Carpokes Porsche Forum

Backstory. I purchased a 951 4 months ago and after replacing the j boot, I discovered that the compressor wheel on the stock k26/6 had damage. The car still seemed to get around 10-11psi even with the bad compressor wheel. I decided to either get the turbo rebuilt or purchase another one. I ran across a recently rebuilt turbo (under 15k miles) for a good deal and decided to go for it. It looked great. No play in the compressor wheel and had been coated and cleaned.

I just got done swapping the turbo along with new vacuum lines, new billet Mode pop off diverter valve, boost hoses, Venturi delete, coolant hoses, new turbo oil feed line, aos seals, various other seals, crush washers and exhaust crush seals, etc. now I only get around 5 maybe max 6 psi boost and I’m hearing almost rattle like sound on cold start. Seems to taper off after warming up.. Maybe from the turbo? Maybe in the exhaust somewhere? I can hear it more in the exhaust under the car. But I can’t confirm and very hard to pinpoint. All I know is it didn’t have the sound before and made around 10-11’ish psi before the work.

Symptoms I’ve noticed
- Intermittent rattle sound seems to come from around turbo I think..? almost sounds like a rattling heat shield. at first I thought that what it is but doesn’t appear to be. still trying to nail down where but seems to be more pronounced under car. Could it be the turbo not getting enough oil from the feed line?
- Slight condensation leak from triangle down pipe flange on cold start. Just a little condensation seeps out of that flange. Assuming this indicates a leak but I installed a brand new donut crush washer and torqued very snug.
- Only 2psi of boost in first gear 3k rpm and maxes out at no more than 5-6 psi in second gear around 4k rpm

Troubleshooting I’ve done:
- Tightened down all vacuum and boost hoses, new wastegate hose, pipes..
- Swapped out cycling valve for a spare. I tested both with applying 9v to the pins and I can hear a click. air passes through the port that is cutoff when power is applied to the valve.
- My boost/vacuum gauge is reading 17-18 hg for vacuum at idle.

Trying to figure out if I need to tear it down again and just get the turbo rebuilt. Or should I keep troubleshooting before I do that? What steps would you recommend? I still have my old turbo that I could just send off to get rebuilt with new compressor wheel, but I’m obviously not happy about forking over the money to buy a ‘good’ used one and it’s not functional or is at least suspect. I’ve seen that clamping off the wastegate is a troubleshooting option but if it gets boost, what does that tell me, and if it doesn’t, what does it mean? I spent a lot of time on the project and tried to take my time and go through every detail so any pointers here on next steps/options would be helpful!

Pulled the j boot again just to double check the compressor wheel and it looks good and I don’t feel any play. Not sure if I have an issue with the turbo or something else reducing boost.

You could pinch the wastegate line before the CV and *carefully* do some 4th gear pulls to see if it climbs higher

Basically take away the wastegate & KLR control and see if you mechnically are boosting

If not... maybe you have a collapsed pipe or something physical after installation

FYi the wastgate spring regulates boost around what you are getting so if the electronics are opening the CV early, that max is about right

danmartinic wrote: Mon Jun 01, 2026 3:02 pm You could pinch the wastegate line before the CV and *carefully* do some 4th gear pulls to see if it climbs higher

Basically take away the wastegate & KLR control and see if you mechnically are boosting

If not... maybe you have a collapsed pipe or something physical after installation

FYi the wastgate spring regulates boost around what you are getting so if the electronics are opening the CV early, that max is about right

Thank you. So to confirm, would you say that 5-6 PSI in second gear is about right? And would it max out at that?. Would we expect full boost only to occur in 3-4th gear? I am still pretty new to the 951 so curious what expectations should be here. I admittedly haven’t been able to take it on a full open road to get to 5th gear yet. Maybe I’m expecting too much. I do know that I was getting upwards of 10 psi with older turbo in upper revs shifting towards 3rd.

No no max boost is possible in any gear.. I just like using higher gears for testing purposes

Something has happened when you did the swap. First step is to determine if it is electronic or mechanical

danmartinic wrote: Mon Jun 01, 2026 4:59 pm No no max boost is possible in any gear.. I just like using higher gears for testing purposes

Something has happened when you did the swap. First step is to determine if it is electronic or mechanical

Gotcha yeah that makes sense. Thanks for the confirmation. Yes I plan to do some testing to dig into this. So just an update. I took it out on an open road and was able to reach 11 psi once I hit 3rd gear. I was at 6 psi in 1st, 8 psi in 2nd and finally 11 when in 3rd. I’m expecting lag but at least I was able to finally get up to 11 and not sure if this is close to spec? And TBH it’s not too far off from how it was behaving before the swap…Guessing also since I got to 11 we’re not dealing with any electronics or cycling valve issue? Btw I have a mechanical autometer boost gauge.

If I pinch the wastegate and if boost climbs rapidly in 1st, am I possibly dealing with a stuck’ish wastegate? I did notice that one of the studs is broken so I’m
wondering if there is a potential leak there.

11 psi is about right (.75 - .8 bar)

If you can get it full boost in high gear maybe look at your diverter valve that it's properly installed & setup

It's hard to test in low gears as they go by so quick

danmartinic wrote: Mon Jun 01, 2026 5:49 pm It's hard to test in low gears as they go by so quick

I know! I had to to a mount my phone close to the gauge so I can take video to catch what I’m hitting.

danmartinic wrote: Mon Jun 01, 2026 5:49 pm 11 psi is about right (.75 - .8 bar)

If you can get it full boost in high gear maybe look at your diverter valve that it's properly installed & setup

So I replaced it with a new Mode billet piston style diverter valve so I hope it’s working bc it wasn’t cheap! https://944online.com/cut-off-pop-off-valve/ i used Lindsey’s vacuum diagram and it’s T’ed from the damper and regulator. https://www.lindseyracing.com/LR/Parts/VACDIA.html

do some research on the diverter valve because some of the non-stock valves are installed in the reverse way compared to the stock style.

chrischrischris wrote: Mon Jun 01, 2026 11:29 pm do some research on the diverter valve because some of the non-stock valves are installed in the reverse way compared to the stock style.

Yeah I ran across that possibility but I’m not finding a lot of Install info on this model surprisingly. It seems like it would at least be odd to install with the swivel vacuum line on the side rather than top. Does anyone run the Mode billet bypass/diverter valve? I guess I could try to install it the other way, any harm in that? Attached is how it’s currently installed.