It's a bit strange, i've owned the car nearly 3 years now (1988 760GLE B280E LH2.2) and not heard this before.
Having gone through a lot of diagnoses on a few other things, something became very obvious, the in-tank pump wasn't pumping.
Thankfully the other problems were all sorted and many improvements have been made, however there was still a niggly little fault that shouldn't have been happening, that of slight hunting on idle after braking on a downhill then turning into a car park - or any similar situation.
By keeping a reasonable amount of fuel in the tank i'd kept rolling through but it was begining to niggle at me, mostly that the underbody pump isn't man enough to pull a head of vacuum to lift the fuel from the tank. I knew if i didn't replace the in-tank pump sometime soon, i'd be replacing both pumps at the worst time possible.
Cue a visit to
https://www.rockauto.com/ and a few days later, an Airtex pump and strainer landed for the princely sum of £35.59 including S&H (USA version of P&P), Customs Import VAT etc.
Waited until the weather improved and i felt up to attempting Volvo Gynaecology Masters Level and then fitted it.
It's cured the hunting after braking downhill and also (unexpectedly) improved throttle response. Since i've had the car it's always felt as if it needs a 2 minute warning of impending beans being given - now i know that there was momentary fuel starvation until the fuel was pushed to the back of the tank giving the underbody pump enough of a boost for it to do its thing.
However, the fuel rail is still dropping pressure overnight so i also ordered one of thse today :
https://www.ebay.co.uk/itm/BOSCH-Dis...9/173415230965
Description is slightly wrong, it's the underbody/main fuel pump output check valve according to the Bosch part number and photo. If my theory is correct and the existing one has failed/gone weak (after 31 years and 225k miles i think it probably has!) then it's allowing fuel pressure to drain back through the filter, main pump and back to the tank through the in-tank pump while the engine is off.
Once it arrives i'll report back but hopefully it will restore instant firing hot or cold (hot starting is improved since the new in-tank pump went in, still not perfect though) as soon as the starter hits the flywheel.
Bottom line is the "funny 'um" is the soothing sound of the in-tank pump doing what it says on the tin!
