Hanging idle when coming to stop..sometimes

[Scott125]

I have this car that is acting all kinds of weird. Its a 98Z, 347, 23x,24x cam...nothing crazy, 6 spd. Its SD tuned. When cold, the car does fine, but when it warms up, the idle hangs. I taken air out of the RAF, idle trims are good. Throttle Cracker is zeroed out and I have increased the Drive and P/N Decay tables...alot. Idle still hangs.

A local hack installed the intake and the car, according to the owner, "idled at 1500 afterward." I thought, vacuum leak. Nope. Still does it.

He has a UMI TB, I thought, lets try my NW. We changed them out and the car ran PERFECT. I thought it was the TB. Later that night, he tells me it is back to doing what it was before. Damn!

He gets an SES light, so he goes to Vatozone to have it read, TPS code. He didn't get the number, but they cleared it and he said the car ran perfect again....then after 10 minutes or so of driving, went back to running weird.

Its like the car has a vacuum leak, but only when hot....but why would it run good after changing TB's and then again after deleting a code. I'm stumped......any ideas are appreciated!

Thanks
Scott

[SSUte01]

First of all log Desired Idle Airflow when you are checking this, but try leaning out your idle just a little. Actual AFR has quite a substantial effect on required airflow a rich car does require the airflow that a lean one does. It's possible that I am way off without a log or tune, but I know for a fact how effect actual AFR has on airflow and this situation.

Best of luck

[comm]

SSute how do u lean or richen the idle? Do u use the VE table?

[5_Liter_Eater]

Use Russ K's idle airflow config. There is a histo that logs the desired idle airflow against the running airflow table. Start from a cold engine with the AC off and fans forced off. Dont touch the gas, let it come up to temp, even a little hotter than normal temp, then just copy and paste these numbers into the RAF table in the tune. This solves >90% of gen 3 idle problems. If that doesn't do it try zero'ing out the idle cracker and idle follower tables.

[jimmypop13]

Use Russ K's idle airflow config. There is a histo that logs the desired idle airflow against the running airflow table. Start from a cold engine with the AC off and fans forced off. Dont touch the gas, let it come up to temp, even a little hotter than normal temp, then just copy and paste these numbers into the RAF table in the tune. This solves >90% of gen 3 idle problems. If that doesn't do it try zero'ing out the idle cracker and idle follower tables.

Would it be better to leave the hood open when doing this? I didn't know fans had to be off. That makes a big difference! I want to make sure I don't damage anything with the fans off, what would you say normal temp is? I'm guessing around 180-190*F? thanks!

-Chris

[Alvin]

I have this car that is acting all kinds of weird. Its a 98Z, 347, 23x,24x cam...nothing crazy, 6 spd. Its SD tuned. When cold, the car does fine, but when it warms up, the idle hangs. I taken air out of the RAF, idle trims are good. Throttle Cracker is zeroed out and I have increased the Drive and P/N Decay tables...alot. Idle still hangs.

A local hack installed the intake and the car, according to the owner, "idled at 1500 afterward." I thought, vacuum leak. Nope. Still does it.

He has a UMI TB, I thought, lets try my NW. We changed them out and the car ran PERFECT. I thought it was the TB. Later that night, he tells me it is back to doing what it was before. Damn!

He gets an SES light, so he goes to Vatozone to have it read, TPS code. He didn't get the number, but they cleared it and he said the car ran perfect again....then after 10 minutes or so of driving, went back to running weird.

Its like the car has a vacuum leak, but only when hot....but why would it run good after changing TB's and then again after deleting a code. I'm stumped......any ideas are appreciated!

Thanks
Scott

Log TPS voltage, i'll bet that the TPS is reading high. If it is the computer woud never come of throttle cracker

[SSUte01]

Yep leave the bonnet up to prevent a heat soaked IAT messing with your fueling and therefore running airflow. I wouldn't let it get above about 200*F as you're unlikely to ever see that temp once you turn the fans back on. Mine sits about 195*F in traffic/highway so I usually let it get a little hotter when setting the running airflow.

Sorry, I didn't read your initial post clearly the first time, it sounds like you have a dud TPS, if you can try swapping it with someone elses and see if that helps, I have had the identical issue that you had and it only happened when hot and intermittently ie twice in 6 months threw a TPS code. New TPS fixed that problem, this may not be your issue but it cannot hurt to swap with a known good sensor. Also with a large cam such as that it wouldn't hurt to disable the P0121 test under Engine Diagnostics ==> General

Good Luck

[kevmar]

what is the proper voltage at idle for the tps??

[kevmar]

no one??? how about anyone haha

[SSUte01]

0.45 - 0.60V is a reasonable spectrum although I would consider 0.60V at the very upper end. Not all applications are the same. What is yours at ??

[mike01z]

Use Russ K's idle airflow config. There is a histo that logs the desired idle airflow against the running airflow table. Start from a cold engine with the AC off and fans forced off. Dont touch the gas, let it come up to temp, even a little hotter than normal temp, then just copy and paste these numbers into the RAF table in the tune. This solves >90% of gen 3 idle problems. If that doesn't do it try zero'ing out the idle cracker and idle follower tables.

you literally make these numbers the same? Why does this work? How is the realtime air incoming at idle affected by the RAF table does it control the IAC?

[5_Liter_Eater]

Yes. Copy and paste the actual numbers. You're logging desired airflow which is essentially taking the actual airflow and adding or subtracting the STIT's and LTIT's.

[kevmar]

0.45 - 0.60V is a reasonable spectrum although I would consider 0.60V at the very upper end. Not all applications are the same. What is yours at ??

stock t/b .63v

[Scott125]

I messed around with this car again. Still doing the same thing. I leaned it out in the decel area of the VE table...no change. I even raised the Drive decay tables to 64 across the board with no change..lol.

One thing that it did do while I was sitting still....it would occasionally register mph...like the car was moving. That has me stumped.

I'll check the TPS voltage to make sure it is within range.

While it is not using the 02's, I did notice B1 is more erratic..it would fluctuate alot more and dive into the 50's sometimes. If the exhaust is sealed good....does that usually mean vacuum leak?