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Discussion Starter · #1 ·
I'm just about to replace a duff turbo bypass valve on my 1989 2 litre turbo and I've got totally confused about which way round it goes.
At the moment, the end opposite the vacuum nipple goes into the metal pipe running from the turbo to the throttle. Looking at the photos on various sites, including Bill J's brilliant site, this is correct. However, if you look at Quasimotor's site, or virtually any site on Google "Turbo Bypass Valve", the valve should be positioned the other way round. Has anyone got any views on this, or better still, does anyone know which way round the factory fits them?
 

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Hi, Dave. Welcome to SaabScene.

Well, I would say this, wouldn't I? The pictures on my site agree with the under-bonnet photo in my '91, '95 and '96 handbooks, and I think we must assume that Saab got it right.

This is scanned from the '95 handbook. The '91 shows the same. I don't know whether your '89 has the valve in that position or down by the turbo compressor, where it was on some earlier models.



Quasi even says that his orientation disagrees with an on-line scan of a Saab technical document, but he reasons that this document is wrong. However, his reasoning is based on a misunderstanding of how the bypass valve works.

Contrary to popular belief, the valve does not have to resist boost pressure while under steady boost as there is (almost) no pressure differential across the diaphragm, and it will not blow open when connected in Saab's preferred orientation. On the contrary, fitting it the other way round will make it slower to respond and reduce performance, possibly even making it less effective in protecting the turbo from shock wave damage on throttle release.

P.S. I have nothing against Quasi or his excellent site, which has been of great assistance while working on my own cars. I simply believe he made a mistake in this instance.
 

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Agree with BillJ: it should always be fitted as shown on his site. To confuse things slightly, Bosch did put the internals in "upside down" in some years (according to Trent TSL and don't ask me why
) but this still means, externally, it should be fitted the same.
 

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Discussion Starter · #5 ·
Thanks a lot, really helpful. I got a bit worried by Quasi, he's usually very reliable. I assume I'm right in thinking that you test the valve by sucking on the small pipe from the throttle body & if there is no resistance, it's faulty. I've got a rough idle & I'm working my way through the possibilities. I've cleaned the IAC valve & all the vacuum hoses look OK. Anyone know a good source for a Turbo bypass valve? Saab want £27 which seems a lot
 

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Don't worry too much about Quasi Everything else he says seems sound.

You are right about testing the valve. However, it is possible to have some resistance, yet it still leaks a bit. If it doesn't hold vacuum completely, then change it. A small leak won't be small for very long.

As for suppliers, I got my last one from Saab because I wanted the newer type which was supposed to be more robust - my Aero runs a lot of boost. It lasted three days. The earlier type is still available from Euro Car Parts and probably many Saab specialist suppliers. ECP charge about £19 + VAT + delivery. I have never had one of these fail on my Aero.
 

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Discussion Starter · #7 ·
Just to update you, I've fitted a new valve and the difference is surprisingly noticeable, better pull & smoother gearchanges.
I got the upgraded valve - I struggle to reach 175bhp so it should last more than three weeks, I hope.
By the way, the Saab part number is 4441895, which I'm told fits all models including 9-3 and 9-5. If you want to save 30% on the price, go to your nearest Bosch stockist and ask for part 0280 142 110. If enough people bypass Saab main dealers parts departments, maybe they might reduce their prices - or am I being naive?
 
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