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Pelican Technical Article:

Fuel Pump Replacement

Tom Morr

Time:

1.5 Hours

Tab:

$210 to $330

Talent:

**

Tools:

Floor jack, jack stands, wheel chocks, wood blocks, ratchets, extensions, nut drivers, 7mm, 8mm, 10mm sockets, screwdrivers, hose clamp tool

Applicable Models:

Mercedes-Benz SLK230 (1998-04)

Parts Required:

Replacement fuel pump

Hot Tip:

Drain the fuel tank if feasible, have suitable containers ready to capture residual gas, wear protective gloves and safety glasses.

Performance Gain:

Ability for the car to start and run.

Complementary Modification:

Replace fuel filter

Luckily, the R170 SLK's pump is relatively accessible: unlike cars that have the pumps in their fuel tanks.

The fuel pump's job is to supply the engine more than enough fuel in order to maintain pressure. Circulating excess fuel keeps fluid temperatures lower and inhibits bubbles: the dreaded vapor lock.

The pump pushes fuel from the tank through the fuel filter and lines/rail to the injectors. The ECU controls the fuel pump via the fuel pump relay.

Fuel system problems can result from many variables. If the car has spark but won't start, then the culprit is more than likely a fuel problem. Rough idle, stumbling and stalling on hills are other symptoms of fuel-delivery issues.

When trouble shooting the fuel pump first you should check that the fuse isn't blown and its relay and other fuel-system components pass their tests.

Fuel pumps often give a whirring sound as they approach the end of their service life. Other tell tale signs are no fuel at the rail's Schrader valve when cranking the engine. (Make sure to have rags and a fire extinguisher handy in case fuel does squirt out.) Also, if the pump is getting power (verified with a multi-meter) but no fuel is arriving at the engine, the pump could be bad. Corroded terminals can also cause fuel pump malfunction.

Potential for fire is always a concern when working on fuel systems. Disconnect the battery's negative cable to minimize possibilities of sparks. Work in a well-ventilated area away from open flame (such as smokers and water-heater pilot lights). Let the car cool down before beginning. Fully heated catalytic converters and ignite combustibles.

Always keep a Class B fire extinguisher within arm's reach. Capture gas in an appropriate container. Relieve pressure from the fuel system before removing the existing fuel pump. Pinching off the suction hose will minimize the amount of spilled gas. Special U-style clamps are made for this purpose, although DIY mechanics often improvised with small C-clamps or locking pliers using something to protect the fuel line from the pliers' jaws. The OEM sliding-style hose clamps are realistically one-use only. Spring-type clamps are DIY-friendly and are normally packaged with the new fuel pump. In a pinch, worm-gear clamps have decades of proven effectiveness, providing they aren't over or under tightened. Many fuel pump kits include new clamps.

Once lines are disconnected, access is eased by sliding the pump out of the bracket. The bracket's hangers can also be unhooked from their rubber mounts to lower the pump. On reassembly, make sure that the new fuel pump's protective sleeve is centered in the mounting bracket's clamp. Otherwise, corrosion can form between the clamp and the pump's body.

You will need to safely raise and support the vehicle and disconnect the ground strap on the battery. Please see our separate article on these projects for help

Begin by raising/securing the car and disconnecting the battery.
Figure 1

Begin by raising/securing the car and disconnecting the battery. Fuel pressure is relieved at the rail's Schrader valve, under a 15mm gold nut.

This SLK's fuel tank had been drained to replace its fuel level sender.
Figure 2

This SLK's fuel tank had been drained to replace its fuel level sender. Otherwise, rags and cups would be used to contain any residual gas while relieving the fuel pressure.

A plastic shield covers the fuel pump and filter on the passenger's side in front of the rear suspension.
Figure 3

A plastic shield covers the fuel pump and filter on the passenger's side in front of the rear suspension. The shield attaches with three flange nuts: the 10mm socket is on one, a second is indicated by an arrow, and the third (top arrow) is on the trunk floor's underside.

Unless the tank has been drained (as it has been here), the suction hose needs to be pinched off above the elbow (green arrow) to keep the fuel in the tank.
Figure 4

Unless the tank has been drained (as it has been here), the suction hose needs to be pinched off above the elbow (green arrow) to keep the fuel in the tank. If the factory hose clamps are still present, they can be clipped with dykes or twisted apart with pliers. Place an appropriate container on the ground and slowly back the suction hose off the fuel pump. Residual pressure can cause a brief spray, followed by fuel trickle.

Unhooking the mounting bracket at its rubber hanger's eases access to the outlet hose and electrical connections.
Figure 5

Unhooking the mounting bracket at its rubber hanger's eases access to the outlet hose and electrical connections.

Loosening the mounting clamp frees the pump.
Figure 6

Loosening the mounting clamp frees the pump. Drain the fuel trapped in the pump into an approved container.

After peeling back the rubber boots, the wires can be disconnected.
Figure 7

After peeling back the rubber boots, the wires can be disconnected. To eliminate confusion, the positive terminal (left) uses a 7mm nut and the negative (right) uses 8mm. Both have star washers to help maintain contact/nut torque on the wires' eyelet conntectors.

Installation is the reverse of removal.
Figure 8

Installation is the reverse of removal. Our existing pump was still good, so we used it for demonstration purposes. Note that the pump's protective sleeve is centered in the mounting clamp to inhibit corrosion.

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