Cold, fast idle
3rd Gear Member
Joined: Sep 2006
Posts: 760
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RE: Cold, fast idle
there is more to idle speed than just the iac. your tp sensor plays a big part in how the pcm contolls the iac. Its voltage at idle should be about 1v. much more than that and the pcm may think you are part-throttle all the time. The computer records the lowest voltage seen by the tp, so if a glitch makes the pcm see .6v, it will think that anything above .6v is part throttle and consequently open the iac for dashpot. The engine may idle high if it does not enter closed loop as well. You may have a vacuum leak as well. Intake gaskets commonly leak on the 5.0. Try blocking off the throttle when it's cold and see if the engine still runs. If so, you have a vac leak. Your base idle should be 525rpm. Obtain this by disconnecting the iac and setting your base timing to 10deg. remove the spout and adjust your throttle stop to give yourself 525rpm. Of course, this will not work properly if the pcm thinks you are part throttle or if you have a vac leak. You can check the duty cycle of your iac (PCM command) by measuring current flow with an amp meter. The iac motor pulls 1a @ 100% duty cycle. Therefore, current flow will be proportionate to duty cycle. (300ma = 30% duty cycle, 500ma = 50% duty cycle and so on). You should be about 30% (or 300ma) at normal 800rpm idle. A lower duty cycle than this @ 800rpm would indicate a likely vac leak. Otherwise if it is higher, you may have the whole tp thing. If the tp voltage is messing with you, just disconnect your battery for about 15 min and retest. This will erase your idle tp voltage that the pcm remembers.
Thread Starter
2nd Gear Member
Joined: Oct 2006
Posts: 327
From:
RE: Cold, fast idle
The TPS is set, IAC is new. Base idle is also set. As for checking for leaks, I have mass air so if I remove the intake tubes it doesnt run. Not sure how you can cover the TB with the tubes in place. Still no fast idle.
ORIGINAL: shaners90lxhatch
there is more to idle speed than just the iac. your tp sensor plays a big part in how the pcm contolls the iac. Its voltage at idle should be about 1v. much more than that and the pcm may think you are part-throttle all the time. The computer records the lowest voltage seen by the tp, so if a glitch makes the pcm see .6v, it will think that anything above .6v is part throttle and consequently open the iac for dashpot. The engine may idle high if it does not enter closed loop as well. You may have a vacuum leak as well. Intake gaskets commonly leak on the 5.0. Try blocking off the throttle when it's cold and see if the engine still runs. If so, you have a vac leak. Your base idle should be 525rpm. Obtain this by disconnecting the iac and setting your base timing to 10deg. remove the spout and adjust your throttle stop to give yourself 525rpm. Of course, this will not work properly if the pcm thinks you are part throttle or if you have a vac leak. You can check the duty cycle of your iac (PCM command) by measuring current flow with an amp meter. The iac motor pulls 1a @ 100% duty cycle. Therefore, current flow will be proportionate to duty cycle. (300ma = 30% duty cycle, 500ma = 50% duty cycle and so on). You should be about 30% (or 300ma) at normal 800rpm idle. A lower duty cycle than this @ 800rpm would indicate a likely vac leak. Otherwise if it is higher, you may have the whole tp thing. If the tp voltage is messing with you, just disconnect your battery for about 15 min and retest. This will erase your idle tp voltage that the pcm remembers.
there is more to idle speed than just the iac. your tp sensor plays a big part in how the pcm contolls the iac. Its voltage at idle should be about 1v. much more than that and the pcm may think you are part-throttle all the time. The computer records the lowest voltage seen by the tp, so if a glitch makes the pcm see .6v, it will think that anything above .6v is part throttle and consequently open the iac for dashpot. The engine may idle high if it does not enter closed loop as well. You may have a vacuum leak as well. Intake gaskets commonly leak on the 5.0. Try blocking off the throttle when it's cold and see if the engine still runs. If so, you have a vac leak. Your base idle should be 525rpm. Obtain this by disconnecting the iac and setting your base timing to 10deg. remove the spout and adjust your throttle stop to give yourself 525rpm. Of course, this will not work properly if the pcm thinks you are part throttle or if you have a vac leak. You can check the duty cycle of your iac (PCM command) by measuring current flow with an amp meter. The iac motor pulls 1a @ 100% duty cycle. Therefore, current flow will be proportionate to duty cycle. (300ma = 30% duty cycle, 500ma = 50% duty cycle and so on). You should be about 30% (or 300ma) at normal 800rpm idle. A lower duty cycle than this @ 800rpm would indicate a likely vac leak. Otherwise if it is higher, you may have the whole tp thing. If the tp voltage is messing with you, just disconnect your battery for about 15 min and retest. This will erase your idle tp voltage that the pcm remembers.
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