High Idle problem
12-08-2016, 02:44 PM
#1
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Join Date: Sep 2016
Location: Conroe, Tx
Posts: 21
High Idle problem
Hey guys... I have a question about a high idle issue.. At start up engine idles high (1500 rpm) for a few seconds and then drops to idle (700-900 rpm). When the car moves at all.. wheels rolling.. the engine rpms goes back up to 1500 rpm and will not drop back to idle until the vehicle comes to a complete stop.. wheels not rolling... Havent been able to get a program or reader yet that I can look at the pcm with .. 96 but previous owner had a 95 odb1 pcm with tuning software.. I dont have the software or the cable .. Any ideas?? Thanks...
12-08-2016, 08:15 PM
#2
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Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,647
Re: High Idle problem
Sounds entirely normal. At cold start the PCM elevates the idle speed. By the time your engine reaches ~160*F the idle speed should be at 800 RPM, which is the stock programming for the manual trans LT1. Note that the stock tach usually reads 100-200 RPM higher than actual engine speed. You need a scanner to read the accurate RPM off the PCM.
The idle speed is controlled by a small stepper motor that moves a pintle in and out of an orifice in the bottom of the throttle body, to control the amount of air that bypasses the closed throttle blades. The PCM controls the stepper motor (called the IAC, or idle air control) by telling it how many steps ("counts") to open. You can use a scanner to watch the PCM adjust the IAC counts to bring the idle RPM to specification.
Once the vehicle starts moving, the PCM increases the IAT counts to admit more air, so the engine does not stall when the throttle blades suddenly close. Hence, as long as the vehicle is moving, as read from the vehicle speed sensor (VSS), the engine idle speed at closed throttle will be higher than 800 RPM used when the vehicle is standing still. The previous owner may have bumped up the tables a bit for the "rolling" engine RPM.
The idle speed is controlled by a small stepper motor that moves a pintle in and out of an orifice in the bottom of the throttle body, to control the amount of air that bypasses the closed throttle blades. The PCM controls the stepper motor (called the IAC, or idle air control) by telling it how many steps ("counts") to open. You can use a scanner to watch the PCM adjust the IAC counts to bring the idle RPM to specification.
Once the vehicle starts moving, the PCM increases the IAT counts to admit more air, so the engine does not stall when the throttle blades suddenly close. Hence, as long as the vehicle is moving, as read from the vehicle speed sensor (VSS), the engine idle speed at closed throttle will be higher than 800 RPM used when the vehicle is standing still. The previous owner may have bumped up the tables a bit for the "rolling" engine RPM.
Last edited by Injuneer; 12-08-2016 at 08:19 PM.
12-08-2016, 08:42 PM
#3
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Join Date: Sep 2016
Location: Conroe, Tx
Posts: 21
Re: High Idle problem
Thank you for the information... So at this point I can expect the 1500 rpm while rolling, with clutch depressed? Just seemed like the engine rpm should drop back to idle while the clutch is depressed and coming to stop.. kinda like the vehicles I had when I was a lot younger... LOL
Guess I need to get the ALDL cable and software to get a better insight into this car.. Once again thank you for responding.. I always learn from you guys..
Guess I need to get the ALDL cable and software to get a better insight into this car.. Once again thank you for responding.. I always learn from you guys..
12-09-2016, 01:51 PM
#4
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Join Date: Nov 1998
Location: Hell was full so they sent me to NJ
Posts: 70,647
Re: High Idle problem
It's so long (16 years) since I had the stock tuning (or even the stock PCM) that I can't remember the exact closed throttle "rolling" RPM. There's a lot going on when you decel. The PCM shuts down the fuel until RPM drops to maybe 1,300 RPM. Then it turns it back on. If you leave it in gear as is slows down, you can feel the fuel turn back on. Logically, the fuel turning back on is what needs the air flow. Maybe your tach reads a bit high. Or maybe the program has been altered in that area.