RWD newsletter - April 2009
Page 6: Fuel Pumps

Symptoms: Low pitched growling sounds, especially in hot weather and when fuel level is low. The car also may feel underpowered or even hesitate or misfire. The sound will come from the main fuel pump located under the car, just in front of the driver’s side rear tire.

Troubleshooting: Try removing the fuse that provides power to it (usually fuse #5). If the pre-pump is functioning, the idle should drop about 2-300 rpm when power is cut to the pump. If the idle doesn’t change, the pre-pump has probably failed. You can also remove the gas cap and listen in the tank when the car is running for a slight humming sound. Sometimes the pre-pump hose inside the tank will crack or fall apart and this will show up as a poor running condition when the tank nears empty. The best way to test the function of the pump is to remove the fuel line between the tank pump and the main pump, at the inlet of the main fuel pump. Put the end of the fuel line in a bucket or container and turn the key on. If the pump is working, it will start flowing fuel into the bucket. If nothing comes out, the pre-pump is dead.

A fuel pump either works or it doesn’t. Be sure to fault-check the wires and the relay before replacing it. A repair to the power source is far less expensive than a misdiagnosed fuel pump. These pumps are the same original Bosch pumps that will last as long as the original.

Prepump failure is common in the early 240 series cars. They work well for the most part and, even when they fail and are replaced with a new one, they usually run like new. It’s the K-jet cars (1978-84) that are most likely to benefit from a pre-pump upgrade. It seems that these 240s may have a main fuel pump noise and lack of full power even with a new original style in-tank prepump. In these cases, Volvo has a bulletin outlining the replacement of the original pre-pump with a later version that has a higher delivery capacity (#3517845 available in our kit).

Unfortunately, this is not a direct replacement pump. Modifications are required to the pick-up tube. The pick-up tube must be shortened by 30mm, because the pump is that much longer. And a clamp is required to secure the pump to the pick-up tube.

If your Volvo runs but runs poorly (as if it is starving for fuel) a common problem area is the prepump (in-tank fuel pump). Another common symptom is that the car will run fine on a full tank, but below half-a-tank it stumbles and bucks, and the main pump will usually be quite noisy.