New Fuel Pumps

My last attempt at trying to fix the high-rpm misfire was changing the fuel filter and spark plugs. In addition to those, I've got new spark plug wires, the distributor cap and rotor are as good as new, fuel mixture has been verified in a machine. None of this fixed the problem: at full throttle, in a long gear, at about 3200rpm (120 kph or faster), the engine begins to misfire and no longer pulls clean. The performance just goes away.

It's starting to feel like this is a fuel pressure issue. I'm saying this because I can pull full throttle accelerations around town all day long, until I go so fast I dare not keep going. It feels like the fuel system has just enough pressure in there for normal driving, and to provide short bursts of acceleration.

All Gen-2 V8-powered W126 cars have twin fuel pumps arranged in series (not parallel, mind you). Fuel system pressure is around 6 - 6.5 bar. These V8 barges take a lot of fuel to propel at full acceleration. I have measured 6.6 bar of pressure in my car:

This is a little high, but then the pressure meter could easily have an error margin of 0.5 bar.

In any case, I'm thinking, one of my fuel pumps may be weakening, which they have a tendency to do with age. The fuel pumps provide just enough fuel and fuel pressure to get around, but not enough to really get the performance out of the engine. When a lot of fuel is needed, the supply just is not enough. The other option is that my fuel pressure regulator has failed, but since it's more expensive than the two pumps, and a fellow enthusiast declared they never fail and I'm stupid for even considering it, I'm first going to try the pumps.

I ordered two OEM Bosch fuel pumps from MB Classics in Vechta, Germany. The pumps are generic so they will fit almost any M-B from the period, in either a single pump or twin pump configuration. Part number is 0580254911 and they cost roughly 140 EUR per pump.

The pumps are shipped with connectors for the electrics and some extra fuel line bits. The latter will probably come in handy, because fuel lines are usually rusted solid...

So, come Saturday, I had access to a garage with a maintenance pit. I got down to business with the pump swap operation...

This is an old photo. It shows the general arrangement of the fuel pumps. As I mentioned earlier there are two pumps in series. After that, the fuel filter (the void in the photo), and after that, the fuel pressure regulator. Fuel inlet from the tank is in the very front of the image, on the right. Exit from the fuel pressure regulator is hidden quite deep within the rear suspension. As you can see in the photo, the frame of the left fuel pump is not connected to the big frame. I didn't notice this at the time but today installed a bolt to hold the pump in place.

From the pumps onwards until the pressure regulator, fuel travels in metal pipes. These are prone to rust and will snap if stressed excessively while opening nuts.  New ones are expensive so I had to be extra careful while swapping the pumps.

All nuts and bolts involved in the swap came off without much of a fuss, except one. The one was in a pump frame, visible above, so I could put it in a vice and work the bolt with CRC until it eventually came loose.

Visible here is the pressure side of a fuel pump. The nuts opposing each other are the power connectors. While changing the pumps you must pay attention which way the terminals are facing as the power leads are rather short. Out of four power leads involved in the swap, one broke, so I had to cut and install a new abico connector. The + and - terminals are sized differently (7mm and 8mm) so theoretically you can't install it wrong. The leads in the car are brown for earth and black (masked) for voltage. The terminals are covered with rubber hoods which, along with new abico connectors and nuts, come with the Bosch pumps. This is good because all of the old rubber hoods on my car had expired.

All the fuel line nuts and fasteners have copper washers which are not supplied with the pump so you would be well advised to buy some before taking on this task.

Since I had a lot of parts off the car, I decided to clean them up a little. This is something you get to do when you work on the car yourself... The piece of fuel line between the pumps had considerable surface rust on it but it's still solid and dry. The pump brackets were also badly corroded but still have life left in them and the basic shape was still in tact. All the parts had dirt and dust on them. The plastic casing had a thick rustproofing coat on it, so at least there was no rust... (tip: white spirit is excellent for removing rustproofing agent.)

Hose clamp cleaned - even that has the M-B logo!

Casing cleaned up.

New fuel pumps installed, wired and tested. Filter in the middle, pressure regulator in the background. In this picture you can see, just right of the right side fuel pump, the fuel pump for the Webasto heater. This is a factory installation so on Webasto-equipped cars, there are three fuel pumps inside the fuel block.

Casing sealed.

I took a test drive and sadly the acceleration misfire is still there. Bummer.

The work was not for nothing, of course. Having new fuel pumps can only be a plus. I've got a lot of life in my fuel block now, I got to empty the fuel tank completely which is good for reliability and I learned how to work on this part of the car. The new fuel pumps are much quieter than the old ones. I can probably sell the old fuel pumps, after I've tested them. At the end of the day, I got to work on the car, which is always nice.