Is it time for a new fuel pump? - PeachParts Mercedes-Benz Forum

Omaha 560SL · #1 07-02-2018, 11:35 AM

I bought a 1988 560SL in 2011 with 134k miles that now has 147k miles. It was well-maintained, but I just replaced the factory fuel filter last year - my Mercedes-only mechanic said it still had some of the "goo" spray on it from when it was shipped from Germany. Regardless, while it always starts right up, even after storing it for 6 months, it occasionally would not start if I tried to start it again shortly after shutting it down at operating temperature. I just thought I was flooding it and would wait a while, then try again and it always started again. However, after about a 125-mile drive yesterday, which included two stops, I got home and shut it off in the driveway, then went back to move it and it wouldn't start. Turns over fine and fires just a little bit at the initial turn-over, but won't keep running. When I took off the air cleaner to vent it, that didn't help, but I found it would fire if I splashed a little fuel in the intake, but only until the fuel was gone. I know I need fuel and spark, and that I have the spark if I manually add a little fuel, but is there a good way to confirm the fuel pump is the problem, versus an electrical problem not powering the pump?

Diseasel300 · #2 07-02-2018, 11:40 AM

The fuel pump RELAY is a very common source of woe on the later 107's and other models that use the same fuel injection system. The relay is actually a printed circuit board and they're known for lousy solder joints and burned traces caused by hot connections. I'd start there. Another common source of problems with hot-starting is the fuel accumulator, however if the fuel pumps are working, the car will splutter to life after an extended crank.

Omaha 560SL · #3 07-02-2018, 11:43 AM

Thanks for the quick response! I see fuel pump relays are listed in the Parts Catalog and would rather try replacing this first, so is this relay also located behind the glove box? (I remember having to replace some other relay that required removing the glove box). Thanks!

Diseasel300 · #4 07-02-2018, 12:21 PM

Not sure where it's located on the Gen II 107's. My '83 just has an ice cube relay in the kickwell so not much use as a reference. I'd imagine if you did a Google search, someone either on this forum or over on BW has done a writeup with pictures. Fuel pump relays are a pretty common source of woe on the CIS-E cars.

rowdie · #5 07-03-2018, 10:17 AM

Fuel pump relay is behind the glove box. Do you hear the pumps for a few seconds when ignition is first turned on? Does it start cold?

Your symptoms point to the accumulator. If is located above the fuel filter. It maintains pressure in the system for a time to assisted in warm starts. When it fails the pressure bleeds off making it hard to start.

Omaha 560SL · #6 07-06-2018, 10:10 AM

Thanks for everyone's input! While I had replaced the original fuel pump relay within the last 5,000 miles (or one that had been replaced with a genuine Mercedes part), I went ahead and ordered another URO relay from the parts catalog and it arrived yesterday. Before replacing it, I tried to start the car once more, but it just turned over and wouldn't fire. So, I replaced the fuel pump relay and when I turned the key, it fired up right away and I took it for a drive around the neighborhood. While I am disappointed my URO replacement part only lasted about 5,000 miles, at least it is a lot less expensive than a fuel pump and much easier to change. I still have the original Mercedes part in my glove box, which I will keep there in case the URO part goes bad again, since the Mercedes part would work occasionally and the URO part that I just replaced would not. Thanks!