Is this a tuning problem or vacuum leak?
Thread Starter
TECH Apprentice
Joined: Oct 2004
Posts: 360
Likes: 0
From: Katy, TX
Is this a tuning problem or vacuum leak? I have a 422 with the 90/90 setup. It still has the stock cam in it. Whenever I put the car into neutral after driving (as in coming to a stop light - even going to shift), the rpm's jump to 2000-2500. I discovered the other day that if I push in the clutch pedal, the rpm's drop like they are supposed to. As soon as I let the pedal back up, the rpm's jump back up. How these are even remotely related stumps me. There are no vacuum leaks that I can find. However, the rpm's also don't go back to normal idle (about 850) until a couple seconds afer I've completely braked. If I coast to a stop, the rpm's won't go down until I mash the brake pedal. I also discovered today that if I drive around with the AC on, that the rpm's don't jump up. I forgot to mention that the car pulls itself (I don't have to give it any gas) when I don't have the AC on. I had the throttle body modified so that it doesn't stick. Now keep in mind that I still have the stock cam. Does this sound like a vacuum leak, tuning issue, or combination of both to you guys? I'm getting around 9 mpg, and it irritates me. If this is a tuning issue, what needs to be done to take care of it? I was thinking about buying some type of PCM software in the future anyway. This would give me a good reason to buy it. Can you guys help me diagnose the problem. This problem is REALLY embarassing as everyone stares at me like I'm revving at them. 
It could be either. If it idles at 850 from time to time its probably tuning related. Get software sooner than later
btw, why does the 422 have a stock cam, its starving for air.
btw, why does the 422 have a stock cam, its starving for air.
This sounds like the throttle body has too much
bypass air, and when the throttle follower is
activated you have -way- too much. This kind
of thins seems to happen on drilled TBs, maybe
the "no-stick" modification also increases the
un-controlled (by IAC or blade motor, what kind
of car is this anyway?) air. You might want to be
playing with the effective-area IAC tables when
you get tuning tools. This seems to be something
that other 90mm guys have found some success
with. Meanwhile might check that the blade is
well seated and not stop-screw jacked, etc. Not
vacuum leak per se, just baseline airflow and airflow
per step outside what the PCM is told to expect.
bypass air, and when the throttle follower is
activated you have -way- too much. This kind
of thins seems to happen on drilled TBs, maybe
the "no-stick" modification also increases the
un-controlled (by IAC or blade motor, what kind
of car is this anyway?) air. You might want to be
playing with the effective-area IAC tables when
you get tuning tools. This seems to be something
that other 90mm guys have found some success
with. Meanwhile might check that the blade is
well seated and not stop-screw jacked, etc. Not
vacuum leak per se, just baseline airflow and airflow
per step outside what the PCM is told to expect.
Thread Starter
TECH Apprentice
Joined: Oct 2004
Posts: 360
Likes: 0
From: Katy, TX
Thank you, sir. That sounds like a perfect explanation because everything was fine before I had the throttle body modified. 
Is it something that can totally be tuned out, or does the throttle body need to be epoxied or something?
Is it something that can totally be tuned out, or does the throttle body need to be epoxied or something? Thread Starter
TECH Apprentice
Joined: Oct 2004
Posts: 360
Likes: 0
From: Katy, TX
To answer your question, it's a 2002 Z28. The screw is all the way out. That was one of the first things I checked. 
I'm going to pull the intake off tomorrow and see what this guy jacked up.
I'm going to pull the intake off tomorrow and see what this guy jacked up. 
