Very recently I started experiencing this phenomenon. In low rev/high boost situations, predominately in 4th-5th gears, I will get a very violent shuddering/bucking that almost feels like I'm hitting the rev limiter. If I start a pull above 3000 rpm this does not happen at all. Car seems fine otherwise.

I have a BBK Full with APS Twin Vent BOV that has never given me issues. Has been a while since I took it apart and cleaned or adjusted the screw/nut.

Some background: last month I was doing some work and forgot to tighten one of the clamps on the upper intercooler pipe (the one connecting it to the APS). Had the pipe blow off and car would not even idle until I put it back on. Seemed fine after that, went out of town for a few weeks and when I got back, noticed this first time out.

Two videos of it. Tach view (4th gear):


Boost gauge view (5th gear). Ignore middle gauge/wideband as I need a new o2 sensor.


Any ideas? First time I've experienced something like this. Thanks guys.

 

disassemble and clean the dv/bov.

 

It's doing that while at wide open throttle? Or is that part throttle?

Part throttle I would diagnose a bov issue.

A cleaning and checking adjustment couldn't hurt.

 

That's close to wot but low revs. If I use part throttle and then nail it above 3000 it's fine. Will look at valve. Thanks guys.

 

It's not wot but definitely over halfway.

 

Sounds like a sure thing as a bov issue. If your foot isn't to the floor the bov is trying to balance pressure. My turbo xs valve did exactly the same thing.

I attributed it to slow reaction of shunting air, then continually osculating / over correcting.

 

Cleaned/lubed diverter valve. No improvement. Also played with the adjuster screw to no avail. Going to do a boost leak test. Can't locate my stock MR diverter valve or I would try that now.

I also have a Spoolinup COP. Could it be a spark/ignition issue?

 

Boost leak test didn't help. The APS was leaking a little and I put a stop to that but issue remains. Getting frustrated here.

 

Does it happen during slight throttle lift or as load begins to drop slightly?

 

Not sure I understand the question. Sorry if my answer misses the point.

I've only noticed it under heavy load at low revs, like so low that I would usually downshift otherwise. I get a second or two of boost building up (to 15 psi or so) and then the violent bucking starts. I'm guessing if I lifted the bucking wouldn't happen because boost would stop building. It doesn't show up under any other circumstances.

I got a PM from someone who experienced something similar and it turned out to be a tune issue. Shortly before this started happening I had the battery out of the car for an hour or two. Is it possible something happened to the tune then? Like a hard reset? Haven't tried reflashing yet but that's my next step.

 

. Maybe you never noticed it before? What kind of boost control method are you using?

If the boost is increasing when it happens I'm going with compressor surge although it's sudden appearance lends an odd note.

Can you log the instance? I've heard 100%wgdc under x load can do this in some cases but really still doesn't cover not there then there, unless a specific factor of environment has changed. Significant air quality etc..

Thoughts ?

 

I'm using a Grimmspeed ebcs. Will log shortly if I can't fix it. Thank you.

 

High load low revs does sound like comp surge as 211ratsbud said. I think you need to post a log at this point. Need TPS, RPM, ign timing, AFR, boost, WGDC.

 

Here you go gents. On the far right (boost) you can see it drop from 23ish psi to 13 psi twice very suddenly even though throttle position doesn't change.

I know AFR is missing. Picked up a new o2 sensor today. Will have proper logs soon.

 

Either going lean, misfire, or your BCS is going bad. I still wonder about the DV too. Sorry it doesn't narrow it down much. You could try plugging your compressor nipple directly into the WGA as a way to eliminate the BCS.