sparkie

Hi,
My car runs and pulls well but i have noticed when i take my foot off the gas at 4000-5000 rpm i can hear a rumble from the exhaust which sounds like small pops and bangs <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" /> (like a small backfire)

what would cause this?

Thanks!!!
sparky <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Bryan

Well, on cars with CIS - the "German carburetor" mechanical injection system - the injectors are always spraying when the engine is turning. There's no way to shut the injectors off during coasting/lift-off like there is in an EFI system.

On US cars with CIS (Audis and VWs anyway), there's a little vacuum-actuated air bypass that opens under high vacuum and lets a little air past the throttle plate. This causes this unnecessarily-injected fuel to burn in the combustion chamber instead of downstream in the exhaust where it causes the popping sound.

So either you have one or more leaky injectors that are allowing too much fuel through for the air bypass to deal with, or your air bypass system isn't functioning (if it even exists on a Euro 928), or the sensor plate in the fuel metering assembly is mis-set slightly so the plate is slightly too high during coasting.

None of which is a big deal to fix.

The air bypass thing may also be called a vacuum limiter.

Good luck,

Bryan

PorKen

I love the sound of exhaust rumble in the morning... it sounds like victory...

I wondered what that gizmo was for, I wonder why they kept it for the L-Jet cars?

Old & New

...and look for exhaust leaks which will allow air to be sucked in during deceleration.

John Struthers

Sparky,
You haven't been messin' around with the timing/Distributor advance have you? <img src="graemlins/nono.gif" border="0" alt="[nono]" />