ZV

Car ran great yesterday, had a blast running around with friends on the 4th. Got up this morning, 3:30 am, driving to work (bloody having to trade in European markets), hit the on-ramp and start accelerating only to get a wooshing sound and no power. Engine runs smoothly and everything's grand as long as I keep manifold pressure at or below atmospheric. Anything over 1.0 Bar on the dash guage (0 on the mechanical guage) seems to be being bled off and the car just stops making power. Feels like it's in overboost mode.

Car has a mechanical boost controller, no cycling valve, GURU chips, K26/8, and a 3.0 bar FPR. It's not running hot and checking it in the parking garage when I got to work didn't reveal excessive heat above the turbine housing so I don't think the turbo is cooked.

Cliff's Notes:
'86 951, modded, K26/8, GURU chips/boost control, no CV, 3.0 Bar FPR.
Ran great yesterday.
This morning the car will not build boost past atmospheric, wooshing sound of pressure being vented after that.
Runs perfectly off-boost.
When boost is being vented, car feels like it's hitting a rev-limiter.

Pretty sure that the car is somehow reading overboost mode, but not sure what would be causing it. There were no warning signs that I could tell of and the car checked out good locally.

mroberts

Overboost shouldn't stop you with an MBC.

Check your intake piping - sounds like something came loose. Wiggle each hose - sometimes they'll be in place, but blowing around the edge. Also, make sure you do it when the car is hot - some things soften with heat, but will be firm when cold.

SamGrant951

Sound like what happened when one of my intercooler hoses had a hole in it. (Turbo->Intercooler hose) It would run fine until you hit boost then it would start venting pressure.

ZV

Even with it venting pressure, shouldn't it still respond like a slighly weaker N/A? Once that venting starts there is absolutely no power. If I firewall the accelerator on the freeway, the venting noise starts and the car does not go any faster, it just holds speed. If I'm careful to stay out of boost and gently give it more thorttle, the car accelerates fine. The feeling is just like a fuel-starvation rev limiter hitting. I'll go over the hoses when I get home this afternoon though, seems like a good and cheap(er) way to start.

Any way this could be due to a failed bypass valve? I'd wondered if there were an alternate overboost trigger like if the bypass valve came 100% open. Just pitching a crazy idea with that though. The "woosh" seems to be coming from the area around the bellhousing, but hard to tell because it doesn't build boost sitting still so I can't rev the engine with the car in neutral and listen for it.

Porsche-O-Phile

I've had all the following happen with similar results:

- TPS sensor disconnected (jiggled loose, not snapped in all the way)
- Turbo outlet hose (big fat silicone one to hot side I/C pipe) popped off I/C pipe
- Blockage in exhaust (but I think you'd probably know it)

mroberts

If it's venting pressure, you'll be running rich, which may rob power.

Bypass valve is easy to check.

ZV

Hadn't thought of the richness, very true and would account for the power loss/mild stumbling. Thanks.

ZV

Rubber connector at the throttle body poped off. I feel stupid...

One tightened hose clamp and one new scar for my hands later she's good as new.

Danno

Yeah, with increased boost, it's VERY common for those hose-connections to blow apart. The intercooler hoses like to pop off as well. Usually it's a good idea to take all the connections apart and clean out the insides with acetone to remove all traces of oil. Then reassemble with a little smear of rubber-cement to hold the pieces together. Then crank down the hose-clamps. Not too tight on the intercooler hoses though because they're reall thin and you can squeeze right through them..

Glad you found the problem!