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Discussion Starter · #1 ·
Just joined up and I’m hoping someone can point me in the right direction for my odd fault. Just bought a 97 Aero, it’s my 5th Saab started out with a 78 99 then 3 9000’s until I found a decent lowish mileage aero (in good condition), finally found one.
Everything works fine (which makes me slightly suspicious!) but I have a curious noise. It sounds like a leak in the intake / vacuum line, a very loud hissing like compressed air being released. All the boost is there, needle goes to the broken orange / red easily and touches the red when giving the car a lot of gas. In the white at idle, all normal? But as you put the engine under load the loud hissing is present all the way up to 6000 rpm then disappears when you release the throttle. I would have thought that if there was a serious vacuum leak the car wouldn’t act normally under boost, idle is very even and there’s no sucking sound at idle.
The hissing is very loud, drowns out all the engine noise, like a leaking compressed air line. Now I’ve inspected all the hoses, all seem okay, not loose, missing or cracked. I’ve put soapy water round all the big elbow joints and it doesn’t get sucked in or blow bubbles, so I’m stumped. The noise isn’t very noticeable with the windows up, you can hear more or less normal engine noises, but down can’t hear anything but hiss!
My first thought was for a vacuum leak, but couldn’t find one, then maybe intercooler problems, or the bypass valve, leaking under pressure. But of course it’s hard to find a definite pressure leak when there’s no ‘real’ load on the engine. Could a restriction in the cat cause this noise when there’s a lot of exhaust flowing through it? Any clues or suggestions would be greatly appreciated, cheers.
 

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If it was a vacuum leak you'd normally expect the idle to be odd, at least high (eg 1,300+RPM) which doesn't mean it isn't a vacuum leak. Are there any other signs of odd behaviour such as low volt on the battery suggesting something is working overtime to compensate (eg cooling)?

I know you've checked the hoses but have a look at the large hose connecting to the inlet manifold at the back, just to the left of the throttle body. This is push fit and if it works it's way loose can give symptoms similar to what you've experienced. Also double check all the jubilee clips around that area.

Hope that helps
 

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Discussion Starter · #5 ·
The idle on this car is more stable than on any other I’ve had, sticks around 8-900. The temp gauge stays just below normal, as on every saab I’ve had, hasn’t even got hot yet, though the cross member above the radiator / intercooler does get hot (I put that down to having four coolers around there). Battery voltage is 10.5 when I first switch on (the battery is a smaller capacity / size than it should be so that will get replaced).
I’ll check the hose you mention and all the other clips again. The hiss is equal in volume whether you wind down the drivers or passenger window.
Thanks, I’ll report back!
 

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Originally posted by SteveN:
[qb].... but have a look at the large hose connecting to the inlet manifold at the back, just to the left of the throttle body. This is push fit and if it works it's way loose can give symptoms similar to what you've experienced. [/qb][/b]
I think someone has reported that this hose can split and a replacement is not cheap.
 

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I'd cjheck all the larger pressure hose (intake hoses) fior splits (which cunningly seal under vaccum, they do that).

If the boost guage is working most of the little vaccum lines will be ok as it is driven from these.

Be careful with the elsow bu the intake, it is £100+......

Andrew (back from Anglesey)
 

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Do check your exhaust - the flexi joint. Bill J will confirm the very odd noise when he was a passenger in my car a year or so ago. It sounded just like a severe boost leak but wasn't. I was fooled. I will wager this is your problem.

If this is the problem then I would suggest that you buy a 3 inch downpipe from Elkparts. Not much more expensive than the standard item and much better spool up.

Alanb
 

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If it is the intake pipe to the trottle body - the large right-angled pipe, try Neo bros first.

I got one there for a fiver !

If it's your exhaust, a check below will confirm if it is blowing. next step, check engine mounts, 'coz if they're worn/shot they will allow the engine to move around under power, which in turn will cause the exhaust to flex beyond 'normal' perameters.

ATB Ric.
 

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Discussion Starter · #10 ·
Thanks for all the advice. Just to be sure I replaced all the small diameter vacuum tubing today and I’m now sure its not those. Can’t be sure it is the exhaust until tomorrow when I will get the car up on a lift and have a good look about / pressure test. I removed all intake piping from the intercooler to the manifold, gave the joints a good clean and couldn’t see any splits. The performance doesn’t seem to be down at all, and the hiss is so loud that I’m hoping I’ll have an answer to the problem when I look at the exhaust tomorrow.
I’ll also swap the dump valve tomorrow and see if that makes a difference.
 

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Easiest way to check for exhaust leaks is to hold an old rag over the outlet while the engine is idling. The increase in pressure will quickly induce any leaks to make themselves heard. Helps if you can get an assistant to hold the rag
 

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It could just be that the bung in the firewall behind and belwo the glovebox has come loose, which would tend to allow more of the noise through from the hissing turbo. I thought I had a leak on my last 9000 and it turned out to be that.

Ideally you need to be able to check the system under pressure- some garages have a handy adaptor that's a section of ally tube with an air valve on that can be attached to the intake system at the outlet of the turbo (you take the outlet hose of the turbo and connect the ally tube to it). This is the best way of finding any leaks that I've come across.
 

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An idle test may not be sufficient to find the exhaust gas leak in this case. You may need a helper to rev it a bit. My car was fine at idle and only made the noise/let significant gas escape under power.

Alanb
 

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Discussion Starter · #14 ·
After several days, fiddling about and changing pipes, I still have the hiss! Had the exhaust checked today and it’s not that, all the sections are pretty new so a leak was looking less likely once I got it up on the lift. Whilst up there I removed the plastic board behind the front valance and check the intercooler pipe fittings, one of the clips was on at an angle so put that right and hoped I’d found the leak. Replaced all the APC pipes, two of them were the type covered in webbing, bit hard to tell if they were leaking. But the hiss is still there! I think the next step is to find someone who can pressure test the turbo pipes. Thanks for all the suggestions.
 

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Has a previous owner done the modification that involves drilling holes in the air filter housing?

(I haven't got a 9000 but they do this on some Saabs)

Some years back I did odd mods to odd (non-Saab) cars, when I did things to the inlet / air filter the extra noise was OK for short trips, but I didn't do those
so I put it back to standard.
 

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Has a previous owner done the modification that involves drilling holes in the air filter housing?

(I haven't got a 9000 but they do this on some Saabs)[/b]
Like 8 valve C900s? .

Does this hiss come noticably from one side of the car? Also, I'm assuming your Aero has ACC, have you tried checking for fault codes in case that's to blame? (IIRC the codes are given in the owner's manual)
 

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Discussion Starter · #17 ·
At last the mystery hiss is solved! Over the past two weeks I've checked / replaced most of the engine but didn't find the problem. Went to my local Saab specialists several times, pressure checked the turbo system, no leaks, checked the exhaust, no leaks. Diagnostic computer said everything was fine. So I thought I'd have to put up with it. Couldn't think of anything thing else, it wasn't leaking under idle or at higher revs when there was no load on the engine. It hissed just when the engine was under load, even light load when the turbo wasn't putting out much pressure. It suddenly came to me that the only part of the engine that is under pressure at all times is the exhaust mainifold, engine to turbo. Looked around there and noticed a brand new gasket on the engine, got the torx bar out and the bolts needed another half turn. The hiss is virtually gone.
The strange thing was that the car didn't suffer from a loss of boost, and tightening them up hasn't changed the performance at all. Maybe the turbo spools up a bit sooner but you can't really tell.
But thanks for all the advice, the car is much more pleasant to drive without the hiss.
 
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