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I've been googling this a lot and most random forum threads that come up just fade off with no resolution, so I'm hoping the brains trust here can help shed some light.
I have a switch for 13psi and 20psi. 13psi runs smooth as. On the 20psi switch, once boost gets over 16psi I am getting oscillation or bouncing. It juuust hits the target but as you can see from the log images below, it's a very bumpy map trace (both 3rd gear pulls). The AFR and timing trace also bounce in line as well. The 13psi log is super smooth.
When it happens there isn't any jerkiness or stumbling. If I didn't have the boost gauge in front of me I wouldn't even know its happening. I haven't had the car long and I am certain when I picked it up it boosted up to 20psi above 5500rpm and held there, but I may be wrong as I believe I was holding on for dear life before I got used to it

Its a built SE with NitroDann twistmount, GT2860RS, 3inch dump that funnels down to 2.5in just before the cat.

I have checked for boost leaks.
Last week my Cat started falling apart inside and I hoped that was what was causing this but after replacing it, it's still the same.
I am running all the vacuum hoses (BOV, Boost Gauge, ECU MAP) off of 1x manifold barb. I have read this is a no-no, especially for the BOV, but I have a flat top manifold and there is only 1x barb on that manifold just behind the throttle body.

Could this possibly be a back pressure issue from the 2.5in exhaust causing issues with the wastegate? (this is about the only logical answer I have found online, but don't know how to confirm or rule this out).

20psi:


13psi:

My first thought is that the wastegate spring might be tapped out. With a single port can and 3 port MAC valve you can only control around 200% of base rate. That is, if the wastegate spring + preload is 7psi, you can only get about 14psi of controllable boost.

I'm not sure what the stock wastegate pressure is on your turbo. Logging a run with disabled boost control would let you know.

If that is the case, then you could either upgrade to a dual port can/4 port MAC, or just upgrade to a stiffer can with a higher base rate. But maybe I'm off base?

It's a 12psi spring so should be within it's limits. 4 port mac valves came up a lot in my searching and seem to be the thing to do now. Perhaps that's worth looking into upgrading to anyway.

It's not just the MAC though, you need a dual port wastegate to go with it. But, if it's a 12psi can as is you should be fine. I guess check the duty cycle of the boost controller at 20psi, see how hard it's working?

Other thoughts I guess is fuel? How hard are the injectors working? When was the filter last replaced? What's the fuel system look like, injectors, FPR?

Also, maybe spark blow out? What plugs are you running, and how are they gaped? Stock coils?

If you have the knowledge, you could lower boost to the point the issue doesn't present simply to identify the level it's occurring at.

ManiacLachy wrote: I guess check the duty cycle of the boost controller at 20psi, see how hard it's working?
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I think you're onto something here, but I'm not sure what I'm looking at with the wastegate duty cycle trace. Just checked wastegate duty cycle in the logs and on 13psi and it looks ok, but on 20psi it spikes like crazy in time with the boost dips. Not sure what this means though. I've google image searched and other peoples traces look very smooth compared to this...

Injectors are at max 70% on 13psi, but on 20psi it does go to 95%...

13psi


20psi

Methinks you might have to take it to a pro to sort out. Too much guessing going on here.

Thanks ManiacLachy, I think you've possibly narrow it down with suggesting looking at the duty cycle. Had a look at the ECU settings and the "Quick Spool" option is activated, "Opens wastegate 100% at MAP readings below target boost" with an offset of only 1.5psi
The actual vs target boost is on that limit just as the first spike to 80% happens, then I think with the sudden increase in boost it's just chasing it's tail up and down after that.
I'll disable the quick spool option and see what happens.

Yeah, could just be close-loop control not fully sorted and it's making wild adjustments. You said it used to be pretty solid, do you have an old log somewhere of a pull? You could compare.

But 95% injector duty is pretty maxed out as well. Might not be a contributor to the issue but something to keep in the back of your mind.

Your closed loop control is not set up well at the higher pressure and isn't maintaining the manifold pressure smoothly. You can see the manifold pressure jumping up and down with the wastegate duty. You'll need to work out why the pwm duty cycle is jumping in such large steps.

I turned off the "quick spool" option and did a log. Nice and smooth now and no noticeable different in spool. I'd say the threshold was too close to the limit. Might have a little play with the closed loop boost PID settings, but that's for another day.

Turn of any of the PID settings and work your way from there is lot of videos on youtube that can help you with pid