Vacuum and Idle hunting issue

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97GreenKouki
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Joined: Thu Feb 02, 2017 6:26 am

Vacuum and Idle hunting issue

Post by 97GreenKouki »

Hello all, just joined KA-T to see if anyone has experience with this specific issue.

I rebuilt my KA24DE almost a year ago with Wiseco Pistons, Eagle Rods and ARP hardware all around. Broke the motor in NA for 3000 miles then Turbo'd it. When I rebuilt the motor I redid all of the intake and vacuum lines. During break in the motor ran perfect, no problems with vacuum or idle. For 3000 miles.

As soon as I got a rough tune from Martin at RS enthalpy to make sure everything was ok and boosted for the first time it would idle very high every now and then, like 2500 rpms. Blipping the throttle or engine braking would bring it down. Didn't think anything of it with the rough tune even though vacuum gauge read lower than pre-turbo levels.

Dyno day. Martin gets it on the dyno and dials it in a little more and it starts hunting for idle, bouncing between 400-1300 rpms. AFR's are bouncing between 15 and Full Lean. To try to diagnose he goes through half a can of throttle body cleaner in the engine bay, nothing at all. At this point all he could do was add fuel at idle to mask the problem. Six cells in the bottom left of the Nistune fuel map were changed to 40 from 10 just to get it to idle between 15-16 AFR.

Once the idle was steady, the vacuum showed it was down about 4 inHg. I'll admit I have a cheap vacuum/boost gauge but it had been installed since before I rebuilt the motor, at idle it always showed 30 inHg. But now with this issue it shows about 25 so I am losing vacuum somewhere but it is not to atmosphere or it would have shown with the throttle body cleaner.

Since then, I have completely went through the intake again, all new gaskets, cleaned the IACV, cleaned the AACV and removed the Solenoid for the Charcoal Canister. ALL EMISSIONS HAVE BEEN REMOVED EXCEPT FOR THE PCV SYSTEM. I only have 4 vacuum lines left: FPR, Brake Booster, BOV and the MAP sensor on the passenger shock tower. The MAP sensor also has a vacuum return that runs under the throttle body to return vacuum outside the surge tank. The car is running a draw through N62 MAF, Deatschwerks 740cc Injectors, and 17psi boost. As soon as the car is off idle it doesn't act like a vacuum leak is present. When Idling the Vacuum gauge reads 25 inHg, if I give it a tiny bit of throttle (just enough to bring the rpms up) I actually get MORE vacuum. It will go back up to 30 with the throttle body cracked open.

Here is the super weird part, if I push the car hard and upshift to bring the RPM's down a little then allow the car to slow the engine back to idle the vacuum goes back to 30 inHg and AFR's drop to 11-12 (essentially closing off whatever is letting air into the intake). This has me completely stumped.

Sorry for the long post, it's just a very strange problem that I've never encountered before. Thanks for any help you can provide!
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