SL500 Fuel System Internal Leakage Test

[Krowley]

Hello, so Ive been having problems with my car. Its never been running right since Ive owned it. It is a 1995 SL500. I have another post trying to figure out an ignition system fault code. This post is mainly about the fuel system. I did a fuel pressure test and I have an attached file showing the test sequence. On the attached file below the nominal values I have my values. First test 3.586 bar (52 psi) second test 4.068 bar (59 psi with no vacuum). So both those test pass. Third test on the bottom (fuel system internal leakage) my car fails spectacularly. It fails instantly going to 0 bar/psi fast. On that test it says “if pressure drops quickly, replace check valve in fuel pumps”. What is the check valve in the fuel pumps? My car is again a 1995 SL500. Under the car I have one fuel pump and one filter. I will attach a photo of what that looks like because I know years are different. When I turn the key on the pressure jumps up but instantly falls off almost to nothing. When I turn the car off it also then drops to nothing. I had a problem with a fuel line (pinhole leak) and replaced it when installing a new fuel filter. I should probably replace all the lines.

Main question is what is the check valve in the fuel pumps? You guys are always awsome and if you could point me in the right direction, I thank you.

 

[JC220]

You have the later type single fuel pump which means the check valve is I integral in the pump and the whole pump should be replaced. (Unlike earlier dual pumps where the check valves could be replaced independently)

Best the avoid cheap ebay China pumps and go for a Bosch or similar known brand. Ideally get a genuine pump from MB.

This is a genuine pump



Looks like your car has had a wrong pump fitted by someone!

 

[gsxr]

Ditto what JC220 said. The check valve is integrated into the single pump. You have to replace the pump, either with OEM Pierbug, or OE/Genuine from the dealer.

In that photo, you have the early-style pump, which is incorrect!! And, your pump package (bracket, pump, filter) doesn't look right. The filter and pump should be tightly clamped by the bracket. Did you loosen them? If not, something is off... probably due to the wrong (larger diameter) pump. The late fuel pump package with single pump has smaller diameter pump AND smaller diameter filter, compared to the earlier dual-pump design.

Additional photos of the correct single-pump design are at this link.

 

[Krowley]

You have the later type single fuel pump which means the check valve is I integral in the pump and the whole pump should be replaced. (Unlike earlier dual pumps where the check valves could be replaced independently)

Best the avoid cheap ebay China pumps and go for a Bosch or similar known brand. Ideally get a genuine pump from MB.

This is a genuine pump

View attachment 157338

Looks like your car has had a wrong pump fitted by someone!

Ditto what JC220 said. The check valve is integrated into the single pump. You have to replace the pump, either with OEM Pierbug, or OE/Genuine from the dealer.

In that photo, you have the early-style pump, which is incorrect!! And, your pump package (bracket, pump, filter) doesn't look right. The filter and pump should be tightly clamped by the bracket. Did you loosen them? If not, something is off... probably due to the wrong (larger diameter) pump. The late fuel pump package with single pump has smaller diameter pump AND smaller diameter filter, compared to the earlier dual-pump design.

Additional photos of the correct single-pump design are at this link.

View attachment 157339

Thankyou for the quick response and knowledge. It was loose when I got in there. I actually dont even have the cover that goes over that area. Previous owner didnt do the car any justice

 

[Krowley]

I replaced the fuel pump today with an “OEM” Pierburg pump off FCP Euro. Car starts first crank and actually primes without immediately turning the car to crank. I may have to replace the fuel line from the filter to metal line that goes to the engine but overall much better response starting the car. Thanks again @gsxr and @JC220