Vf48 not holding boost for long

[Ninjacheezit]

So like the title says. I just got back from a “check up” with my tuner. Everything was normal. Anyways I asked him about my tune and he told me that my car peaks 18psi where it should but it won’t hold as much boost as it should at redline. He was telling me sti turbos usually peak 18-19psi and only taper to about 15-16 psi at redline. Mine is dropping between 10-12psi.

 

Anyone got any tips on finding out what the problem is? Actuator spring? Boost leak? Turbo gone?

 

Edit: just got off the phone with him it’s 18psi at about 3200 rpm then tapers to 10psi by redline on a 3rd gear dyno pull

Edited June 17, 2019 by Ninjacheezit

[SonoftheBread]

I'm gonna guess you guys have searched it over for vacuum leaks? My other guess would have to be an exhaust leak between the headers and the turbo. Check the uppipe and crosspipe for leaks, cause a leak between your exhaust ports and your turbo is a boost leak effectively. Also check your intercooler piping for leaks and then additionally if your wastegate is getting stuck you're just gonna dump your boost, especially at higher rpms. Film or watch the actuator so that it's moving correctly. If all else fails, send your turbo in somewhere for an overhaul.

[Ninjacheezit]

I'm gonna guess you guys have searched it over for vacuum leaks? My other guess would have to be an exhaust leak between the headers and the turbo. Check the uppipe and crosspipe for leaks, cause a leak between your exhaust ports and your turbo is a boost leak effectively. Also check your intercooler piping for leaks and then additionally if your wastegate is getting stuck you're just gonna dump your boost, especially at higher rpms. Film or watch the actuator so that it's moving correctly. If all else fails, send your turbo in somewhere for an overhaul.

 

Ya I checked for vacuum leaks and my intercooler piping all that fun stuff. This weekend I guess I’ll look for some pre turbo exhaust leaks. I did reuse my turbo gasket. Thought it would be fine since I didn’t run them only just tightened it down so I might just go ahead and replace those and maybe the exhaust manifold gaskets just to be safe.

[Marvick08GT]

Wouldn't it be faster to just drop the downpipe? You can check the turbo and verify the wastegate is closed, no cracks etc.

[Ninjacheezit]

Wouldn't it be faster to just drop the downpipe? You can check the turbo and verify the wastegate is closed, no cracks etc.

 

Ya if I just replace the gaskets to be sure the downpipe will be off anyways where I can check the wastegate and for cracks. But when I put it on which wasn’t many miles ago maybe 100 there was no cracks but something could of happened on the dyno.

 

If everything is good no boost leaks/exhaust leaks, no cracks or anything should I try a avo 15psi actuator or go straight for a turbo rebuild?

Edited June 19, 2019 by Ninjacheezit
Edit: just remembered vf48 doesn’t have an adjustable actuator so changed the question

[Ninjacheezit]

So I just went over a few things today since I got back from work. Checked the exhaust manifold, crossover pipe, and manifold to up pipe gaskets didn’t feel any leaks. Also put a thin piece of paper around it to see if it would move and nothing. So if there is a leak it would have to be the turbo to up pipe gasket

 

. Second thing I tried is a smoke test. Took off the pipe from the turbo inlet and maf put a bag over it and taped it up. Blew smoke in it saw none anywhere kept going for a good amount of time still nothing. Took off the bag and all the smoke came out. So I’m gonna say it’s not a vac/boost leak.

 

Third Took off the downpipe to see if the turbo cracked. Nothing still fine.

 

Now from here I guess my only options would be replace the turbo to up pipe gasket. Or change the actuator to see if maybe mine has worn and won’t hold over time.

 

Will also be doing a compression test tomorrow to check cylinder 3. Has the slightest misfire at idle. Throws no codes and wouldn’t of known It was missing if I didn’t ask my tuner why my car randomly started surging at idle and a couple times died out. He suggests a leaking lower seal on my injector or vac leak. But I’ve read online Horror stories about the same problem and it was a failing Ringland. So compression test it is just to be sure.

[Ninjacheezit]

Update: just finished compression test on the engine warm. All of them were at 150psi. So that wouldn’t be the misfire problem thank god.

[Ninjacheezit]

So here’s an update:

Installed a boost gauge like my tuner recommended so I can trouble shoot this problem a little easier.

 

Did a 3rd gear pull and fair enough saw it with my own eyes. Gauge peaked at 18psi held 16psi up until 4000-4500 then started falling to around 12-12.5 at redline. Surprised how accurate the gauge was based on my dyno psi readings but that’s a different story.

 

Anyways got home after coming from Subaru and installed the vent valve solenoid. Took it back out and tried it again since it was stuck open boost would of been going in my evap lines. Hoping that fixed or helped the problem. Nope same results. Good thing is it did fix my terrible idle every time I get gas. So one problem down.

 

My vac reading at idle in drive is 19hg and in park it’s around 20hg my elevation is 774 above sea level so I believe those readings are right. I wanted to make sure the manifold still wasn’t leaking so I sprayed everywhere I could with carb cleaner. No idle change so don’t think it’s a vac leak since that didn’t change my idle and my readings are good.

 

Guess from here I replace the turbo gaskets and hope that’s the problem. If not I’ve been talking to jmp and will be getting my vf48 rebuilt sooner than I was expecting.

 

Anyone got any other ideas that I might be over looking?

[ehsnils]

Can you do a data log? That could tell a lot more.

 

 

 

You may have a bad knock sensor that starts to give false indications of knock when you go up in rpm.

 

 

Also have the injectors checked to ensure that they work well and are matched. Mismatched or bad injectors can give strange issues and lead to more serious issues later like burnt valves.

[Ninjacheezit]

Can you do a data log? That could tell a lot more.

 

 

 

You may have a bad knock sensor that starts to give false indications of knock when you go up in rpm.

 

 

Also have the injectors checked to ensure that they work well and are matched. Mismatched or bad injectors can give strange issues and lead to more serious issues later like burnt valves.

I can’t datalog sadly really should get a cable so I can.

 

Also I repeated the smoke test. This time I did fine a leak. I forgot to clamp down the coupler on the back of the sti tmic. Went out again and did some 3rd gear pulls peaked around 19-20psi redline 14.5-15psi. Plugged in my code reader and my map readings were about the same readings as the boost gauge peak was 19.5 redline was 14.8. Hopefully this was the problem.