Recently acquired #1915/2000 up in Canada. It's a fun little car and get a lot of positive reactions to it.
A little history and introduction:
From what I understand of its history is it was brought up north, rally raced, and sold to various POs before me. It's obviously been a little abused in the past with some interesting modifications (not all bad). I'm slowly putting the TLC into it to restore it to its former glory and smooth out some of the rougher edges it has developed.
My New Problem:
It seems the engine has developed a stutter/violent loss of power above 4000RPM when pulling through the gears.
What I know: Off of boost there is no problem revving. In neutral I can rev to redline with no stutter whatsoever, which makes me believe this is related to the forced induction.
I have unfortunately yet to invest in various aftermarket gauges or sensors to help analyze this any deeper so all I have to go in is what I see and feel.
The stutter occurs just before 4000RPM and opening the throttle further will cause it to bog or choke while pushing full boost (based off the factory meter on the dash). I can milk it gently with the throttle above 4000 but as the turbo continues to wind up it'll eventually stutter and more throttle will cause it to bog and RPMS to drop (boost gauge still reads full though). Lot's of throttle causes it to start bucking as if about to stall.
Interestingly, within the first 5 minutes of coolant reaching normal operating temperatures I can pull through all the gears no problem. After about 10 minutes and the entire engine bay heating up is when this problem arises. If I shut the car down, let it cool, and start again, I can pull through all gears without stutter but then eventually run into the problem again.
The first thing I did was replace the plugs with BPR6ES and gapped to .028, thinking it was a spark blow out. That didn't fix it, so now I'm onto the next steps.
It'd be nice if someone has maybe seen this/had experience with this and has the surefire guaranteed answer, but I doubt that. So instead I'm hoping to get a little feedback as to where to go next and my best options.
When I got the motor it had a coil on plug setup which I converted back to the regular coilpacks and wires. My plan is to reinstall the coil on plug setup and, if that cures the problem, I know it's related to ignition and will continue to replace the wires and coilpacks to try and fix it.
If that doesn't work, my next option are to look towards air and fuel delivery, as I'm guessing there's something wonky happening there. Maybe if it's starving for fuel it'd cause that stuttering followed by the ECU retarding the timing drastically to avoid knocking. I'm thinking maybe fuel pump and fuel filter as next on the list to try and replace, followed by taking a look at the injectors.
It currently has a K&N cone filter on and that seems to be the only modifications on the intake side. I'm wondering if it could possibly be a MAF issue or TPS or TB issues on the intake side either letting too much air/not enough air in. I don't think a turbo leak would cause these problems and I can't hear any noticeable whine or leaking sound but I could always do a leak test to see if there is a savage leak causing these problems.
After that, I'm not sure. Maybe someone here has some ideas or more advice to get the ball rolling?