Boost limited to 5PSI - Possible Tests?

[benzor]

I have an 1986 951 with ~85k miles. Was recently off the road for 2 months while I did the clutch. Now it is limited to 5PSI, and I'm really scratching my head on what could be wrong. Car is completely stock.

Before the clutch, car would boost to ~9PSI, but clutch would slip when really on power, esp. on shifts 2-3 and 3-4. I had a long history of testing prior to 9PSI (car would only run 3PSI), turned out to be a clogged muffler.

Here are the tests I've run since the clutch:
- Pressurize intercooler and intake at intercooler inlet: No leaks, very slow (2-3 min degradation from 15psi to 10 psi). Caught lots of small leaks previously.
- Checked accuracy of aftermarket boost gauge when intake pressurized - same to +/- 2 pis
- No exhaust from downpipe down : limited to 5psi of boost
- Check for exhaust leaks in crossover pipe: some wind, hard to differentiate from flywheel
- Startup vaccuum is 12, warm is 16.5.

Prior to the clutch, I had also done / checked:
- Wastegate open & close with external pressure : worked fine
- Replaced CV valve with known good valve, including bench test.
- Replaced the BOV with new OEM
- Block off plate on waste gate - was not the solution (was clogged muffler)
- Clamped wastegate line - was not the solution (was clogged muffler)
- Replaced all vacuum lines with new, venturi delete
- Capped the test port on the crossover pipe with a new lug nut
- Rebuilt throttle body to fix air leaks
- New fuel filter
- Put new muffler and catalytic converter in

As I see it:
- There could be a leak in the turbo outlet pipe. (Has been recently replaced)
- Turbo could have suffered a low-oil or low-coolant event, is no longer working. To my knowledge, turbo is a rebuilt unit put in 10 years / 15k miles ago.
- The computer could be opening the WG early, due to a faulty signal
- Any others?

I'm really at my wits end here !

Bruce

[flash968]

start with pressurizing the manifold (engine not running)

if you don't find it, then do a leakdown test

i'm betting you're losing it into the crankcase

[benzor]

Thanks. I did pressurize the intake manifold, so you're suggesting the leakdown test. Will take a look.

[Jfrahm]

If it wanted to, could it boost? Clamp off the WG line again and carefully boost the motor in a higher gear (right foot boost control).

If it can't boost then you run a different set of tests. Bad turbo, internally collapsed crossover, bad WG, etc. You can check if the WG bypass pipe/cat gets hot right away despite no boost as a quick test.

If it can boost but is choosing not to it could be the CV, KLR, knock sensor... I know some of these things have been tested but it's clearly time to test the tests and re-test.
-Joel.

[968 GUY]

Jfrahm is probably pretty close to correct, but you may want to go to the 951/turbo forum side. tThat way you can look at the troubleshooting guide and more people that own turbos ore on there.

It could be as simple as something got broken or knocked off during the clutch repair.

[benzor]

Will re-run the WG clamp and block off tests, good thinking.

[mikepellegrini]

+1 on the internally collapsed crossover pipe. That's something that does occasionally happen and produces the exact same results.

[benzor]

I actually had the crossover off before the clutch, on that hypothesis. It was not collapsed at the time, so I doubt it has since collapsed - but thanks for the check.

[Techno Duck]

I would recheck your wastegate. Make sure the vacuum lines are all routed correctly.

A boost leak limiting you to 5psi would be huge and the car would run like **** off boost.

[benzor]

Woohoo! It was the cycling valve. I clamped the line, and power came back, then re-arrnaged the wires to cycling valve and gave it a sharp tap, and it works. So now I have to figure out if it is a permanent solution.

Boost is now at 9PSI, so I've got some more sleuthing to figure out how to get to 11psi, but feel that is a more manageable problem.

Thanks for all your help!

[Porsche-O-Phile]

I had a similar, maddening problem with mine when I first bought it a few years back. I swore it was the collapsed interior section of the crossover but that is very, very rare and it ended up being a bad wastegate spring. Change your wastegate (don't just clamp it - change it out).