ExitWound

I'm dumb. I have noid lights in my tool chest. Didn't even think about them.

I might just switch it out with a working one to see what happens.

I'm trying to trace all the wiring back to the DME as best as I can but it's not easy. I know pins 33 and 34 on the DME are the swapping grounds that power the ISV but I have limited experience reading electronic schematics. My contact is an electrical engineer so that helps me a lot. He's just not available during the days until after work so I don't get much done by myself other than simple tests.

peanut

I know you have probably done this a hundred times already but just to satisfy me..... was there any fluctuation in rpm when you pinched the vacumn tube flat before the ISV ?

Any fluctuation would indicate that you are still getting air into the vacumn system somewhere.

Have you got a schematic of the ICU with the input/output connections ? I've got one somewhere if you need it.

not sure if this is of any help
http://www.miata.net/garage/isc.html

ExitWound

When the hose is pinched off, the idle is rock steady.

I have the Haynes manual which has the complete wiring diagram, and I've come across the innards of the transistors that power the ISV circuit as well as the DME wiring diagrams.

Bri Bro

What did you replace the transistor with, a TIP120? Did you check that the circuit board trace didn't get burned? Is 0 volts an open or a short to ground?

With the ignition on, you can see the valve flutter as the PWM signal changes state. One ISV signal is always the opposite of the other, ie one is ground, the other is open. The frequency of the PWM signal is around 70 Hz, might be hard to see with a light.

I all else fails, get a $70 scope.

ExitWound

I can't remember which model number they were. They were Darlingtons though, 4A max if I remember. Got them at Radio Shack. We checked over all the traces to make sure that they were clean and solid. It's possible we missed something.

Are you saying I should visually see the valve inside the ISV fluttering? I can't feel it vibrating when it's plugged in. I currently have the intake off and the ISV out. If I plug it in with the key on, I should see it fluttering?

Bri Bro

The duty cycle of the PWM pulse determines which way the valve will rotate. The PWM signals are always inverted to each other, one side on, the other off at 70-80 Hz rate. You should see the valve flutter a small amount (and feel a faint vibration) when it is working correctly. Do you have 12vdc on pin two of the idle controller connector?

peanut

if the idle is steady and you can then adjust the idle screw to reduce the idle to a stable 800 rpm then that must mean that you still have a leak somewhere on the inlet side of the ISV

ExitWound

Can you walk me through your reasoning that there's a leak on the inlet side of the ISV? I don't seem to follow.

I'm reading less than 12V at pin#2 with the key turned. Only about 8V.

Bri Bro

12V with ignition on sounds correct. You need to look at the PWM pulses.
https://rennlist.com/forums/7184543-post1.html

ExitWound

Wow that looks like a sweet little product. I don't like that it's fully exposed though. I don't have a garage and I'm sure many others don't either.

ExitWound

I put the intake back together today. I'm sick of the car being in my living room and working on the car in 20F weather.

• Whether the ISV is plugged in to the harness or not, there is NO change in the situation which leads me to believe that I'm right when I conclude that the ISV isn't turning on in the car, but works outside the car.
• Bypassing the ISV via the diagnostic port does nothing as well.
• Changing the throttle screw does nothing to change the idle if the diag port is jumped or not or whether the ISV is installed or not.
• TPS tests perfect.
• AFM tests perfect.
• New engine temp sensor.

Just for reference, here's a video of the hunting idle. This is taken without the ISV attached electronically, but still connected via the intake hoses. I suspect that the door is open on the valve. I had cleaned the ISV with carbcleaner and it operated perfectly. However, now that it's out in the cold, I don't believe the door is operating again on battery power.

peanut

nice vid.

thanks for the summary'

thats quite interesting actually because the 'hunting' idle is very very regular. The revs rise steadily to 1600rpm then immediately drop to 1200rpm then cycles again .
Sounds like something is causing the revs to rise steadily which is counteracted by another mechanism which compensates and allows the revs to drop momentarily.

I'm sure that once the complete electrical/thermo/mechanical idle operation is understood the solution should be obvious.

peanut

if it were my car I think I would try and get hold of an air pressure gauge and connect it to the inlet vacumn side . It could confirm or not if there is an air leak .

The other thing I noticed is that your MPG guage showed a marked drop in mpg immediately before each rise in revs which might indicate that you are getting variable fuel delivery.

Have you tried putting a fuel pressure gauge on the fuel rail to check fuel delivery pressure is constant ?

Is there any change if you pull off the electrical connector to the cold start injector ?

this thread might be useful
http://www.civicforums.com/forums/18...e-problem.html

ExitWound

I've pretty much eliminated fuel and vacuum as the causes. As I understand it, the ISV, when it was hooked up, was partially open. And, if my assumption is correct, it's not getting power to turn on from the DME for whatever reason. That open passage bypass for the air at idle raises the idle above threshold at which time the DME thinks that it's overrevving at 1600rpm (at idle on the TPS), cuts the fuel delivery to compensate. At this point, it sees that the rpms have fallen enough to kick them back on to idle delivery rate. But because the air leak through the ISV is there, when it sets the fuel delivery rate to be what it should be, it's actually too high due to the excess airflow. At 1600, it cuts the fuel and it starts over.

I put the intake back on today and closed everything up and warmed up the car. What I did was set the ISV to CLOSED by attaching a 9V battery to the close ground pin. Started the car and found it would not idle. I then upped the throttle via the throttle screw and got it to idle at 800rpm STEADY. And when I mean steady, it's rock steady. Absolutely NO fluctuations. This says to me that fuel delivery is perfect and vacuum is perfect. With the ISV closed, there is no airflow (or as little as possible) through that bypass and the computer sees a proper idle rpm and doesn't continuously drop the fuel delivery.

If the ISV were to power up suddenly and open that valve, there would be too much air and the idle would raise. However, I don't believe the ISV is getting power because nothing I can do electronically changes the effect that the ISV has on the airflow. The ONLY thing that has changed it is allowing more air in via the throttle screw. I think that if the ISV saw power and properly was monitored by the DME, it would work. But because it's not turning on and opening & closing that valve accordingly, the dme fights the open airflow and I get surging.

At least, those are my thoughts. Testing today confirmed that I have ZERO amps of current through the ISV when it's plugged in. Why? I have no clue.

At this point, I'm back where I started, with a nonfunctioning ISV and a low idle on cold engine. I have to keep my foot on the gas for 2 minutes to keep the rpms above 400 to keep the car running. But, once warm, it idles fine due to the throttle screw adjustments made.

(Last summer, I checked the fuel delivery rate and it was fine, as was pressure. We put in a new fuel pump and regulator.)

We will still do the vacuum leak test as soon as I can secure the gauge.

peanut

good explanation . Its complicated isn't it. ! Sounds like you are nearly there, you just have to work out why the ECU isn't pulsing the ISV .

I wish I could spend some time on this because its bound to benefit me sooner or later but my central heating boiler packed up yesterday and the boiler is in bits all over the kitchen.

I have my 90 year old Father staying with us and no heating no hot water with 12" of snow and -6 outside I've got to concentrate on that for now. Good luck I'll be keeping an eye on the thread.