matticakes

i wanted to check the operation of the bac valve on my 87 t2... i took it off the UIM while keeping the electrical connector connected and turned the key to on. I heard clicking but the inner "valve" wasnt opening or moving. Is it SUPPOSED to move or open back and forth?

Also, is that the only idle screw on 87 t2's. Do they also have the "mechanical idle screw" on the throttle body (if so anyone have a pic as to where this is located)? If not, the bac screw is the only means of setting idle?

Im aware of the throttle stop and fast idle screw adjusters which ive verified set correctly and replaced thermowax and tested.

I also tested resistance across the bac terminals, it read 0 which is why i took it off to visually inspect the operation...

The Wankler

iTrader: (14)

Follow the FSM for direction on setting the idle. You can find this on ine as well. Do a search for FSM, and look for a link.

matticakes

That doesnt work very well when you suspect your bac to be malfunctioning, i have read the FSM and followed its instructions

HAILERS

The resistance should be b/t 16 and 20 ohms. Zero ain't too good. Remove and replace it.

matticakes

ohms or kohms?

what about visually, is the valve inside supposed to move, hailers?

MRX_Rotary

Make sure your ohmmeter's fuse isn't blown. I mistakenly replaced my good BAC valve because of this. oops...

matticakes

ive used this ohmeter to set the tps, it works =P

johnnyg

Yes, the valve is supposed to move, but it moves back and forth very quickly, so on first blush, it might appear not to be moving.

HAILERS

ohms

matticakes

and finally before my big rx7 work day tomorrow, do the turbos have the mechanical idle screw on the throttle body?

Edit: one other note: after hailers verifying the ohms and not kohms im pretty sure i had my multimeter set wrong, which could have given me the "0.0" reading. My bac DOES click. My only other test i did with it was adjust its idle screw. By turning clockwise all the way i was able to get the car to stall. However, i wasnt able to greatly increase the idle, the adjusting screw didnt seem to have an effect in that direction. How high should you theoretically be able to set the idle with the bac screw?

RETed

Yes, there is a throttle stop screw.
It's very hard to see with the intercooler still on top of the engine.


-Ted

HAILERS

A decent bac should be opening enough to increase your idle to approx 750.

I've a suggestion. Get the engine hot. Idle the engine. Pull the bac plug off while idling. It should idle close to 500 to 750. IF you reinstall the bac plug the engine should chug up and down a few times before coming to an approx 750 idle.

But on the whole, the thing should idle close to 750 with the bac plug OFF. If it does not, then adjust the bac screw along with the variable resistor. Keep in mind the variable resistor has stops on it and won't turn more than approx 3/4 of a full turn. Clockwise to increase fuel at idle and counterclockwise to do just the opposite.

HAILERS

The bac will stay wide open only if you go to START with the key and hold it there. NOONE holds it there. But if you reach under the car and remove the small wire on the starter and go to START and look at the bac while it's removed from the intake with it's plug ON, then you'll see it wide open and staying there as long as the key is held to START. If you let the key return to ON, it will vibrate around 110-120hz AND this would be a good time to spray some carb cleaner into it and then dump it and spray again and dump it and do that routine several times. It MIGHT help. Might not. DO NOT PRY on the valve in the bac with anything. Just spray and dump.

matticakes

Will do what you suggest. Btw, im fussing with all this because after having a high idle of 1500, realizing the fast idle cam wasnt coming off the roller, replacing the thermowax and solving that problem I now have an idle thats TOO low on occasion almost to the point of stalling...

johnnyg

Did you re-adjust the TPS after replacing the thermowax thing?

matticakes

yes, ive always checked tps via resistance method after messing with any of the adjustment screws etc... i have yet to try the dual bulb method though

HAILERS

Next time get the engine HOT. Then put the meter on DC Volts, the negative lead on a good ground, then backprobe the GREEN/RED wire on the harness side of the TPS plug. You should see one volt. If not turn the TPS screw til you see 1vdc. Apporx is fine. Like 0.95 or 1.05 vdc. Done. This is done with the engine running and the tps connector connected up. It's in the fsm. Been doing it for years this way and I've done it the other ways also.

R37ribution

So what exactly should the voltage be? The reason I ask is because with most Ford cars/trucks it should optimally be .96vdc. But with Mazda it should be 1.00vdc exactly?

Also on www.explorerforum.com we found that it is best to make the measurement with the ground wire that is supplied to the TPS in the harness, since the resistance was slightly different when making the measurement from the chassis to the harness as opposed to using the ground wire and the hot wire on the harness.

I have not made the TPS adjustment on my RX7 as of yet as I just bought it, so I don't know all the wires that are supplied by the harness. I still think that it would be a better idea to use the ground (if supplied) in the harness when making the adjustment as opposed to any other ground.

my 2 pennies

matticakes

hailers, ive also read that this method can also be affected by a crappy battery/alternator, etc?

In your experience, have you set your tps via the voltage method to your liking then checked the resistance? Have there been times where the voltage is in spec but the resistance isnt?

HAILERS

I've done it with the two LED's then checked the output of the TPS to the ECU. It's always withing the figure I gave earlier. The FSM says ***APPROX 1VDC*** and that's with the engine hot and idling. Too much is made of the TPS setting. As long as it does not drift off the approx one volt everything should be fine and dandy. I mean really drift off to like 0.75 or something like that.

When you start off in the cold morning it'll be something on the order of 1.75vdc because of the thermowax/fast idle cam etc. That's why you check the tps when the engine is HOT.

I really don't mess with the resistance as long as the voltage goes up/down with no aberrations

matticakes

one other interesting tidbit. Approx how long does it take for your car to "warm up"? As ive stated, i replaced the thermowax (and while i was at it cleared the coolant path that was clogged) and then verified coolant flow by pumping water through the throttle body and watching it come out the other end. After all that, it still takes a rather long time (15-20 min) for the roller to come off the cam. Is this normal? Since its difficult to see the markings for setting the fast idle cam, would a good alternate technique be to set the fast idle screw so that your tps is at 1.75 on a cold engine (assuming tps has already been set correctly on a hot engine to ~1.0)?

HAILERS

If your sure water is flowing like you said, I think the sensible thing to do is FIDDLE with the fast idle screw/cam so it comes off the cam earlier. I wouldn't do the voltage thing you mentioned.

Twenty minutes sounds waaaaay to long for driving. I never timed how long before.

johnnyg

One of the hash marks on the fast idle cam is for 21 degrees celsius, which is basically room temperature. Since it's probably pretty close to that in NY right now, it should be easy to set. But whatever the weather outside, you could always take the throttle body inside, let it sit for about an hour, then adjust it.

J-Rat

iTrader: (2)

After you do the TB mod, checking your TPS is a MUST.

matticakes

just an fyi. I did the voltage tps check method and got .631vdc at idle, hot. However, via resistance method its nearly dead on at 1.006kohm. Alternator? Battery? Grounding issues?

Also, as a simple way to tell if the overall electrical draw on the car was a factor in my voltage reading I connected/disconnected the thermo switch (top of the thermostat housing) to make the little electric fan turn on and off. this caused about 0.01vdc difference in my readings...

thoughts?