Checking Fuel Pressure: Delorean's Vital Maintenance Guide

Checking the fuel pressure in a DeLorean is a crucial aspect of maintaining the iconic car's performance and longevity. The fuel pump, powered by the RPM relay, plays a central role in this process, drawing fuel from the gas tank towards the engine. While the car is running, the pump typically operates at around 75-80 PSI, and a faint hum or buzz should be audible from the front of the vehicle. However, there are instances where the fuel pressure may deviate from this standard, indicating potential issues with the fuel pump or related components. Understanding how to test and troubleshoot fuel pressure is essential for DeLorean owners and enthusiasts, ensuring the car's reliable performance and preventing unexpected breakdowns.

Check fuel pressure with the engine off

Checking the fuel pressure with the engine off is a recommended method outlined in the DeLorean technical manual. However, it is not an easy task and requires a fuel pressure testing kit. The process is further complicated by the fact that the engine is not running, which means that it is challenging to get a complete system pressure test.

To check the fuel pressure with the engine off, start by consulting the technical manual for specific procedures and safety precautions. Ensure that you have a fuel pressure testing kit and any other necessary tools. It is important to note that the fuel pump needs to continuously pump fuel to cool itself, so blocking off the return line is not recommended.

The fuel pressure testing kit will allow you to measure the pressure in the fuel system. Connect the kit according to the manufacturer's instructions, being careful to avoid any leaks. You may need to consult the plumbing diagram for your specific DeLorean model.

Once the kit is connected, you can begin testing the fuel pressure. With the engine off, the fuel pump will not be running, so you will need to activate it manually. This can be done by removing the RPM relay from the car and shorting pins 30 and 87 together. If the pump runs, it indicates an issue with the relay.

It is important to note that the fuel pressure may be different when the engine is off compared to when it is running. The voltage across the fuel pump can affect the pressure, with higher voltages resulting in higher pressure. However, as mentioned in the technical manual, the excess pressure is bled off by the PPR once a certain set level is reached.

By following these steps and referring to the technical manual for specific procedures, you can check the fuel pressure in your DeLorean with the engine off, helping you identify any issues with the fuel system.

Voltage and fuel pressure

The fuel pump in a DeLorean is powered by the RPM relay and the fuel relay when the car is on. The fuel pump should run at about 75-80 PSI. You should hear a faint hum or buzz from the front of the car when the key is switched to the run position but the engine is not started. Once the engine is running, you shouldn't hear the pump anymore. If you don't hear the faint hum or buzz when the car is not started, this may indicate a problem with the fuel pump or related components.

The voltage across the fuel pump can affect the fuel pressure. For example, when the alternator kicks in, the system voltage goes from 12V nominal to about 14V. However, the fuel pump pressure is regulated down from a much higher dead pump pressure, so changes in voltage may not make a visible difference. The pump can still reach close to 75 PSI, and the car will still start.

To check the fuel pressure, the technical manual recommends doing so with the engine off. However, it can be challenging to perform a full-system pressure test in this way. Running the engine makes it easier to see what the warm-up regulator is doing, as it needs to be warming up. Alternatively, you can simulate warm-up by powering the electronics and the regulator heater and using a small hand vacuum pump.

If you want to test the fuel pump with the engine running, ensure that nothing leaks. You can use a fuel pressure testing kit to check the pressures on a running engine.

Testing fuel relay circuit

To test the fuel relay circuit in a DeLorean, you will need to locate the fuel pump relay in the fuse box. The fuse box is usually located under the hood near the front of the vehicle, or under the steering column. The fuel pump relay is typically coloured green or grey. Once you have located the relay, remove it by pulling it straight out of the fuse box. You can then clean the relay prongs with a wire brush to remove any rust or corrosion.

To test the relay's resistance, you will need to refer to the circuit diagram on the relay to identify which prongs to connect the power and multimeter to. The power prongs are usually labelled 85 and 86, while the prongs to be tested are labelled 87 and 30. Set your multimeter to the ohm setting and attach jumper wires to the terminals of your vehicle's battery. Connect the jumper wires to the positive and negative relay terminals, and listen for a clicking sound to indicate that the relay is working. Hold the multimeter probes against the remaining prongs on the relay and check for a 0 reading on the multimeter to confirm that the relay is functioning correctly.

If your relay is functioning correctly but you are still experiencing issues with your fuel pump, you may have a problem with the fuel pump itself or another component of your electrical system.

Repairing a fuel relay

The fuel relay provides power to the fuel pump and is activated by the RPM relay when the car is on. This is also known as the Lambda relay or the main fuel relay. It is not a standard automotive relay, though it may look like one, and can often be identified by its metal case (plastic cases also exist).

Volvo Adventures has instructions on how to repair a fuel pump relay used in Bosch K-Jet systems like the DeLorean's. The instructions include the basics for dismantling and repairing a malfunctioning relay, which is usually due to bad solder joints.

Troubleshooting a non-running fuel pump

Part 1: Understanding the Problem

Firstly, it's important to verify if the issue is fuel-related. If your engine won't start or is exhibiting performance problems, there could be several reasons, and you need to narrow it down. Start by checking the fuel pressure and volume to ensure that enough fuel is making its way from the tank to the engine. This can be done using a mechanical fuel pressure gauge, which is inexpensive and readily available. Follow the instructions in your repair manual to connect the gauge properly and take accurate readings.

Part 2: Advanced Testing

If you have access to an oscilloscope or a professional-grade scan tool, you can run some advanced tests to pinpoint the issue. Use the oscilloscope to measure the current draw of the fuel pump. A good pump should produce a pattern of even "humps," while a bad pump will show jagged and uneven waves. Alternatively, a professional-grade scan tool can be used to remotely activate the fuel pump to see if it runs.

Part 3: Checking the Fuel Pump Electrical Circuit

In most older continuous-style fuel systems, the Engine Control Module (ECM) controls the fuel pump via a relay. Check this relay before assuming the fuel pump is faulty. Newer, returnless fuel systems use a Fuel Pump Control Module (FPCM) that controls the pump based on information from various sensors. Ensure that the control system is functioning correctly before blaming the pump. Additionally, check the wiring in the pump circuit for any issues, such as continuity, power, ground, or excessive resistance.

Part 4: Inspecting the Fuel Pressure Regulator and Fuel Filter

If your vehicle has a continuous fuel system, it's worth checking the fuel pressure regulator and fuel filter. Disconnect the vacuum hose from the regulator and see if the pressure increases. If it doesn't, either the regulator is faulty or there is an insufficient vacuum supply. Continuous fuel systems also have an external fuel filter that can be easily replaced. Try installing a new one to see if it improves the fuel flow.

Part 5: Considering Other Possibilities

Keep in mind that several other issues can mimic a faulty fuel pump. For example, a clogged fuel filter, a blocked fuel line, a blown fuel pump fuse, or electrical faults in the fuel pump wiring could all lead to similar symptoms. Before condemning the fuel pump, rule out these potential problems by checking the fuel pump circuit and other components in the fuel system.

Remember to always refer to your vehicle's repair manual for specific instructions and safety precautions when performing any diagnostics or repairs.

Frequently asked questions