High idle after PCV upgrade
06-08-2020, 08:59 PM
#1
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High idle after PCV upgrade
Heyo! So I welded in -10 AN bungs to my valve cover for the PCV and the breather, and I drilled out the internal hole between the chambers on the PCV from 1/4" to 3/8". I hooked up the PCV to the catch can and used a -6 hose to connect the outlet of the catch can to the factory PCV barb on the manifold, turned slightly to face the catch can. There is a -6 check valve between the manifold and the catch can to prevent my boost from boosting the crankcase.
Lo and behold the car now has a high idle of 1500-2000 RPM. Of course it does! It's now sucking air through the breather system and using that air to idle. The holes have gone from the tiny 1/4" to a much larger 5/8", with the hose directly on the PCV at 3/8". Setting the minimum IAC duty to 0% still has the high idle.
What can I do? Perhaps introduce some kind of restrictor in the system? There's a decent vacuum on the breather port, I could probably afford to turn it down a bit.
Lo and behold the car now has a high idle of 1500-2000 RPM. Of course it does! It's now sucking air through the breather system and using that air to idle. The holes have gone from the tiny 1/4" to a much larger 5/8", with the hose directly on the PCV at 3/8". Setting the minimum IAC duty to 0% still has the high idle.
What can I do? Perhaps introduce some kind of restrictor in the system? There's a decent vacuum on the breather port, I could probably afford to turn it down a bit.
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06-08-2020, 09:05 PM
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There should be an idle screw on the side of the throttle body. I like to get the car warm then unplug the IAC valve and turn the screw in until I reach around 700 rpm with no loads. IMO setting it like that makes for easier idle tuning.
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06-08-2020, 09:52 PM
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I did a thing, I took off my check valve and put the factory PCV valve in it's place. It works now and doesn't do the high idle, but there's barely any pull on the breather port.
I'll try reducing the -10 AN hoses on the valve cover/catch can to -6 AN and see if that improves things, reduce volume and get more flow.
I'll try reducing the -10 AN hoses on the valve cover/catch can to -6 AN and see if that improves things, reduce volume and get more flow.
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06-09-2020, 07:53 PM
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A PCV valve isn't a check valve, it's a metered air leak under vacuum conditions in the direction of the intake. Using only a standard check valve will create a massive vacuum leak from too much flow. Put the stock PCV valve back in and then put your check valve on the IM side of the PCV valve in the right direction to allow airflow towards the IM.
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06-10-2020, 12:07 AM
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A PCV valve isn't a check valve, it's a metered air leak under vacuum conditions in the direction of the intake. Using only a standard check valve will create a massive vacuum leak from too much flow. Put the stock PCV valve back in and then put your check valve on the IM side of the PCV valve in the right direction to allow airflow towards the IM.
But I feel like the stock PCV might be a little small given that I'm turbo and all that. More crankcase pressure. What do you all think? Is there an upgrade better than the 323 GTX?
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06-10-2020, 02:16 AM
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PVC Valve only vents under vacuum. When on boost it's closed and does nothing (no venting at all - it's only the exhaust size that vents under boost) so size of PCV hose is irrelevant.
When in vacuum, the vacuum from the IM would far exceed the blowby being created under those conditions.
Donyt worry about hose size on the PCV side. If you're venting the PCV side to atmo only, then go big hose.
Do you have an excessive blowby issue? Hook a boost gauge up to your oil cap and see what it says on boost. I had 2-3psi till I put a single 12an vent on the exhaust side. Problem solved.
When in vacuum, the vacuum from the IM would far exceed the blowby being created under those conditions.
Donyt worry about hose size on the PCV side. If you're venting the PCV side to atmo only, then go big hose.
Do you have an excessive blowby issue? Hook a boost gauge up to your oil cap and see what it says on boost. I had 2-3psi till I put a single 12an vent on the exhaust side. Problem solved.
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06-10-2020, 02:46 AM
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PVC Valve only vents under vacuum. When on boost it's closed and does nothing (no venting at all - it's only the exhaust size that vents under boost) so size of PCV hose is irrelevant.
When in vacuum, the vacuum from the IM would far exceed the blowby being created under those conditions.
Donyt worry about hose size on the PCV side. If you're venting the PCV side to atmo only, then go big hose.
Do you have an excessive blowby issue? Hook a boost gauge up to your oil cap and see what it says on boost. I had 2-3psi till I put a single 12an vent on the exhaust side. Problem solved.
When in vacuum, the vacuum from the IM would far exceed the blowby being created under those conditions.
Donyt worry about hose size on the PCV side. If you're venting the PCV side to atmo only, then go big hose.
Do you have an excessive blowby issue? Hook a boost gauge up to your oil cap and see what it says on boost. I had 2-3psi till I put a single 12an vent on the exhaust side. Problem solved.
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06-11-2020, 07:14 AM
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There is a big crankcase ventilation/catch can thread on this forum.
I'm glad you solved your vacuum leak without messing with the throttle body bleeder settings which should never be touched.
I'm glad you solved your vacuum leak without messing with the throttle body bleeder settings which should never be touched.
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06-11-2020, 12:14 PM
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https://www.miataturbo.net/megasquir...-tuning-92690/ Post 3 is where it is suggested and plenty of others who know what they're doing agreed.
Last edited by SpartanSV; 06-11-2020 at 12:40 PM.
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06-17-2020, 11:41 AM
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Quote from the thread:
Hold on, where is this idle screw? My car won't even START without the IAC connected.
I have a 2002 NB
Also disagree. Set the base idle just under where you want it with the IAC unplugged... set for warmup pwm then raise your idle DC to hit your target for the coolant temperature you're at... and use timing to reach your target... set up your timing table to stabilize idle by using rows just above and below your target. You can have a car that idles Pretty damn good on warmup PWM, which should have your IAC at a minimal DC when warm. You can have a 90% awesome idle using this method. Once you can't seem to improve it at all, THEN use closed loop idle to start fine tuning things.
... this is how I got my street car with 272 degree cams to idle rock solid, even on a hot summer day with A/C. Hell I can even slowly release the clutch from a stop, and watch the IDLE DC increase and maintain slow speed in traffic without touching the gas. At idle the IAC should not do anything unless load increases from electrical system, power steering, A/C, etc.
unplug the IAC and open up the bypass screw until you idle at 800 or so, and that's a start.
... this is how I got my street car with 272 degree cams to idle rock solid, even on a hot summer day with A/C. Hell I can even slowly release the clutch from a stop, and watch the IDLE DC increase and maintain slow speed in traffic without touching the gas. At idle the IAC should not do anything unless load increases from electrical system, power steering, A/C, etc.
unplug the IAC and open up the bypass screw until you idle at 800 or so, and that's a start.
I have a 2002 NB
Last edited by dsamani; 06-17-2020 at 11:53 AM.
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