Weird Boost Problem

nobbie_nz · Post by **nobbie_nz** » 04 Jul 2007, 15:41

Hi guys, all the way from New Zealand

Have just hooked up a temporary boost gauge on my new 950R 1996 AUTO and I have made some interesting findings.

After hooking up an Autometer boost gauge my vacuum at idle is around 15-17, during deceleration (no throttle) i get 20. Do you guys still think I have a vacuum leak? or do these numbers sound normal?

Now heres the interesting part...SOMETIMES for some reason I only get 8psi of boost, then other times I get 11psi??? During the shift from one gear to another the boost spikes to around 15-17psi (auto box) - please note this is BETWEEN shifts when the boost spikes.

The other thing that happened was that in 2nd gear at wide open throttle boost is at 11psi and then when the car changes to third the boost drops to only 8psi and feels quite sluggish.

Any ideas on what might be the problem here....I did some research on the site and read a little about wastegates?? but not sure if the wastegate would be the problem??

feedback and thoughts really appreciated

Cheers
Rob

Post by **MadeInJapan** » 04 Jul 2007, 20:18

Everything sounds fairly normal unless your ECU has been chipped. Other than that, I hope you meant 850 and not 950.

nobbie_nz · Post by **nobbie_nz** » 05 Jul 2007, 02:44

Right, i think im a little thick lol...having never owned a turbo car...something just dawned on me.

If the car downshifts to a lower RPM then the boost will start lower and then rise. IF the car downshifts to higher RPMs then the boost will be higher.

So...when changing from 3rd to 4th (auto) I would assume its normal for the boost to drop from 11psi to 6psi when it goes from high revs 3rd to lower revs 4th. The boost would then rise with the revs as the car gets faster.

Have I just solved my weird boost problem? or is this not normal

Post by **JRL** » 05 Jul 2007, 05:11

Not normal.
At idle you should be in the negative area for one thing

zenmervolt · Post by **zenmervolt** » 05 Jul 2007, 08:55

JRL wrote:Not normal.
At idle you should be in the negative area for one thing

He is. Re-read his post. He's getting 15-17 inches of vacuum at idle and 20+ inches of vacuum on decel. Both perfectly normal and in an acceptable range.

His boost should vary with both RPM and throttle position, though the spiking between shifts seems to indicate a failed bypass valve.

Since my 951 has a boost gauge and my S70 doesn't, I'll use numbers from my 951 to illustrate, but they should illustrate how a properly-functioning turbo setup should work (Note that the 951 is set up for 15 PSI of boost instead of the ~10 PSI of a stock Volvo T5 engine and that the 951's turbo spools much later than the Volvo's):

Warm Idle: 18 inches of mercury vacuum.
Cold Idle: 16 inches of mercury vacuum.
WOT (Wide Open Throttle) 2,000 RPM: ~5 PSI boost.
WOT 3,000 RPM: ~15 PSI boost.
75mph @ 3,000 RPM: ~10 inches of mercury vacuum.
Between Shifts: 0 (atmospheric) due to the bypass valve venting manifold pressure to keep the turbo spooled.

The 951 also has a mechanical boost controller and it's typical to see a lower maximum boost pressure in 1st/2nd gear than the others because boost also increases with engine load, the Volvo's system is electronic and should manage boost a little better. If I remember correctly, on a Volvo T5 engine, the turbo should be producing full boost by 2,500 RPM assuming WOT. Remember though that even backing off the throttle slightly can significantly reduce boost even if you're still accelerating.

nobbie_nz · Post by **nobbie_nz** » 05 Jul 2007, 16:39

Right,

Thanks guys for the help so far. I think im slowly drawing conclusions that there is something wrong with my boost and with your help so far I think im isolating the problem

Ok so the turbo is spiking between shifts = possible failed bypass valve

Just to clarify my maximum boost is not achieved until high RPMs close to the redline, like 4500+. From what I can work out, you guys are saying that I should be achieving maximum boost at around 2500rpm with WOT

zenmervolt · Post by **zenmervolt** » 05 Jul 2007, 21:34

nobbie_nz wrote: Also is the bypass valve also know as the wastegate?

No. The wastegate diverts exhaust gas pressure away from the turbine to limit overall boost.

The bypass valve is supposed to open whenever the throttle closes quickly to prevent the manifold pressure from spiking. It is sometimes also called a diverter valve, or, incorrectly, a "Blow Off Valve". A bypass valve will vent excess pressure back into the compressor intake side of the turbo, while a blow-off-valve vents excess pressure directly to atmosphere. Both a bypass and a blow-off valve perform the same function: preventing a boost spike during shifts and preventing compressor strain and stalling of the turbo (not related to engine stalling).

You should be hitting full boost well before 4,500 RPM with these engines, you're leaking boost somewhere. Check the bypass valve, and the wastegate as well, and the hoses. From your description, something's not right.