Vacuum reference off the intake plenum

I took the opportunity to re-plumb all my vacuum hosing a while back, to get the MAP sensor off the same vac reference as various other things like boost controllers and BOVs and FPRs.

Anyway. I'm sure people say, and I'm just as guilty, something like "just use any outlet from the plenum" or words to that effect.

Well I found that my fuelling was all to cock afterwards, especially under boost. After some strategic datalogging I found that the stock MAP sensor's voltage was up and down like a yoyo, which means it's either kerbuggered or the hose to it is bad.

After swapping out a known good MAP sensor, nothing changed. The hose checked out with the "red cheeks eyes bulging puff" test. On a whim I put the reference on a different barb and the signal actually cleared right up. So I put it back and it mucked up again, so no coincidence. Ace, problem fixed quickly and with no cost (for once).

The barb I used was the takeoff from the idle speed control valve area. It comes from the internal tunnel within the plenum and I can only conclude that some strange pressure waves were going on in there which got worse under boost. At 1.2bar it was fluctuating between 0.95 and 1.2bar, which is bad for fuelling I can assure you.

So the moral of the story is:

http://www.ian.chisholm.clara.net/pix/manifoldtakeoff.jpg

-Ian

The correct take off has a pulsation damper within the fitting, you are seeing cylinder pulsations on the undamped take off.

Just to confirm, the correct take-off is teeing off from the MAP sensor itself, is that right?

The correct take off has a pulsation damper within the fitting, you are seeing cylinder pulsations on the undamped take off.

So that's what that black blob is for Every day's a school day!

So my conclusion would be to run the FPR and any boost pressure sensors off the damped output and the BOV and whatever else off anything else? That doesn't mean T'ing off the MAP sensor hose but if done properly should be safe...

-Ian

I always thought it was just a filter to keep crap outta the map sensor, but it obvisously provides some sort of damping then.

in that pic, can you take the pipe from the top of a bov and tap it on to the nipple that comes out from the back, (one on the far left) as when going single that becomes free and i have done this and im sure my bov doesnt work..

in that pic, can you take the pipe from the top of a bov and tap it on to the nipple that comes out from the back, (one on the far left) as when going single that becomes free and i have done this and im sure my bov doesnt work..

my FPR is connected to that nipple on the back so it would be good to know if that is ok

My Greddy pressure sensor, BOV, and FPR are tapped into that rear one and I'm about off into the garage to take the FPR and Greddy sensor off it and on to the dampened port. All this has explained some odd readings I've been getting off it as well I'd say BOV yes, any boost pressure sensor, no.

-Ian

hmm not really what i wanted to hear, wonder why my bov dont bo

Would a vacuum plenum (one of the mkivstore group buy ones) connected to that port give a true reading for any gauges etc. attached to it?