bac valve question
 

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...

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.

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

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

ohms or kohms?

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

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

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

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.

ohms

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?

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


-Ted

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.

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.

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...

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

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

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.

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 :D

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?

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

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)?

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.

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.

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

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?

I don't see why the fan would make a difference. The TPS uses a ref voltage of 5vdc put out by the computer and I would have though that that figure would have stayed the same.

I don't quite under stand you voltge reading. You didn't leave it at .631 did you? You adjusted it to approx 1vdc I would have thought.

About the tps and the lights/ground signal coming and going at the check connector: When the tps is adjusted right, the ECU will put a GROUND on the Relief Solenoid and take it off the Switching solenoid.

That is what is being represented by the LED lights coming and going at the tps check connector (the Green, three socket connector).

Look at the wiring schematic for the EGI EMISSIONS and the tie of the Relief and Switching solenoids to the tps check connector.

I'd bet I could set a tps by putting my fingers on the Relief and Switching solenoids and adjust til I feel them click on/off.

I did leave it at .631 because the resistance method reads correctly. From my reading of this board the voltage method CAN be thrown off by some faulty electrical related issue. Since i was getting such a huge difference in correct readings from one method to the next i figured id put ADDITIONAL strain on the electrical system by tripping the electrical fan (by removing its connector on the thermo switch). This proved that adding additional electrical strain further threw off my voltage reading at idle.

What is wrong with my electrical system? How can i verify the solenoids that the tps controls at idle are beng managed correctly?

Well, I took my advice in my last post.

I fully heated the engine up.

Turned the tps screw virtually all the way in.

Slowly turned it counterclockwise til I heard a CLICK. Kept turning it counter clockwise til I heard another CLICK.

Then turned it clockwise til I heard it CLICK again. Done.

I put a meter on the green/red wire. Result: 1.06 vdc. Started the engine: 1.02 vdc

The CLICK above is the sound of the Relief and Switching Solenoid coming and going.

I even checked it with the LEDS and came to the same result. Approx 1vdc which 1.02 volts qualifys (qualifies?).

Don't get carried away with the tps. Set it to approx 1vdc with a meter with the engine HoT and the engine idling.

Have I told you about the tps setting method using the two vacuum input lines to the ACV? I thought not. It's redundant to the LED light/clicking of the solenoid methods.

I guess its also within the realm of possibilities that my multimeter is WAY OFF.

Im not really getting carried away with the tps, id like to "confirm" its set properly and move on. It seems like theres 3 ways to do it, 1 of which im good, one of which im way off, and one which i havent tried (lights).

Just make a light thingy and call it a day! It's simple and perfectly accurate because it reports what the ECU is 'thinking', which is more important than the voltages coming out of the TPS.

i did the light tps check. Adjusted it until 1 light lit. Pumped the throttle, etc. When i went back and checked via resistance it was 1.274. Multimeter?

Again, the problem at hand is a low, sometimes rough, idle...

Without reading back through the whole thread (which I should probably do), have you checked the operation of the fast idle cam? Is the ACV still on your car? If so, you should replace the check valve. That made a big difference with the quality of my car's idle and it was cheap and easy to do. Other than that, a vaccum leak is still a big possibility.

The engine MUST be HOT/VERY WARM when you set the tps due to the fast idle cam/thermowax etc.

yes, the engine is fully warmed up...