944 Fuel Supply — Pump, Filter, Tank & Lines

Fuel-supply service for the 944 — the low-pressure side that feeds the L-Jetronic / DME injection: the filter, the electric pump, the tank, and the lines. Fuel-pressure, idle and CO setpoints are a separate job and live on the fuel-injection adjustment page; when a supply fault points at the management side, cross-check the DME self-diagnosis.

Warning: The whole system is under pressure and holds fuel even with the engine off. Relieve pressure, pinch the feed line before opening a joint, catch the escaping fuel, disconnect the battery ground lead before electrical work, and observe your local safety regulations for working on fuel systems.

Replacing the fuel filter

The filter sits in the feed line. It is directional — a flow arrow on the body must point the way the fuel travels.

1. Pinch the fuel feed line shut with a standard hose clamp. Unscrew the fuel lines, counterholding so you don't twist the filter body, and catch the escaping fuel.
2. Loosen the clamp and remove the filter.
3. Install the new filter with the flow arrow pointing in the direction of flow.
4. Remove the pinch clamp, start the engine, and check the lines and the filter for leaks.

Checking the injection lines for leaks and tightness

1. Inspect visually for leaks.
2. Tighten the coupling nuts and hose clamps.

Checking fuel-pump delivery rate

This measures how much the electric pump moves in a fixed time. Do it only with the fuel filter and the power supply in perfect condition — a clogged filter or a weak supply reads as a failing pump.

1. Unscrew the fuel return hose at the pressure regulator and catch the escaping fuel.
2. Connect a standard hose to the return and hold the other end in a measuring glass of roughly 1500 cm³ capacity.
3. Pull the fuel-pump relay off the fuse/relay board.
4. Bridge relay terminals 30 and 87 b with a piece of wire — the pump should now run.
5. Let the fuel run into the glass for 30 seconds and read the volume.

Tip: The pump can also be run by feeding battery voltage to terminal 4 (black/green) of the multiple-pin plug.

Test value: at least 850 cm³ in 30 seconds. The speed-reduced pump (green colour code, Bosch No. 0 580 464 028) delivers at least 750 cm³ in 30 seconds. The matching fuel-pressure and CO setpoints are on the fuel-injection adjustment page.

Removing and installing the fuel pump

The pump sits under a guard at the bottom right of the car, ahead of the tank.

Removing

1. Disconnect the battery ground lead.
2. Take off the fuel-pump cover at the bottom right. Pinch the feed hose with a standard hose clamp, loosen its clamp, pull the hose off the pump, and disconnect the electric plug.
3. Unscrew the fuel pipe from the pump at the fuel filter, counterholding with a second wrench, and catch the residual fuel.
4. Loosen and detach the right fuel-tank strap at the bottom, then take off the lower guard together with the pump.
5. Unscrew and remove the fuel pipe and the electric connector at the pump, loosen the hose clamp, and remove the pump.

Installing

1. Always fit new seals.
2. Tighten the capped nut to 22 N·m16 ft·lb, counterholding.
3. Remove the pinch clamp, check for leaks, and refit the guard.

Removing and installing the fuel tank

The tank comes out from inside the car; the exhaust and the transmission have to come off the underside first.

Removing

1. Disconnect the battery ground lead.
2. Draw out the fuel.
3. Unscrew the hexagon-head bolts on the exhaust flange and both final-muffler suspension points, and take the assembly off.
4. Unscrew the final-muffler shield at the fuel tank and the rear suspension point, and remove it.
5. Take off the fuel-pump cover at the bottom right. Pinch the feed hose, loosen its clamp, pull the hose off the pump, and disconnect the electric plug.
6. Unscrew the fuel pipe from the pump at the fuel filter, counterholding with a second wrench, and catch the residual fuel.
7. Unscrew and detach the right fuel-tank strap at the bottom, then remove the lower guard with the pump.
8. Remove the transmission — see Group 34 in the repair manual.
9. Unscrew the fuel pipe at the fuel filter, counterholding with a second wrench. Unscrew the transmission carrier from the body and take it off together with the fuel filter.
10. Remove the trunk floor cover and the cover for the fuel-level transmitter. Pull the fuel return hose and the electric plug off the level transmitter.
11. Remove the right rear side window. Carefully loosen and take off the side trim panel, pull off the wheel-house trim and the insulation sheet up to the rear cover of the filler neck, then loosen and remove that cover.
12. Loosen the hose clamp on the filler neck and pull both fuel vent hoses off the neck.
13. Loosen the left fuel-tank strap at the bottom, unscrew both M 8 nuts, and remove.
14. Remove the tank by pulling it back and down.

Installing

1. Make sure the hose connections and hose routing are perfect before fitting the tank.
2. Insert both inner sealing hoses (with the bead) into the connecting hose. Coat the inner sealing hoses with assembly oil (for example Capella WE32 or Omnis 32; tyre mounting paste such as Contifix may also be used).
3. Slide the pre-assembled connecting hose onto the tank neck / filler neck, seating the inner sealing hoses correctly and avoiding creases.
4. Seal the filler-neck cover thoroughly with sealing compound before installing, to stop fuel vapours leaking.

Fuel-system line routing

The supply loop runs pump → filter → pressure damper → pressure regulator, with the injection lines feeding the injectors and a return line carrying the surplus back to the tank. Alongside it sits the idle-speed / evaporative control plumbing.

No.	Component		Item	Component
1	Fuel pump		A	Air-flow sensor
2	Fuel filter		B	Throttle housing
3	Pressure damper		C	Ejection pump
4	Pressure regulator		D	Intake pipe
5	Injection line		E	To brake booster
6	Fuel injector		F	Idle-speed control
7	Fuel return		G	Temperature valve (opens above 58 °C136 °F)
			H	Shutoff valve
			J	Control valve

Active-carbon evaporative canister

Fuel-tank vapours are trapped in an active-carbon canister instead of venting to the air, then drawn back into the intake and burned. The tank breathes through the expansion tank and a vent line; a safety valve and a flushing-air line manage the canister, and a temperature valve, shutoff valve, and control valve on the vacuum side meter the purge.

No.	Component		No.	Component
1	Fuel tank		7	Temperature valve
2	Expansion tank		8	Shutoff valve
3	Vent line		9	Vacuum line
4	Safety valve		10	Throttle valve
5	Active-carbon canister		11	Vacuum line
6	Flushing-air line		12	Control valve

To leak-check a removed expansion tank, plug the venting connection and apply about 400 mbar of vacuum at the draw-off connection with a hand vacuum pump; the gauge reading should hold and not drop.