| Author |  Topic  |
melle
V4 Guru
United Kingdom
3833 Posts |  Posted - 26 Aug 2017 : 21:05:51  
| quote:
Originally posted by EdinburghJoe
according to the crankcase ventilation diagram on page 27 of Haynes it looks like the flow is in through this valve and out through the other rocker cover and up into the air filter
No, the other way around.
www.saabv4.com |  |
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pchristy
V4 Guru
United Kingdom
1790 Posts | Posted - 29 Aug 2017 : 07:18:39
| That's weird Joe! The manifold is normally full of vacuum - er, if you get my drift! At full throttle it will have little or no vacuum, but unless you have a super- or turbo-charger, there should never be positive pressure in the manifold!
Are you sure that's the original valve? Someone hasn't replaced it have they?
As a quick test, you could try removing the pipe from the rocker cover and blocking it off completely (cork or something bunged in the pipe!), and see if that fixes the problem. You will get some oily fumes escaping from the rocker cover into the engine bay, so don't treat this as a permanent solution, but it could help diagnose the problem.
--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!" | |
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EdinburghJoe
V4 Fan
United Kingdom
162 Posts | Posted - 29 Aug 2017 : 12:52:42
| It certainly looks original, and it can't be the wrong way round as it's screw threaded on one end and lipped on the other for the hose. Also, I've had the car for a year and it has always run fine, the problem I'm having is new.
Has anyone else ever had reason to do what I've done with the sucking/blowing through it in situ? If I'm the only person who's tried it this way (use a long-ish piece of hose to avoid inhaling any gunk) then perhaps I'm just misinterpreting what I've found.
Joe | |
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EdinburghJoe
V4 Fan
United Kingdom
162 Posts | Posted - 29 Aug 2017 : 13:06:48
| I've been trying to understand what this crankcase ventilation is all about. If it's what it says it is, then is it to create a flow of air through the crankcase?
If so, then it'll need to flow from high relative pressure to low relative pressure.
If the pressure in the air filter is at or just below atmospheric pressure, and the pressure in the manifold a varying amount lower still, then air would flow through the crankcase, going *in* to the manifold, which would mean my valve would be doing the right thing?
I'm more than happy to be corrected - I'd not even heard of crankcase ventilation before last week, but I'm just trying to understand it, so please put me right on what is happening and why!
Thanks again,
Joe
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pchristy
V4 Guru
United Kingdom
1790 Posts | Posted - 29 Aug 2017 : 18:49:50
| Yes, Joe. The idea is that the manifold will suck air through the crankcase and burn the fumes. However, you don't want an air leak into the manifold, so a restrictor valve is fitted which will restrict the flow when the manifold is at low pressure - essentially, at tickover. As the manifold pressure rises with the throttle opening, a light spring will push the valve open allowing more fumes to be drawn into the manifold.
If the valve sticks open, there will be a massive air leak into the manifold, causing lean running at idle, and possibly (probably?) stalling.
Therefore if you *blow* into the pipe that goes to the valve, the valve should restrict the flow (but not cut it off completely). If you suck, there should be little or no restriction.
Hope that makes sense!
--
Pete
"Duct tape is like the Force: It has a light side and a dark side, and it binds the Universe together!" |
Edited by - pchristy on 29 Aug 2017 18:50:45 | |
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EdinburghJoe
V4 Fan
United Kingdom
162 Posts | Posted - 04 Sep 2017 : 12:29:47
| That ties in then. There does seem to be a restriction when I blow into the manifold. The snapping shut when I suck hard is probably not something that ever happens in real life if there's never any positive pressure in the manifold.
Anyway, I finally took the car out for a motorway drive at the weekend (ie steady cruising, where the 'lumpiness' had been most pronounced) and it was fine the whole way, so with a bit of luck it was the tightening of the carburettor mounting screws that did it.
Thanks to everyone who contributed to this thread!
Joe | |
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melle
V4 Guru
United Kingdom
3833 Posts | Posted - 05 Sep 2017 : 19:24:30
| quote:
Originally posted by EdinburghJoe
The snapping shut when I suck hard is probably not something that ever happens in real life if there's never any positive pressure in the manifold.
The valve should snap shut in case of backfires.
www.saabv4.com | |
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| Topic | |
Intermittant uneven running when warm
