Strange throttle/idle problem... Could IAC be the cause?
05-22-2008, 05:43 PM
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Strange throttle/idle problem... Could IAC be the cause? Hello,
I was first going to post this in "Gen3 External engine", but now im more leaning towards it probably being tune-related. I've searched around but didn't find anything that really matched my problem so here goes:
My engine has just been started after a major makeover. From stock to TR224,224-112 cam, Patriot LS6 ported 243 heads, FAST 90/90 tb+manifold, LS7 lifters, yellaterra rockers, manley pushrods, ls2 timing chain etc.
When i start the car and give it throttle to about 3500rpm then let go (TPS reports 0% open), it goes down to 2000rpm, stops for like 3-7 seconds then slowly goes down to around 1000rpm again. It happens every time and gets worse as the engine becomes warm. However, if i disconnect the IAC the problem goes away. What could cause this?
I don't know if it can have anything to do with this, but at complete random, the BRAKE, ABS INOP and TCS OFF lamps turns on (sometimes it doesn't happen at all), and stays on until i turn off and on the ignition.
I had to install a spring between the TB and chassi, to make the tb close properly. Without it, when i press the gas-pedal, it will get stuck at like 3500 rpm and not move until i manually shut it even tho the wire is slack. I'm not sure if this is related to this problem, but might instead be something mechanically wrong with the FAST TB...
I have not done a tune since the install, and i cannot because i live in sweden. I do have EFILive (but not skilled enough to tune yet) and can extract any data if that could help solve the mystery...
Do you think its tune related? If so, any help will be greatly appreciated. Are there any negative side-effects of running the car without the IAC connected?
Thank you,
Corday
1999 Pontiac TransAM, LS1, T56.
I was first going to post this in "Gen3 External engine", but now im more leaning towards it probably being tune-related. I've searched around but didn't find anything that really matched my problem so here goes:
My engine has just been started after a major makeover. From stock to TR224,224-112 cam, Patriot LS6 ported 243 heads, FAST 90/90 tb+manifold, LS7 lifters, yellaterra rockers, manley pushrods, ls2 timing chain etc.
When i start the car and give it throttle to about 3500rpm then let go (TPS reports 0% open), it goes down to 2000rpm, stops for like 3-7 seconds then slowly goes down to around 1000rpm again. It happens every time and gets worse as the engine becomes warm. However, if i disconnect the IAC the problem goes away. What could cause this?
I don't know if it can have anything to do with this, but at complete random, the BRAKE, ABS INOP and TCS OFF lamps turns on (sometimes it doesn't happen at all), and stays on until i turn off and on the ignition.
I had to install a spring between the TB and chassi, to make the tb close properly. Without it, when i press the gas-pedal, it will get stuck at like 3500 rpm and not move until i manually shut it even tho the wire is slack. I'm not sure if this is related to this problem, but might instead be something mechanically wrong with the FAST TB...
I have not done a tune since the install, and i cannot because i live in sweden. I do have EFILive (but not skilled enough to tune yet) and can extract any data if that could help solve the mystery...
Do you think its tune related? If so, any help will be greatly appreciated. Are there any negative side-effects of running the car without the IAC connected?
Thank you,
Corday
1999 Pontiac TransAM, LS1, T56.
05-22-2008, 09:35 PM
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I had similar issues also. I have a spring on my TB also the spring was too heavy and the TB plate was closing thru the Cruise/Traction Control box. The spring was pulling the throttle cable back. I got a new cable for a non-traction control car and bypassed the box, and streched my spring a bit for pedal feel. Make sure you check this and check that your TB plate is opening 100 percent. My IAC is also causing problems. I had the tuner look at it today. I had it disconnected at the track and just idled high. Today the tuner set the idle a little higher to begin with and changed the cycles the IAC was looking for i beleive. Everything has to be coordinated, the MAF, IAC, TPS, TB idle screw.
05-22-2008, 10:29 PM
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Sounds to me like you have a lot of bypass air, and on top
of this is laid the throttle follower (jacking the IAC open in
response to throttle position). The sum is more than it takes
to idle. If you fix the bypass air problem you may be able to
leave the tune alone. If it is just "different" and not fixable
then you probably want to take out some follower airflow
and maybe make the decay rate faster.
of this is laid the throttle follower (jacking the IAC open in
response to throttle position). The sum is more than it takes
to idle. If you fix the bypass air problem you may be able to
leave the tune alone. If it is just "different" and not fixable
then you probably want to take out some follower airflow
and maybe make the decay rate faster.