I am a shade tree mechanic and have self-maintained several "performance" cars but my EVO 9 is the first with forced induction and I've never experienced what I am about to explain...

If I drive the car on the highway and keep the revs stable on level ground, I can feel a slight acceleration followed by a slight deceleration and then it repeats endlessly. I have no idea where to start troubleshooting this but feel it has to do with the combustion event deep inside the engine.

If anyone has experiences this, please comment on how you resolved the issue. Thanks!

 

Are you in vacuum when it does this?

 

Yes - no boost as if I was driving a naturally aspirated car.

 

Probably the BOV opening and closing. What BOV is on it?

 

Tial 50mm

 

Yeah, you're probably coughing the BOV. Put a lighter spring in it, the -5psi or -6psi spring.

 

>> you're probably coughing the BOV

What does that mean?

 

yeah, sounds like the blow off valve being closed.... what happens is that you cruise with small throttle but your turbo tries to build boost. Since the throttle is at a small opening, the air builds up in front of the throttle and goes back trough the turbo intake... In fact it is a wave... This then hits the MAF annd messes up its reading... as a result you get a hiccup in power..

 

This same scenario causes the BOV to rapidly open and close, causing the shutter/stutter.

 

sure, can be... I had the similar issue when we installed the hks turbo and some cams... it would surge like mad even on stock bov and was undrivable during cruising.. Converting to SD solved it even though I can still hear the surge. Yeah, the OP should experiment with BOV spring and preload..

 

That's also happening!

 

Thanks for your insight. I will work with a lighter spring to see if it helps

 

^ This, try that out first and see if it fixes it. GL