Engine stalls when slowing down quickly

[Maurits Jonkergouw]

Hi list,

When I quickly slow down my '85 Coupe GT - cold or hot - (ie. release
throttle, engaging clutch and/or brake) the idle drops rapidly and
the engine stalls, but not constantly. My fuel pump has been making a
lot of noise for years now and when the engine stalls, I clearly hear
the FP noise-pitch goes down accordingly (similar like the noise
change when switching on all electric gear, or when the radiator fan
goes on), so my first thought is to replace the FP. But could there
be something else to look for?

Like I said, sometimes it does stall and sometimes it doesn't, so I
guess that rules out any intake manifold leak, bad ignition timing or
idling adjustment, right? What I do know is that the 5th cylinder has
an uneven compression of 7~8 bar, while the others do 10 bar.

I've read some time ago somebody had the same problem but I can't
find the tread anymore. As I recall someone replied there could be
excessive carbon build up in the engine and absorbing the fuel
somehow, causing the stall. Does this make any sense? I've already
add some additive in the fuel a few months ago with no effect
obviously.

Any thoughts please, thanks.
--
Regards,
Maurits

'86 quattro GV (WR)
'85 coupé KV
'82 coupé DD