R33 RB25DET Not reaching set boost + jerking etc

[Jakeb.]

Engine: RB25DET s2 (in a 1996 R33 gtst)

ECU: Stock

Engine Mods: •Gktech fan/adapter •Gktech master cylinder stopper •BoostDoc billet half moon seals/oil splash plates •OG valve cover bolts •Greddy FMIC w metal piping •Greddy Trust BOV •red top coils (dunno the brand tbh) •Greddy iridium plugs (0.08 I believe pregapped) •aftermarket coil harness •Metal head gasket (on before I bought the car) •HKS EVC IV (I think) boost controller •cone filter cai metal •Tomei forged Downpipe •Viper 100cpsi 4” high flow cat (cerikoted) •nur spec exhaust •almost every single hose has been replaced with new/improved silicone ones (aside from the fuel lines and a couple breather hoses), all couplers are within a years old aside from one or two on ic piping

probably forgetting some engine mods but whatever if you have any questions just ask me please.

So, over the last week or two I’ve been having this issue with boost that I can’t figure out. My target boost is .85 bar, which I’ve been safely running and hitting for over a year and a half with no issues. Basically, if I floor it in 1st to redline I’ll hit .85 bar no problem and every gear after that it just doesn’t want to make it to .85 bar…. It’s like whatever I set the boost to, I can expect to hit .1-.05 bar below my target boost. Usually I let off when I feel it, because the engine sounds “different” once it’s getting closer to .6-.7-.8 bar. It’s like a dull sound that sits at the same decibels basically, and when it gets to this spot it’s like a dead zone for power, the boost feels like it’s completely gone. When I’m at this point in boost it sounds almost like I have a major vacuum leak from the drivers side, shhhhhhh! If I shift out at this point I usually hear a pop of some sort. And if I try this again the boost will feel fine up to the same area, and like I said 1st usually runs fine and hits target. 
 

so, today I decided to send it. I brought it to the point where the boost does this thing, like every other time it struggles to get closer to target (about .1 bar under set boost of .85 (the higher gear the less boost I get to) & (I set it to .8 bar and it seems to do the same thing about .05-.1 under,  if I set it to .9 bar it seems to hit .85 if I remember correctly) BUT this time I had it set to .8 bar and held my foot down even though there was boost issues, same stuff occurred but while I was holding my foot down through it the car JERKED HARD once, the boost looked like it hits target and went way down for a split second and then started coming back up, the engine sound and power loss issues still present. But that jerk and pop the car makes is unsettling to say the least. I tried it a couple more times and no matter what the same situation happened.

Fuel pump seems good, injectors are blowing their loads, I’ve reset the ecu, checked for leaks, reset the boost controller and reconfigured it after the most recent mods (cat and downpipe) it has never done this before and even after the downpipe install it’s been handling anything I throw at it. Turbo spools and sounds beautiful, oil and filter are new as of a day ago, fuel filter was replaced 6 months ago. I can’t figure this out. My gas tank is cracked slightly and I’ve been busting my ass for weeks trying to fix it but even still the cars been running just as god intended up till now.

The shhhhhhh! I hear out the drivers side makes me think it’s a leak… like a leak that really just shows up at .6-.7 bar and up or something, because I don’t hear it at all any other time. Someone please save me from myself.

[GTSBoy]

It's going to be a plenum gasket leak, isn't it?

[Jakeb.]

On 2/10/2022 at 4:53 PM, GTSBoy said:

It's going to be a plenum gasket leak, isn't it?

You think so? The middle gasket on the plenum has been kinda iffy looking and I have a replacement ready to swap in for it but it hasn’t done this before and I haven’t ever changed the gaskets on the plenum… but I’ll change the gasket I have today and report back if anything changes or not. Thank you.