Can someone enlighten me on the function of the "vacuum limiting valve" on the 308
From page D17 in the Mondial8/QV WSM: Image Unavailable, Please Login Not sure at what intake vacuum level (applied at the small fitting) it "opens", but the air path that flows thru it (when it's open), via the big fittings, is plumbed in parallel with the throttle plate air path, so when it opens, it's like pushing the accelerator pedal down a little (and "limits" the vacuum level in the intake manifold from going to a super high value and always keeps enough air entering to keep the airflow metering plate deflected and some fuel flowing).
And if yours starts sticking, it will manifest itself like a vacuum leak, with the car refusing to settle all the way down to a normal idle at traffic lights. Happened to me last year. And GOD help you if you need a new one. They are Unobtainium from Bosch so the only place to get them is NOS from Ferrari parts places. I paid north of $200 for one. I took the Bosch part number off mine and called Bosch. They looked it up and told me it was a Ferrari-specific model (even though they are used on tons of different cars with K-jet) and they had none in stock, so NOS from Ferrari was my only option.
What would be the harm in it's deletion? I can't quite understand the reasoning behind it's function. I understand the reasoning for limiting man. vacuum on decel for emission purposes, but not about curbing the leaning aspect.
Does the VLV perform the same function as the O2 sensors in that the function appears to be somewhat simillar? That is, they both seem to keep the engine from stumbling when throttle is being reduced. Refering to an US 87 TR with Lambda.
This is the aspect that confuses me.......richening on decel. I need to find my jag notes......there was an article explaining the rational of bleeding of manifold vacuum on deceleration.
Wasn't there some other car of the time that used the same exact part, only with a different p/n ? I remember reading about it somewhere in FChat, but unfortunately can't find it and didn't make a note at the time. Was it Porsche ?
I've found someone that says it's the same as the one from a VW Scirroco. Can anyone confirm this? The purpose of this valve, as I understand it, is to let some air bypass the butterfly (but not the fuel distributor air sensor plate) so that you don't have a lean condition when off the throttle, which could cause a backfire and serious damage to the fuel distributor P.S. - Could this one be the same? Not a nice price ... http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=911-110-220-00-OEM&catalog_description=Decel%20Valve%20%28or%20vacuum%20limiter%29%20%39%31%31%20%28%31%39%37%38%2D%38%33%29%20
Yes.......i assumed it was a backfire issue........thats what the jags use it for......but i always thought backfire was a rich condition.....oh well.......thanks for all the info....
As a secondary function I believe it also acts like somewhat of a damper for the air flow meter for CIS cars - in other words, the butterfly shuts rapidly off throttle and extremes of vacuum allow a little air to flow through the valve to cushion the motion of the flow meter a little. Other who work daily with the CIS system will know better than I but had heard that was the case and it seems to make logical sense.
Well i got the scoop from a porsche mechanic........it is for over run.......just like the overrun valve in jags........to keep mixtures from being to rich during deceleration......this is probably more critical in cars with cats.......mine does not have any cats or mufflers.
