Joe Perez

iTrader: (8)

As I’ve been upping the boost lately I’d started to notice my BOV doing a lot of venting at part-throttle operation, particularly if I lifted off partially while in boost. This has a somewhat detrimental effect on drivability. So I decided to try plumbing the system for reticulated operation- I’ve now got the BOV venting into the pipe between the AFM and the compressor inlet.

I’ve now noticed that every time I shift under boost, the fuel pump relay clicks off and then back on. Makes sense I suppose as the door in the AFM slams shut (this opening the switch) but it really bothers me. If nothing else, this can’t be good for the relay contacts.

Has anyone else noticed this happening with a recirc setup?

bripab007

Never seen it. Are you running an RX-7 AFM?

Joe Perez

iTrader: (8)

Nope. Stock 1.6 AFM, unmodified.

bripab007

Sounds pretty weird to me. I've got an RX-7 AFM with a loosened flapper door and recirculated bypass valve and have never experienced this. Never experienced this on any of the turbo Miatas I've driven.

How do you know it's the flapper door closing that's causing the fuel pump to cut off?

magnamx-5

iTrader: (7)

yeah i vote for flapper door slamming shut but i dont see how are you absolutly sure it is the pump? WAIT A MINUTE YOU HAVE YOUR BOV PRE TURBO? if so there is your answer you are stopping the suction process that keeps the flapper open, how close is the bov to the afm if it is to close and you dont have the bov on the charge pipe then it would blow the door shut.

samnavy

iTrader: (7)

I've never heard of your problem... it looks like as your BOV dumps the pressureized air into the intake, it's enough force to blow the flapper door back closed??? That's weird?

The only way to know for sure would be to take off the black plastic cover on the MAF, and mount a bullet cam in the engine compartment aimed at it to watch if it pushes it far enough closed to activates the switch...

PS: does anybody have a picture of the SCREW I need to turn on the MAF to back off the spring tension... maybe a little writeup on how to do it correctly?

Atlanta93LE

iTrader: (5)

Sam, it isn't really a screw that backs off spring tension. It's clockspring, essentially. You need to pry the retainer spring back, rotate the spring, and put the retainer back into another notch on the spring.

magnamx-5

iTrader: (7)

mark your start point for reference.

Braineack

iTrader: (62)

here:
https://www.miataturbo.net/forums/sh...00&postcount=6

There is an air idle mixture adjustment screw, never fiddled with it though.

olderguy

iTrader: (19)

Mine is after the AFM door, and I probably haven't hit that problem because my FM BOV blows off first to atmosphere so that the first "shot" isn't able to affect the door. Very interesting.

jayc72

iTrader: (14)

If the line off the BOV is plumbed in between the AFM and the Turbo how could it possibly be causing the door to shut? if the line off the BOV was plumbed in before the AFM I could see it maybe being an issue.

Jay

Al Hounos

Can't you just get rid of the damn thing? You've got an EMU.

olderguy

iTrader: (19)

Look at my vb garage photo. I can see where a sudden blast of air could be more than the turbo was requiring and could affect the AFM door if I were not first venting to air the major amount of the blowoff.

jayc72

iTrader: (14)

I can maybe see it with how yours is setup Bruce, with the hose from the BOV going INTO the AFM. Joe's setup is just like mine and enters much closer to the turbo than the AFM.

TurboTim

iTrader: (9)

Your BOV is installed backwards. It still works, but relies only on vacuum to pull the piston up instead of both vacuum and positive pressure below the piston. If you install it correctly it would work better. I told you this before, but I digress...

Actually maybe your BOV doesn't work and that's why you are not experiencing this problem. hehe just kidding

Eitherway however, the FM BOV vents first to the recirc port, then to the slotted vent-to-atmosphere port. If you loosen the adjustment screw on the FM BOV (at least my older unit, same as Bruce's) until it basically falls out, at idle (high vacuum) you can easily visibly see thru the two flanged ports (aka huge leak). This is why I use the FM supplied air filter on that port. If I had a air flow meter, I'd have to make it recirc. obviously.

olderguy

iTrader: (19)

I don't really remember the discussion on it being backwards. Chaulk it up to old age memory loss.

I installed it this way(without directions) because it made sense to me that the BOV was blocking any blow off until I actually hit vacuum on the control line.

The other way it seemed that it could creep up slowly as the pressure decreased and maybe dump early.

What happens now is that it snaps open with a pretty decent blast when I pull over 20in/hg and then settles to recirculate at about 20in/hg. The filter seems to be superflous because at over 20 in/hg it isn't going to be pulling air into the intake through a closed TB and the major route for air to the turbo will be through the intake filter. I'm just sayin'

TurboTim

iTrader: (9)

True. I guess that's the reason for the spring tension adjustment screw on the top. And why I screwed mine all the way out, as I need it to dump as early as possible with these tiny turbos. (I actually removed a spacer from inside the BOV too.)

samnavy

iTrader: (7)

You sir, live up to your screen-name. Thanks for that diagram, I'm sure I won't be the only one to benefit from it.

Joe Perez

iTrader: (8)

Whoa- lotta opinions on this thread!

I'm using the old-style FM BOV which does not have any open vents. Here's a picture of it: http://www.flyinmiata.com/index.php?...umber=05-90105

The bottom (intake) of the BOV is fed from a point just before the throttle body. The side (exhaust) of the BOV goes into the pipe that connects the AFM to the turbo inlet, at a point just a few inches from the turbo.

I theorize that it's not so much that the BOV exhaust is blowing the AFM door shut, but rather that when I snap the throttle closed, there is little demand for new air entering the system and so the AFM door closes naturally. Basically, the same thing ought to happen with an NA car, but perhaps because the difference between WOT airflow and idle airflow is much greater now (meaning the flapper is open much farther at WOT under 10PSI of boost than it used to be at atmo) it has further to travel to get to the idle position, and thus its inertia keeps it moving all the way to fully closed before it snaps back to the idle position. The flapper itself has mass, remember.

Looking at the schematic for the circuit opening relay, it looks like there's a capacitor in there wired such as to prevent this very thing from happening. In other words, it'll hold the relay coil on through momentary closures of the AFM door. Perhaps that 16 year old cap is just dried out and in need of replacing. I think I'll try to open the COR and see if it's servicable.


Al Hounos; I have tried AFM removal via EMU about a month ago, but was unsuccessful due, I believe, to the limitations of my 2bar MAP sensor. I finally broke down and ordered a Greddy MAP sensor from Mohd, should be here Monday. So I'll definately be trying again.

Braineack

iTrader: (62)

maybe your dashpot needs adjusting so the butterfly valve doesnt slam shut?