c900 boost gauge at idle

where should the boost gauge sit at idle, neutral or in the white? The gauge barely gets into the yellow at full throttle on mine, and in an effort to figure this out I disconnected the wastegate hose to the solenoid valve and test drove it. The fuel pump cut out as the gauge approached the orange but before it entered it. Could the gauge be reading low? I do get boost and the car doesn't really feel sluggish, though I suspect that it might be capable of more than it has right now.

Reply to
ebarter
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in article snipped-for-privacy@g47g2000cwa.googlegroups.com, ebarter at snipped-for-privacy@yahoo.com wrote on 26/06/2005 21:10:

The dash boost gauge shows a white section, a very short white section, a yellow section and and orange (or red) section.

With the engine off, the gauge should show just at the start of the broken white section. That is 0 bar pressure. Okay, yes, 1 full atmosphere at sea level, but not to confuse things; "0" :)

When you start the car and sit at idle, the needle should drop into the white section. This is normal. The gauge is showing that your engine is "sucking" ... This is called vacuum pressure. A vacuum is generated at the intake manifold by the combustion powering the crank and following through with an exhaust cycle.

Okay ... Boost :) When you drive and use the turbo-charger to "blow" air into the engine, rather than rely on the crank to suck air into the engine, your needle will enter the yellow zone. The exact pressures differ from car to car, but the end of the yellow zone represents anywhere between 0.8 bar and 1.0 bar of pressure. The C900 has a safety cut-out switch to cut the fuel when a pre-set pressure is reached - 0.95 bar on the T8 or 1.1 bar on the T16, IIRC (and "dunno" on the LPT 16V).

You are experiencing that fuel shut off. It would appear to be set correctly, since the car should not really enter the red (orange) zone on the gauge. The manual does say that under certain circumstances, it will, but should not be for long. Once the default cycle of the APC system is superceded by the internal tuning cycle, the boost will be lowered to the correct pressure and maintained by the APC system.

It would appear, therefore, that you are getting higher boost than standard. This means that the "default (or passive) cycle" for the turbo computer is over-shooting. The most likely reason is that your wastegate has been setup a little too high. There are a couple of remedies ... One is to bypass the fuel shut off system and the other to lower the wastegate setting. The former is preferable :)

Since APC will lower boost when it detects knock, inefficient fuelling (which will lead to knock) is not strictly a problem.

Paul

Var tog vägen vägen? SAAB : Nothing on earth comes close

Reply to
Paul Halliday

What I was trying to say is that that cutoff happened only when I disconnected the wastegate hose, which as I understand it is effectively bypassing the APC system. When I did this I was getting significantly higher boost than normal, almost to the end of the yellow. Normally the needle will barely enter the yellow. I would assume that something in between is normal?

Reply to
ebarter

in article snipped-for-privacy@g43g2000cwa.googlegroups.com, ebarter at snipped-for-privacy@yahoo.com wrote on 26/06/2005 21:52:

Yes .. That will happen :)

How far into the yellow does it go? About a quarter of the distance? Sounds like you're running on "basic boost" ...

Two things to check:

  1. The APC box, which you'll find the LHS wing, must be connected. A bad connection there will lead to "basic boost" ... Poor performance :)

  1. The APC solenoid valve on top of the radiator is another likely culprit. Check the power is properly connected. There is a further, more complicated procedure to check the function, but for now, just make sure it has power.

Remove power from either and if your boost is exactly the same, then that confirms my basic boost theory.

Paul

1989 900 Turbo S
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Reply to
Paul Halliday

I removed the APC fuse and the boost was ideed exactly the same as before. I pulled out the APC box, it looks good and the connection, at least right there at the box, appears good and uncorroded. I then put everything back and just disconnected the wiring the the valve, again everything was as before. Do you know how to determine which part it probably is? Thanks for all your help,

Emery

Reply to
ebarter

I also just realized that the red switch next to the APC box is disconnected entirely, the vacuum hose is just dangling there with its t- shaped connector. The hose that I think that connector would be in line with is just going directly into that other black cylinder. Also, there is another connector coming from that cylinder that seems like it should have a hose hooked up to it, but there is nothing and I see no hoses floating around that look like they should connect to it... perhaps this is a part of my trouble?

Emery

Reply to
ebarter

It's a good thing you didn't blow up your engine, the fuel cutoff is a last resort and hitting it is very hard on the engine, good thing the cutoff worked too or you'd likely be picking up parts of piston rods off the road.

Reply to
James Sweet

Well, I didn't really expect to hit it because I hadn't hit the red yet, though I was close...

Reply to
ebarter

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