Strange fuel supply problem - 500SLC (M117 K-Jet)

[Greg in Oz]

The car:
1981 500SLC 107.026 with 117.960 "Euro" spec (standard K-Jet without lambda O2 sensor).

History:
Has been in storage since it moved from Europe to Australia around 7 years ago. I purchased it late last year and have been preparing it as a club car with a view to having it attend its first event next weekend.

The problem:
With low fuel level in tank (around 15 to 20 litres remaining - not low enough for reserve light) it suffers fuel starvation symptoms when driven. Will perform well including strong full-throttle accelaration but will suffer intermittent power loss (stumbling or even stalling) after braking or cornering.

What I have checked:
Drained and flushed fuel tank. Cleaned strainer in bottom of tank. Tried another fuel pump (used one from my 190E parts car). Replaced fuel filter. Checked small strainer at inlet to fuel dsitributor. Temporarily replaced fuel pump relay with jumper wire. Checked fuel pressures in line between fuel distributor and warm-up regulator (1 bar at cold start rising to 3.6 bar at normal operating temperature. Around 5.5 bar with line closed between fuel distributor and warm-up regulator). Checked distributor cap, rotor and ignition timing all OK.

Other observations:
Both original fuel pump and replacement become noisy after about 10 minutes operation (yet both reach pressures listed above). Engine runs fine until car is driven. Vigorous shaking of car (whilst stationary) can cause engine to stumble (or stall) and hot fuel pressure to momentarily drop back to 2.5 - 2.8 bar. Listening through tank filler it is possible to hear a sound like air being drawn into a vortex when this occurs suggesting air being drawn into strainer and pump. This is worse with less fuel in tank and can only be prevented with fuel gauge showing tank more than 1/4 full (at least 25 litres in tank). If fuel level is reduced to the point where reserve light illuminates (less than 15 litres) car becomes undriveable due to engine stumbling and stalling. I have also noticed an intermittent sound like a relay clicking in the passenger's footwell near fuse box (where all relays are located), although I believe I still heard this sound with the fuel pump relay removed and replaced with a jumper wire.

Any suggestions as to where I go from here? I am considering purchasing a new fuel pump due to those I have both being noisy even though they show good pressures. This will eliminate any doubts about the pump.

Thanks,
Greg

[joseph c]

the fuel tank pick up tube is cracked or has a pin hole in it. inspect it very carefully, ha dthe same problem with a circle track car. just a thought

[Greg in Oz]

Joseph c,

Thanks for that suggestion. I'm not sure that it could apply to my situation though. I have attached a diagram of the SLC fuel tank and you will notice that there is no pick up tube exiting from the top of the tank. Instead there is a swirl pot (not visible in diagram) in the bottom of the tank into which the strainer is fitted. The return line from the engine returns fuel to this swirl pot. An outlet hose screws into the strainer which allows fuel to flow directly to the pump mounted below the tank. A hole in this hose would show as a fuel leak under the rear of the car (which I don't have).

Thanks,
Greg

[david s poole]

have you done a fuel quantity test.line at the fuel distrib should flow 1 litre in 30secs.

[Greg in Oz]

David,

Thanks for that suggestion and sorry for the delay with my reply. No I haven't performed a fuel quantity test yet (mainly due to the lack of a hose with the required fitting to fit in place of the return line from the fuel distributor). Unfortunately, since the problem really only shows when the car is driven, a fuel quantity test may not have shown a problem as will become evident as I describe how I eventually found the cause of the problem.

When I previously drained and flushed the tank, it was still fitted to the car. To flush it I removed the strainer and poured fuel and methylated spirits in through the filler and collected it as it drained from the opening the strainer fits into. I used methylated spirits to hopefully absorb any water that may have been present in the tank. I should have noticed that no matter how quickly I poured liquid into the filler, the rate at which it drained from the bottom was slow as this was actually a clue to cause of the problem, as was the fact that the car had been in storage for many years.

On Saturday morning I fitted a new fuel pump hoping this would fix the problem. For a while it ran fine but after about 20 minutes of driving the engine again stumbled and I noticed the new fuel pump had also become noisy. Obviously there was a restriction, presumably at the strainer, and the pump noise was due to cavitation. It was time for the tank to come out for a better look.

With the tank removed and the fuel guage sender (transmitter) and strainer removed from the tank I could have a look inside. Firstly I noticed the strainer was indeed blocked with what looked almost like mud. The bottom of the tank was also not clean. Obviously with the car stationary all this dirt would settle and allow it to run fine. When driving the car it would get stirred up, eventually blocking the strainer.

I plugged the opening in the bottom of the tank and added a few litres of fuel and gave the tank a vigorous shake. By then draining it quickly through the filler I was amazed at how much dirt washed out. The dirt included fine muddy silt and a small amount of rust. There was also still a small amount of water present. I repeatedly shook and flushed the tank with both fuel and methylated spirits until it looked fairly clean inside and no more dirt could be flushed out. By filtering the fuel and methylated spirits and allowing time for the dirt to settle, I was able to use much of it for several flushing attempts. Flushing and shaking the tank took an hour or more so I felt like I'd had a good workout at the end of it all!

I was also now able to see and understand how the swirl pot which sits on the bottom of the tank surrounding the strainer functions and understand why the strainer was drawing air with the tank less than about a quarter full. The fuel return line (from the engine) enters the bottom of the swirl pot via a venturi which is designed to draw fuel into the pot from the tank. This way the swirl pot is kept full and overflowing even when the fuel level in the tank is below the top of the swirl pot. The dirt in the tank was blocking this venturi such that the swirl pot could only get fuel from the tank if the level was above the top of the pot. It did of course still also receive fuel from the return line, but this had to have entered the swirl pot from the tank to have got to the return line in the first place.

With the strainer cleaned again and the tank refitted, I finally had the car running well that night just in time for it to attend its first club event on Sunday morning. Unfortunately I did not get to clean the rest of the car as much as I would have liked as I had spent all my time sorting out the fuel problems. The car drove to and from the event without a problem although I did notice that by the time I got home the pump was again a bit noisy. Presumably there is still some dirt present in the tank so I may have to look for another tank, not a bad idea considering there must also be some rust present.

Thanks David and Joseph for your suggestions. Hopefully this thread may help someone with similar problems one day.

Greg

[david s poole]

i read your post with much interest and your explanation was well put but had you run a fuel volume test[just take supply line loose from fuel distrib and stick it in a 2 litre coke bottle] i think it would have pointed you straight back at fuel tank.

[rmasteller]

Originally Posted by Greg in Oz

Hopefully this thread may help someone with similar problems one day.

I've got my tank disconnected right now as I believe my 450SL has the same problem. How do you get the filler neck out of the fender? Must the rubber gasket be cut/destroyed? Am I just not pushing hard enough from the outside of the fender?

Thanks!