the pumps , [ and I am assuming you mean right at the pumps ], then the pumps are most likely gummed up from the year of sitting or they are bad.
I would take them off , drain them and then put some carb cleaner in them to soak, then battery/bench test them..sometimes they will free up [ even a little tapping can help ]..
You have power to relay 30 and have manually jumpered to 87, so that assures V to pumps.. you also want to check the terminals/connectors at the pumps too.
A 12v test lamp will work better at the pump than a meter b/c you can see if the connections load drop with a test lamp.
The schematic shows both pumps 12v pos feed to have a connector X36 and the neg of each pump goes to ground..so make sure that is where you are taking your power readings from...could be simply a bad ground...

[rdooley]

Mr. Dalton

I removed the pump set and checked the individual pumps on the bench and they would bump but not run. After a great deal of carb cleaner and tapping / bumping they finally started running and sounded good. I put them back together and installed on the car. They sound and feel like they are running but I still have no gas on the fuel rail. My questions are:

Do you have to " prime " the fuel after servicing the pumps ? I drained all the fuel out of the system to work on it.

Can the pumps run but not pump the fuel - can I test ?

Restriction in the fuel line ? Can I blow air through the fuel rail test port back to the pump discharge ( disconnected ) ?

Any other recommendations other than these items ?

Thanks for any help.

[MBZ86]

rdooley--Hi, I am having some of the same problems that you have described on my 86 420 sel,which has also been sitting for almost a year.
Question for ya, after you changed your filter and drained all the fuel out of all your gas lines, did you have to prime or attempt to refill fuel lines or did you just slap the new pumps on and immediately have pressure again.

I wonder how the air that was in the lines would affect the fuel distributor or other parts of the system. I just changed my filter yesterday ( that was plugged) checked that both pumps were working (they seemed to be) but no pressure at the distributor

Just wondered if you did something that I should also do!!

Glad to hear you are up and running again. Ya done good!