Problematic RB25DET Overboosting - Sputters over 5psi

[Nismo34GTT]

I just got a R33 and have been working on it just over a month now with a friend. Since received the car has had boost issues whenever it's under throttle. Sometimes the boost gauge doesn't go past 5-7psi with the car sputtering like crazy under acceleration. Other times the boost gauge shoots way past 10psi almost instantaneously. First we checked the compression to find 170-180psi across all 6 cylinders (most were 180). We then removed the plugs, cleaned them, gapped them (0.035-0.040) and re-installed. No difference. We heard an air leak from the intake side when we first got it, so we took off the manifold and replaced all gaskets and hoses. The air leak was gone but the problem persisted. We took out the factory fuel pump and filter, put in a new filter and a 340lph fuel pump. Problem persists. We tried two aftermarket manual boost controllers. We used every vacuum diagram on the web for a R33 boost solenoid (sadly there are literally guides stating that every way to run the hoses is correct). We removed the heat shield to the turbo and could move the wastegate manually, so chances are it isn't stuck. Problem persisted. We then ran a hose directly from the charge pipe (coming off the intercooler to the intake pipe), directly to the wastegate (bypassed the solenoid) which should give us 5-7psi. The problem persists.

When you're driving anywhere below 3psi the car runs like normal. It has a factory ECU with zero codes. Stock turbo, stock intercooler, stock piping. The only aftermarket mods are HKS plugs, Apexi Filter, and downpipe/exhaust. Neither of us has any idea why this car continues overboosting causing a horrible sputter. We appreciate any help this community can offer.

[GTSBoy]

If it's misfiring at 5 psi or so, then it is weak ignition. Your coils are the most likely culprits there.

The boost control issue is not something that will be easy to diagnose across the 'net. Especially if you have had it apart lots. It will be something weird.

[Nismo34GTT]

Thanks for the feedback.  I'll try swapping out the coilpacks tomorrow and see if that fixes it. 

Edited January 6, 2020 by Nismo34GTT

[robbo_rb180]

Also test the wastegate actuator using an air compressor. Did you remove it to test wastegate movement? 

[Nismo34GTT]

I didn't remove it, but took a pair of pliers and was able to move the rod back and forth without any issues.  I'll try a compressor today as well. 

[Nismo34GTT]

Today I hooked up a 5psi source to the wastegate, and it moves just fine.  No idea why it was overboosting.  We checked the fuel pressure and it's a bit higher than it should be, which I've read happens sometimes when you go with a higher lph pump due to the extra fuel being returned to the tank, causing a bit of a pressure buildup.  We took some coilpacks off another RB25DET, swapped them on, and sure enough that was it.  Set the timing and now she runs perfectly!   Thanks for the help