Fuel Pump and Relay

Fuel Pump

The fuel pump pulls fuel out of the gas tank and toward the engine. This pump is powered by the RPM relay and the fuel relay when the car is on. The fuel pump runs at about 75-80 PSI.

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Troubleshooting

You should hear a faint hum or buzz from the front of the car when the key is switched to the run position but not started, and when the engine is running. If not, you may have a problem with your fuel pump or related components.

Fuel Pump Doesn't Run

These are some common possible causes for a pump that doesn't run.

Failed RPM Relay

The fuel pump runs when the car is on. This is controlled by the RPM relay, which also turns on the main fuel relay and the Lambda system. When this relay fails, the fuel pump will not run.

The RPM relay can be tested by removing it from the car and shorting pins 30 and 87 together. If the pump runs, then the relay is bad. See the RPM relay article for more information on manually running the fuel pump, and the RPM relay in general.

There may also be a wiring fault that is causing the pump to not run. This requires using a multimeter to test the wires.

Blocked or Collapsed Pickup Hose

The pickup hose and/or filter screen may have become blocked by debris and other contaminants in the fuel tank. The fuel tank should be cleaned, as should the pickup hose and filter screen. Note that even after you clean the tank, debris may still be in the hose and on the screen. Shifting the car (such as by towing it) or letting it sit for a while (just an hour or two) may cause the debris to clear from the screen, at which point it will run better for a while, but will eventually get plugged up again.

A collapsed pickup hose will be "mushy" to the touch. It can be reinforced with a spring, or replaced with a new one from a DeLorean vendor. Special T Auto also sells an alternative hoseless pickup and baffle that can be used to alleviate this problem.

Failed Fuel Pump

The most severe issue, this requires you to replace the fuel pump with a new one from a DeLorean vendor.

Noise from the Fuel Pump

A quite hum or buzz from the fuel pump is normal. This can often be heard when you turn the key to the run position without starting it yet, but once the engine is running it cannot usually be heard. It's quiet enough that you may have to listen for it, or be in a quiet location.

A loud buzzing or whining noise from the front of the car is often a symptom of a fuel pump problem. Usually this is heat related, either due to the temperature of the fuel tank itself, or from the pump working too hard, both of which can damage the pump. There are a few common causes.

Very Low Fuel Level

Pump noises can occur when there there is approximately half a gallon or less of fuel in the tank, resulting in the tank not being able to reliable suck in fuel through the pickup hose. Note that this may not happen in all cars. Simply filling the tank will fix it.

Excessive Heat

The location of the fuel tank is such that hot air from the radiator blows directly onto it. This excess heat causes the fuel pump to operate above it's normal operating temperature, which in turn generates noise. This is often the case when the fuel pump is initially quiet when the car starts, but gets progressively loader the longer it is driven. Usually the pump continues to deliver fuel without issue, but still makes this loud sound. However, this excess heat can eventually damage the pump and cause it to fail sooner than it normally would.

To see if heat is causing the problem, try pulling into a gas station and filling the tank. The cold gas from the pumps will cool the fuel pump, which will stop the buzzing if it is indeed a heat issue.

Some vendors sell special solutions to this problem, such as Special T Auto's hot air dam, which redirects hot air from the radiator away from the fuel tank. You can also build one yourself. Some owners have added rubber or other insulation around the tank to protect it from the heat.

Collapsed Pickup Hose

If the noise occurs when the tank is less than half full, the problem could be a collapsed pickup hose. Excess heat and age can cause the normally stiff rubber hoses in the tank to become soft and collapse. This can restrict fuel flow, causing the pump to work harder, or cause it to fail to suck in any fuel at all. This can also lead to premature pump failure, as the fuel lubricates and cools the pump.

The suggested solution is to pull the pump out of the fuel tank and see if the pickup hose is "mushy". This hose lies in the tank and sucks up fuel. If it is not stiff as it should be, either replace it or install a spring along it to stiffen it. Special T Auto also sells an alternative hoseless pickup and baffle that can be used instead.

Tank Contamination Restricting Flow

If there is debris or other contamination in the fuel tank, it can get sucked up and block the filter screen that protects the fuel pump. Eventually, more debris will build up on the screen, further restricting fuel flow. This may result in constant or intermittent pump noise irrespective of the tank temperature or fuel level, and will likely cause performance issues and sputtering due to lack of fuel.

The best solution here is to clean the fuel tank, as well as cleaning out and possibly replacing the fuel pump pickup hose, and making sure the filter screen is unobstructed.

Failing Fuel Pump

If all else fails, it is likely that fuel pump is failing. The solution is to pull the pump and replace it with a new one from a DeLorean vendor.

Fuel Relay

The fuel relay provides power to the fuel pump, and is itself activated by the RPM relay when the car is on. This is also commonly known as the Lambda relay or the main fuel relay.

Note that this is not a standard automotive relay, although it may look like one. It can often be identified by its metal case, although plastic ones also exist. It is number 30 in the relay compartment diagram. Replacement relays can be purchased from DeLorean vendors, or you may be able to find a cross reference.

Solid State Lambda Relay

Bitsyncmaster on DMCTalk posted a schematic and pictures of a prototype version of a solid state Lambda relay. This unit produces no heat and has a very low resistance.

Testing the Fuel Relay Circuit

Walter posted on DMCNews about how to bypass the fuel (Lambda) relay for diagnostic purposes, allowing you to test if the relay circuit itself is functioning. He explains how to do this both by removing the metal relay cover to access the relay's internals, and by removing the relay itself to access the socket below.

Repairing a Fuel Relay

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.

See Also

• Fuel System
• Relays