Lift-throttle lurch - how bad is normal?
 

I'm still getting a feel for my (new to me) 930, and can't get enough of it.

One annoyance I've run into a few times is that, when under boost, if I lift off the throttle even moderately quickly I get a brutal hard engine-braking-style lurch. I've found I either need to lift off throttle REALLY slowly or clutch-in and lift once in boost.

Is this normal? Is it something I can rectify? I'm used to other turbo cars that might make some sounds and a *normal* engine-braking lurch, but this is particularly brutal - almost as if we hit a brick wall before settling into normal engine braking.

Car has ~50k miles with 1bar wastegate spring, big intercooler, and exhaust. Had engine out reseal within 18 mos, and a full PPI inspection within the two months - all checked out.

Is there a chance your boost pressure switch is turning the fuel pumps off? What year car? Intake recirculation still in there?

mine does it too.
just change the way you drive.

its like it happens in the lower RPM range and dont remember if it is c complete lift or just partial

you mentioned you run a big I/C - long or short neck?

what you running for a bypass valve - OE or aftermarket? If aftermarket, is it vented to recirc or dump?

Mine would get real hokey @ the top of throttle / boost when I'd go to lift, running fullbay longneck & BPV to dump... now running BPV to recirc and muuuuuch better on / off throttle (for a # of reasons).

just some thoughts SmileWavy


ps. congrats on the purchase / welcome to the insanity here - obligatory NFG pics, stat :D

I wonder if your recirculation valve/bypass valve is not opening? Should open when you lift when vac is high. I would test that first.

Daniel

Yeh, that doesn't sound right. You should be able to feather the throttle - or even just close it with a bang - and have "normal" engine braking/deceleration.

Yeh, kind of what it sounds like. Based on what others have said, as my car hasn't had an overboost switch connected to anything in my ownership...

Another possibility; if the recirc valve dumps to atmo on CIS, doesn't the mixture go overly rich because you've dumped all the metered air or something? Thinking it might go so rich it can't run anymore...

Do you have a lightweight flywheel?

With stock recirc valve etc, mine had opposite - rubber band effect - would sprint foward slightly then slow. Very cool. Liked it.
Something not right here. Not normal.
Alan

Others have mentioned the recirculating valve. I don't see whether you have the stock valve on the car, but if yes, there are a few great threads on Pelican illustrating how to replace the gaskets. Cheap and easy. They disintegrate into a gooey mess.

There is an element of clutch/throttle coordination that is needed but the result of a chop throttle event with the clutch engaged should not be violent. What you feel may be turbo surge which causes bucking. Could be that your BOV piston is hanging in the bore. As others have stated it is common for the gaskets to disintegrate with high milage or simply age. As the gaskets fail pieces of them can get smashed between the BOV piston and cylinder. This can cause the piston to hang or seize completely. These gaskets are very inexpensive and very easy to change. Easy wear check is to stab the throttle while you are sitting at idle. Listen for a clanking sound after the stab. That clanking sound is the BOV piston hitting the end cap with no gasket left to cushion the impact.