Porsche 924, 924S & 944 Cranks But Won't Start? Fuel Pump Fault Finding Guide

Porsche 924, 924S & 944 Cranks But Won't Start? Fuel Pump Fault Finding Guide

Porsche 924, 924S & 944 Cranks But Won't Start? Fuel Pump Fault Finding Guide

Introduction

Few things are more frustrating than turning the key on your Porsche only to find the engine cranking happily but refusing to start.

While ignition faults can certainly be responsible, one of the most common causes is a problem within the fuel delivery system. Before spending money on parts, work through this simple diagnostic checklist.

This guide covers the Porsche 924, 924S and 944 range. Where specific checks only apply to the 924S and 944 models, we'll point that out as we go.

Quick Check – Is The Rev Counter Moving?

This applies to the Porsche 924S and Porsche 944 models fitted with Motronic engine management. It does not apply to the early 2.0-litre Porsche 924.

Before diving into the fuel system, watch the rev counter (tachometer) while cranking the engine.

Tacho Bounce Present?

If the rev counter flickers or lifts slightly while cranking, the DME is receiving a signal from the flywheel reference sensors.

This is a good sign and generally means the engine speed and reference sensors are working.

No Tacho Bounce?

If the rev counter remains completely dead while cranking, there's a strong possibility the fault lies elsewhere.

Common causes include:

  • Failed flywheel reference sensor

  • Failed speed sensor

  • Damaged sensor wiring

  • Incorrect sensor gap

  • Corroded sensor connections

  • Damaged sensor bracket

Without a valid reference signal, the DME doesn't know the engine is turning and therefore won't trigger fuel injection or ignition.

In simple terms, the engine can crank all day long but it will never start.

Before condemning the fuel pump, always check for tacho bounce on a 924S or 944. It can save a lot of unnecessary parts swapping and point you in the right direction much more quickly.

First Quick Check – Can You Hear The Fuel Pump Prime?

Before reaching for the tools, there's another quick test that can often point you in the right direction.

On many Porsche 924, 924S and 944 models, the fuel pump will briefly run when the ignition is switched on or when the key is turned towards the start position.

In a quiet environment, you can often hear a short humming sound from the rear of the car.

A simple trick is to flick the key towards the start position and listen carefully.

Pump Primes?

Good news.

The pump is receiving power and is at least attempting to operate.

This doesn't guarantee the pump is healthy, but it does suggest the fault may lie elsewhere.

No Pump Noise?

If the pump doesn't prime at all, work through the checks below. The fault could be:

  • Failed DME relay (924S & 944)

  • Fuel pump

  • Wiring fault

  • Poor earth connection

  • Fuse or relay issue

Parts - https://stuttgartspares.co.uk/products/porsche-964-993-944-924s-968-dme-fuel-pump-relay-oe-manufacturer-94461522700-99361522702

Don't Leave Home Without A Spare DME Relay

If you own a Porsche 924S or 944, carrying a spare DME relay is one of the smartest things you can do.

The DME relay is a known weak point and can fail without warning, often leaving you stranded with a car that cranks normally but refuses to start.

Many experienced owners keep a spare relay in the glovebox for exactly this reason.

In our experience we've seen far more DME relays fail than fuel pumps.

Emergency DME Relay Test

A quick roadside test is to bridge terminals 30 and 87 in the DME relay socket.

This bypasses the relay and supplies power directly to the fuel pump circuit.

If the fuel pump suddenly comes to life, there's a very good chance the DME relay is faulty.

This is also a useful way of testing whether the fuel pump itself is capable of running.

Important

Only perform this test if you are confident working on vehicle electrical systems. Always take appropriate safety precautions and remove the jumper wire once testing is complete.

Step 1 – Check The Fuel Pump Fuse

Start with the basics.

Check the fuel pump fuse for:

  • Corrosion

  • Heat damage

  • Loose connections

  • Blown fuse element

A surprising number of electrical faults are found here.

Step 2 – Check The DME Relay

The DME relay is one of the most common causes of Porsche 924S and 944 no-start faults.

A failed relay can prevent power reaching both the fuel pump and engine management system.

Typical symptoms include:

  • Engine cranks normally

  • No fuel pump operation

  • Intermittent starting

  • Vehicle starts occasionally then cuts out

Many experienced owners carry a spare DME relay in the glovebox.

Porsche 924S 944 fuel pump relay

Step 3 – Test For Fuel Pump Power

Access the fuel pump underneath the rear of the vehicle.

Using a multimeter:

  • Check for battery voltage at the pump.

  • Have an assistant crank the engine.

Power Present?

The pump is likely faulty.

No Power?

Continue diagnosing the circuit.

Step 4 – Check Fuel Pump Earth

A poor earth can mimic a failed fuel pump.

Inspect:

  • Earth connections

  • Corrosion

  • Loose fixings

Clean and retest.

Step 5 – Inspect Wiring And Connectors

Repeated heat cycles, moisture and age can all take their toll on wiring.

Inspect:

  • Fuel pump connectors

  • Wiring near the fuel tank

  • Fuse box connections

  • Relay terminals

Look for corrosion, overheating or damaged insulation.

Step 6 – Test The Pump Directly

If you're still unsure, disconnect the pump and apply power directly from a suitable 12V source.

Pump Runs

The fault lies elsewhere in the circuit.

Pump Doesn't Run

The pump is likely due for replacement.

https://stuttgartspares.co.uk/products/porsche-911-924s-944-968-fuel-pump-1x-magneti-marelli-94460810204

Porsche 944 924S FUEL PUMP

Step 7 – Check Fuel Pressure

A running pump doesn't always mean adequate fuel pressure.

Common causes of low pressure include:

  • Blocked fuel filter

  • Weak fuel pump

  • Internal pump wear

  • Restricted fuel lines

If pressure is low, investigate further before replacing other components.

Common Causes We See At Stuttgart Spares

Over the years we've found the most common causes of Porsche 924, 924S and 944 fuel delivery problems to be:

  1. Failed DME relay (924S & 944)

  2. Failed flywheel reference or speed sensor (924S & 944)

  3. Corroded fuse box connections

  4. Poor fuel pump earths

  5. Aged fuel pumps

  6. Blocked fuel filters

  7. Vehicles that have stood unused for long periods

  8. Perished fuel hoses and electrical connections

Need Parts?

At Stuttgart Spares we stock a range of fuel system components for Porsche 924, 924S and 944 models, including:

  • Fuel pumps

  • DME relays

  • Fuel filters

  • Electrical components

  • Window switches

  • Hard-to-find transaxle Porsche parts

If you're unsure which part is right for your car, get in touch and we'll be happy to help.

Final Thoughts

When a Porsche cranks but won't start, it's easy to jump straight to replacing the fuel pump. In reality, many no-start faults are caused by relays, wiring issues or failed reference sensors.

By following a logical diagnostic process, you can often identify the fault quickly, save money and avoid replacing perfectly good parts.

A few simple checks such as listening for the fuel pump prime, checking for tacho bounce and testing the DME relay can point you in the right direction before a single spanner is lifted.

Happy fault finding, and good luck getting your transaxle Porsche back on the road.

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