RPM Fluctuates 800-1800 with hot engine and brake depressed????

[pecanflyboy]

1996 Stock 1.8L

I've had an inconsistent RPM fluctuation when at a stop light with the brake applied. Tonight, I was able to islolate the failure mode.

MODE:
Hot engine
Everything is off (lights, radio, AC)
Set 1800 RPM with throttle
Apply brake
RPM will fluctuate betweeen 800 and 1800 in 1 second intervals
When throttle is closed, fluctuations will fade to idle
When brake is released, fluctuations will cease

What I've done, with no help:
Sprayed carb cleaner into Idle Speed Control valve
Removed vacuum line and capped fitting for brake booster. Not the booster or lines
Checked all vacuum lines
Disconnected brake lights
Disconnected battery and ECU, reconnected

I've read, and continue to find articles on this issue. The only thing left to do is to go through the Idle Speed Control troubleshoot as detailed by Ross Kuhre in the Garage section of Miata.net. This is a big job. If there is something else I should check before jumping into the ISC, please give me some input.

Thanks,
Jimmy

[POS Racing]

I could be wrong but isn't the ISC only on the 90-93 1.6 liter Miata??

By Ross Kuhre (Stock 93 MX-5, C package, 130k mi)

If you have an idle problem with the unmodified NA 1.6L engine, this article is intended to help you understand how the ISC (Idle Speed Control) system should work and how it may fail (my experience).

[brandonmpace]

You may want to peek inside the throttle body as it only takes a few minutes and see if there's any black buildup there. If there is, you may want to check other hoses/holes on the intake for this same gunk. My throttle stuck open a bit intermittently at ~2000 RPM til I cleaned it.
I say this because you say it's not the booster. I plan to pull the top part of my intake off and clean it as I know it's dirty in a few spots.
Seems like it would be a vacuum issue to me. CEL off?

[pecanflyboy]

You may want to peek inside the throttle body as it only takes a few minutes and see if there's any black buildup there. If there is, you may want to check other hoses/holes on the intake for this same gunk. My throttle stuck open a bit intermittently at ~2000 RPM til I cleaned it.
I say this because you say it's not the booster. I plan to pull the top part of my intake off and clean it as I know it's dirty in a few spots.
Seems like it would be a vacuum issue to me. CEL off?

BINGO! We have a winner!

I checked the issue with the TEN to GND, no help. No CEL.

I was going to abandon the issue until after my track day, and then I took a second look at my 1996 1.8L and realized the throttle body and the ISC come off the front of the throttle body with only 4 bolts! In 20 minutes I had the entire unit on mywork bench. I removed the ISC from thethrottle body, and thoroughly cleaned everything. I checked the resistance at between 12 and 13ohms. I also visually checked thesolenoid operation. Everything was OK.

I cleaned a pretty dirty TB and ISC. I think the port from the outside of the TB to the interior of the unit was partially clogged. I think this is the idle circuit.

Then I looked into the Intake Manifold. All of the deposits were in the forward part of the IM, and easily accessible. I stuffed some paper towels aroundthe dirty area and started to clean. To my surprise I found a 1/2inch port that was completely caked over with sludge. I cleaned it completely.

Reassembled, and initially, everything seems to run well,with no surges.

I am about to go on a long drive as a definitive check. If you don't hear from me, all is well!

Jimmy