The Deputy

What does the vacuum limiter do that is attached to the throttle body beneath the manifold plenum? Mine is missing and both large ports from the throttle body have been plugged off, I'm not sure what happen to the little hose that comes to the center of the pod portion of the vacuum limiter. Is the limiter necessary or better yet...what does it do (besides it name sake)? The engine runs fine cold and warm.

Thank you,

Brian.

The Deputy

This throttle body below has a vacuum limiter...which got me wondering about my vehicle. Since I noticed the two ports plugged off while doing the fuel lines...and it didn't look right to me.

http://www.ebay.com/itm/Porsche-928-...0fce51&vxp=mtr

This throttle body has the ports plugged...like mine.

http://www.ebay.com/itm/83-84-PORSCH...049c0f&vxp=mtr

So, now I'm not sure about its importance.

Brian.

The Forgotten On

I'm wondering about its importance too as I intend to swap to a S2 intake system soon and it doesn't even have the ports for the vacuum limiter even drilled out.

I have a 4.5 l-jet which is basically the same as your 4.7 so I hope some one with a bit more knowledge will chime in and answer this for the both of us.

Eric Buckley

This part is also known as the Deceleration Valve. It's main purpose is to reduce the amount of unburned fuel sent out the tailpipe when you suddenly lift off the gas. The valve opens when you slam the throttle shut and vacuum becomes high, it then provides some air to burn the fuel, instead of passing it out less completely combusted. It has the effect of making the revs drop a little slower when you lift off the gas. Many folks have removed theirs, but I have fairly recently reinstalled mine. I think the only sort-of good reason for taking it out is if it is broken (leaking air all the time) and you want to save some money. Some think that having the revs drop faster makes for better heel-and-toe shifting, but I have not found that. What I have found is that having the Decel Valve working properly causes the oil pump to keep working harder deeper into a corner after a long straight, and that is a VERY good thing. Without the decal valve, when I would enter a corner that required shifting down the oil pump would slow down too much as I was shifting and I would get ugly low pressure spikes. With a properly working Decel valve the oil pressure in corners drops gradually somewhat, but without the ugly low spikes.

GlenL

Eric has covered the function well.

I've got mine disconnected as the behavior is fairly touchy and, with engine mods, it was holding the RPMs up too much.

It may have additional benefits with a catalytic convertor as it reduces unburnt fuel going into the cat.

Mrmerlin

Thanks for that tidbit of info Eric

mark kibort

you need to keep the revs a little higher during the turn.
there is a reason to disconnect the fuel cut off switch, and that does help the revs from falling too far should you have a slow shift near a corner.

I spent a lot of time and money putting the US vaccum diaphragm on the newer euro throttle body (which doesn't have it) by modifying it, so everything would run the same. BUT, when I did Scots euro 5 liter Ljet, after learning what the thing actually did..... I left it off. it ran the same in all ways as far as I could tell.

The Deputy

Interesting and at least I have a concept of its purpose now...and can weigh out the challenge or expense of replacing one.

Thank you guys for the info.

Brian.

Nate Miller

my decel valve is is not horribly gunked up but it has me worried. how would one test if it works properly or not or is there a way to clean it?