Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

These may include hard starting, inexplicable poor idle,
louder than normal main fuel pump noise and a loss of
power under load. To check the operation of the pre-pump
without removing it from the tank, locate the pre-pump fuse.
Remove this fuse with the car idling. You should be able to
detect increased noise from the main pump and decreased
vibration of the fuel tank itself.
Fuel Pickup and In-Tank Pump
Diagnosis of pre-pump: Are there any short cuts to check to
see if the in tank pump fuel pump is working? I opened the
gas cap and did not hear any noise.] You should really be
able to hear a "buzzing" sound in the fuel tank if the in-tank
pump is running. Sometimes a length of heater hose
inserted in the filler neck and placed against your ear will
help isolate the sound. Try it with the fuse in place and
removed and listen for a change in the sound. You might
also find another Volvo owner who might let you probe their filler neck with your length of hose
and listen to their in-tank pump . Assuming their in-tank pump is working you would be able to
determine the sound you are listening for. If you do take it out of the tank:
Replace the short segment of rubber hose with a new section of fuel hose.
Replace the "sock" that is on the end of the pickup tube (about $15 from Volvo)
DO NOT bench test the in-tank pump. When the pump is removed from the tank arcing
will occur when powered up and can result in a fire due to the gasoline vapors present.
Certainly don't remove the pump from the fuel level in the tank and give it a quick bump
on the starter to check out its function- that can result in one hell of a fire (and explosion).
The pumps are designed to run submerged in gasoline.
Symptoms of worn fuel pre-pump: My experience with a worn fuel pre-pump is that it will cause
the engine to miss and lose power under conditions of high flow (high throttle and high rpm).
The problem grew worse very gradually (over 10's of 1000's of kms). It certainly wasn't going to
leave me stranded anywhere. [Another example:] My wife drives an '83 760, which I have
maintained as needed. Recently, the car started to vapor-lock in warmer weather, and/or when
the fuel level dropped to around a 1/4 tank or lower. Oddly enough, when I refill the tank, the
problem seems to go away. [Test Tip:] As far as a definitive test goes, I guess it would be
necessary to set up a pressure gauge and flow meter as well as a valve in the circuit that would
allow a specified volume to flow while reading pressure, but see the above fuse and noise
procedure for an alternative. [Editor] Hard starting and poor idle have been traced to a bad prepump,
if other symptoms are also present such as main pump noise.
[Car Stalls During Turn: Fuel Prepump Failing.] [Inquiry:] I bought a used '87 760t. It ran fine
when I first got it but after a few months it started stalling when making a left turn. Not every
time, but its worse (more likely to stall) if I'm decelerating, in fact I can usually avoid the stall if I
slow down before the turn and then give it some gas during the turn. When it does stall it almost
always starts right back up no problem. [Response: MikeW] The stalling during turns is usually
indicative of a problem with the in-tank fuel pump. Does it happen when you have a full tank of
gas, or only when your down around 1/3 tank or less? Anyway, it sounds like your in-tank pump