Cold and A/C idle issues and my attempts thus far

[tails]

It's booked to get the o2 sensor done in a couple of weeks. I don't have the means at home to get under the car.

In the mean time, for my own curiosity, I checked the plug that leads to the ISC while switching the AC on to make sure the ISC was even getting a signal from the car. With AC off and engine warm the plugs reads about 12.7v. When switching the AC on the volts drop to about 11.5v.

Am I correct in my thinking that this is what is meant to happen? I figured if it defaults to open when unplugging it, then dropping the volts would partially open it.

I can't seem to edit my post so I'm just double posting.

To clarify what I meant above, and as a bit of an edit due me doing some more research. With the ignition on (but car not running) I got a reading of 12.5v off the white and red wire leading to the ISC. When plugged in I got 8v on the blue and yellow wire running out of the ISC. According to the research I just did, this is what they're meant to be showing (it did say 7v for the blue yellow, but figured 8v is close enough, right?).

[93_Clubman]

It's booked to get the o2 sensor done in a couple of weeks. I don't have the means at home to get under the car.

ADM 1992 NA6 - if like the ADM 1993 NA8 there's no need to get under the car as the single wire O2 sensor for the NA6 is just slightly down from the headers.

[tails]

It's booked to get the o2 sensor done in a couple of weeks. I don't have the means at home to get under the car.

ADM 1992 NA6 - if like the ADM 1993 NA8 there's no need to get under the car as the single wire O2 sensor for the NA6 is just slightly down from the headers.

I've got aftermarket headers so my o2 sensor is a bit further down.

[93_Clubman]

(it did say 7v for the blue yellow, but figured 8v is close enough, right?).

Believe +7v is the spec:
See 2nd last (& 5th last) posts here:
http://forum.miata.net/vb/archive/index ... 27287.html

[tails]

(it did say 7v for the blue yellow, but figured 8v is close enough, right?).

Believe +7v is the spec:
See 2nd last (& 5th last) posts here:
http://forum.miata.net/vb/archive/index ... 27287.html

Yep, that's the same thread I found last night. Is +8v close enough or does that indicate an issue? Everything else has been spot on (12.5ohms on the ISC side and +12v on the plugs hot side).

[93_Clubman]

Is +8v close enough or does that indicate an issue?

Our interpretation of +7v appears to differ - I read it as '7v or greater', while you appear to read as 'positive 7v' - don't believe in the context of auto electrics there is negative 7v.
Btw, just checked the Haynes manual, but it doesn't go into enough detail, so checked the Rod's Enthusiasts manual, but I have 1.8 version which is different, so no help there.

[tails]

Is +8v close enough or does that indicate an issue?

Our interpretation of +7v appears to differ - I read it as '7v or greater', while you appear to read as 'positive 7v' - don't believe in the context of auto electrics there is negative 7v.
Btw, just checked the Haynes manual, but it doesn't go into enough detail, so checked the Rod's Enthusiasts manual, but I have 1.8 version which is different, so no help there.

I hope you're right based on what I just measured. I just measured the cold side (blue orange) wire again to be sure, and started the car to check the variation. I get about 8v with ignition on but engine not running. When I start the car it jumps to over 13v coming out of the ISC. the white/red hot side wire is still showing 12.5ish.

That seems awfully high.

[93_Clubman]

"Mazda MX-5 Miata 1990 Workshop Manual" (the factory manual)

I believe the manual states...voltage at the return side of the ISC Valve should be about +7 volts

http://forum.miata.net/vb/archive/index ... 27287.html
If you want to feel more comfortable about the conclusion, suggest a check of the above factory manual to see exact wording. Can do a bit later today, or if you need it now you can download it yourself just Google the tittle.

[93_Clubman]

Ref states: 'Approx. 7V'.
You can check here - searched on '7v' & got 3 hits on p274, p844, & p873
viewtopic.php?f=29&t=65789&p=819375#p819375

[tails]

I guess "about 9v when engine running" could also mean "13v when engine running". Fingers crossed it's cool!

[93_Clubman]

Factory manual states: 'Approx. 9V'.
So getting 13v could be an indication that there's an issue.

[tails]

Thanks for the help, 93_Clubman.

I cleaned up a spare ISC valve I managed to get my hands on. It passed the resistance test so I plugged it into the car (just electronically) and switched the ignition on. The valve turns as it should, opening when cold and clicking open when losing a signal (being unplugged). It also shifts when the aircon is turned on.

I'll swap them out this weekend and hope for the best.

[tails]

For anyone who might care...pulled the TB & ISC off this morning. There was a dead end gasket between the TB & ISC, so that explains all my cold and AC idle issues.

After fixing and putting it all back together, when trying to set base idle, with TEN and GND bridged I can't get it to idle lower than about 1150rpm. So I suspect an air leak somewhere. Will hunt for that soon. Without TEN and GND bridged, the ECU seems to be making up for it, so it runs pretty much fine now (idles at about 950rpm, so a little high, but close enough for the time being).

[tails]

I thought I'd update all my old threads to let people know what I did to fix my issues (as there's a lot of threads on a lot of forums where people don't let others know what fixed the issue).

After fixing the ISCV (by removing a dead end gasket) I found a vacuum leak in the intake tubing which after sealing fixed the high idle as well.