9two8

Recently I,ve noticed when I switch the engine off, there is a light hollow knocking type noise coming from the rear of the wheel well driver side,( RHD ) UK model.., if I exit the car quick enough after switch off, I can just hear the noise dissapear, in all about 10 seconds from switch off.
I,ts got me stumped as I,ve owned the car twenty years and never heard it before.
Has anyone come across this strange noise.., it,s definately not engine related.

Ken
80 928 S
5 Speed

UK

Dave928S

If the 80 is the same as my 82, all you have in the top of the well above the tank inlet is what PET calls a compensator, in the front of the well you have the fuel accumulator, and in the rear there are just hoses and the filler neck. That's all that I have in my wheel well on that side, so it would seem that it must be fuel related.

The compensator seems to me to be just a tank to take gurgles from the upper tank and neck, so that you don't get blow back out the neck as a result of trapped air bubbles .. so you wouldn't expect that sound from there.

It sounds a bit like a pressure equalisation issue. Maybe it's the accumulator/line/pump/tank section of the fuel system and a check valve which can't decide if it wants to open or close. Check your fuel system pressures ... maybe you're getting some fuel pressure bleed down after turning off.

Any difficult or slower starting lately? Noisier pump? Any other signs? Any chance of dirty fuel recently?

Could also be an animal that you've scared knocking to get out

Landseer

Or sloshing fuel.

Dave928S

Yeah ... maybe something loose in the tank being moved around by that. Can't think what, but after all these years it wouldn't hurt to have a look in the tank and WYAIT siphon any sediment out.

Landseer

The fuel itself, if at a certain level, will slosh for 10 seconds or so. I sat there contemplating it one day before figuring it out and being a bit ashamed about it.

Dave928S

You probably just drive too fast ... seriously, though, I know what you mean and that is a possibility. Slopping of fuel on the tank wall does give a bit of a knocking sound. I guess we'll find out soon enough when the OP gives some more info.

9two8

........, sorry guys I did,nt word that too accurately..., when I said the knocking/tapping sound was coming from the ( rear of the wheel well driver side ) , what I meant to describe was that the noise is actually coming from the rear section of the driver side ( UK RHD ) front wing ( fender)...., so between the door hinge area and front suspension area.
Sincere apologies !!

Ken
80 928 S
5 Speed

UK

Dave928S

OK .... at least if anyone's getting a knocking in the rear they'll know what to look for

It's worth removing the splash shield on that side as you just might be enlightened as to what's causing the noise. What you have behind the shield is the washer tank and pump at its base, a heap of hoses, and a carbon canister for fuel vapours up the top, which goes partially into that area. It's worth taking the splash shield off anyway, because that area can get full of road crap and it's a good idea to clean it out, and you can check all the lines to make sure that none are about to self destruct. Check fuel vapour lines carefully and the y piece as well as this is a common failure and a worthwhile WYAIT. I can't imagine what would give that sound in that area, apart from something loose or floating in the washer tank knocking on the side, after you stop, until it calms down ... or something loose and broken swinging back and forth.

Other than that it could conceivably be something on the engine side of the inner guard ... but I can't think of anything obvious. Remember that sounds can often seem like they emanate from a certain area but come from somewhere different.

Check behind the splash shield first .. and check the washer tank.

9two8

Thank,s Dave.., and for your perseverance..., best way to describe the knocking/tapping is to get your knuckle and rapidly tap it on the fender, sounds just like it !..., you would think there,s something in there trying to get out

I,ll try filling the resevoir with water and see if it does anything regarding something stuck ? but it only does it when you have done a run, come back and switch off, it does,nt effect the car otherwise.

Ken
80 928 S
5 Speed

UK

Dave928S

Good first step ... although the sound being that sharp it seems a bit less likely. I'd still take the splash shield off and have a look if it's not that. It's worth it if only to clean out the road muck and check everything ... and go from there.

9two8

An update.., I took off the fender splash shield, and could,nt find anything unusual.., however today.., after a run.,I switched the engine off, and jumped out as quick as possible in order to hear where the noise was coming from.
It seems to come from the rear of the plenum, then stops, this is after it,s switched off.
Is it possible that the butterfly disc is sticking and vibrating in those last few seconds after switch off.., and if so, is there a recommended cure ?? or am I miles out ?

Ken
80 928 S
5 Speed

uk