XP Loss of boost.

Colin Stamp gurgled happily, sounding much like they were saying:

I can't remember for certain if it'd apply to an NG9-3, but the boost gauge on most recent Saabs isn't actually a "boost gauge", but a sort of ECU-controlled "load gauge", the calibration of which can be amended via the diagnostics...

First port of call, though, would be to see if it still does it on another tank of fuel, in case it's knocking back boost after seeing pinking. I'd also go over all the pipework - make sure there's no bleed- off from a duff vac hose somewhere.

Yep, now you mention it, I think I remember the service manual saying it shows injector pulse-width, so it's more of a torque gauge than a boost gauge. It hasn't been re-calibrated though, so the differences in the readings are down to some real effect.

I wondered about knock too, but it's been doing it through several tanks from several petrol stations. I've only had a quick look at the pipework so far, but I might get a better chance later today...

Cheers,

Colin.

A blocked air filter can do all manner of nasty things to any engine. I'd try changing that before anything else.

Si

Well, I had some time to do a bit of poking around today, so I tried taking out the air filter - no change, so that was that off the list of suspects.

The vacuum pipes for the dump valve looked a bit dodgy, so I thought maybe they ware causing the valve to leak air round the bypass loop. I tried it with the control pipe to the valve disconnected and plugged, which gave me low boost right through the rev range, which was inconclusive. So instead, I plugged up the bypass pipe. That gave me the original symptoms back, so I guess the dump valve and it's control system is off the hook too.

It could still be the dodgy vacuum tubes though. They share an inlet manifold tap with the fuel pressure regulator so I guess, if they were leaking, they might cause knock through low fuel pressure.

I've also finally RTFMd a bit, and I found a section entitled "Air mass limitation" which has a graph showing the torque reaching its maximum value at 1.8K RPM and then starting to ramp down at 5K. That doesn't really tie up with what I'm seeing now *or* how I remember it!

I think I'll just change the vacuum hoses and then forget about it, whether it fixes it or not. I'll be trading it in soon anyway...

Cheers,

Colin.

I must admit, I had my hopes pinned on it as a nice easy fix, but I've since tried it without the filter in and it's still the same :o(

Cheers,

Colin.

It's not got a little bit more rorty has it? A leak on the manifold/turbo sounds quite good until it's gross and then it sounds like a tractor. An exhaust leak before the turbo will reduce flow though turbine giving late spool and low boost. Grope all the nuts on the turbo flanges to check security.

A leak after the turbo reduces backpressure and thus pressure drop though turbo giving more boost. A 3" drainpipe exhaust does the same.

IF the air filter is "getting on" maybe its time for a thorough service (and as its a turbo - "decent" high quality Oil)

The exhaust seems solid and there aren't any audible leaks. I'll have a closer look when I change the control hoses for the dump valve.

Cheers,

Colin.

The filter is about 30K miles old, which makes it just due for replacement, though I've always thought 30K miles is quite old for an air filter. It's out of the picture now though. I do the oil/filter around every 6K miles against the 12K recommendation. The other service items are generally up-to-date. Are there any other routine service items that might affect this?

Cheers,

Colin.

Does it have a MAF sensor that could be gummed up? I know they're famous on VAGs for manking easy and just giving random symptoms including things like these... I'm not a specialist though, I'm betting others can give better advice.

Hmmm. Sounds plausible - I guess anything that could c*ck up the mixture might be the cause. I'll check that too.

Cheers,

Colin.