Idle Connected To Speed?
Thread Starter
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St. Jude Donor '07
Idle Connected To Speed?
Is there a sensor that connects idle speed to vehicle speed?
Something I have noticed:
While I coast downhill in neutral and/or the clutch pushed in, the idle stays high, even if it is a 3 minute coast.
Also, as I drive normal, the idle stays high as I come to a stop but usually slows to proper after ~15-30 seconds.
Is there a sensor that can cause this? A wiring issue? High rpm exposed vac leak?
Something I have noticed:
While I coast downhill in neutral and/or the clutch pushed in, the idle stays high, even if it is a 3 minute coast.
Also, as I drive normal, the idle stays high as I come to a stop but usually slows to proper after ~15-30 seconds.
Is there a sensor that can cause this? A wiring issue? High rpm exposed vac leak?
Safety Car
Joined: Oct 2007
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From: :45 minutes from everywhere / E-I-E-I-O
Yup.
That's normal. I think the vehicle speed sensor buffer controls that.
ed.: It will idle a couple of hundred over normal idle until you're at a full stop. This is absolutely normal.
That's normal. I think the vehicle speed sensor buffer controls that.
ed.: It will idle a couple of hundred over normal idle until you're at a full stop. This is absolutely normal.
Last edited by schrade; May 25, 2008 at 12:59 PM.
Pro
Joined: Oct 2004
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Mine does exactly the same - if I put the car in nuetral at 30mph it shows about 1300 - 1500 RPM; when the car comes to a stop it takes a few seconds to settle back down around 900.
One theory I have seen is that the throttle-blade 'shaft' is sticky in its housing (i.e. not returning fully closed when lifting off the throttle pedal).
One theory I have seen is that the throttle-blade 'shaft' is sticky in its housing (i.e. not returning fully closed when lifting off the throttle pedal).
Thread Starter
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St. Jude Donor '07
Didn't used to... I could push in the clutch oin a hill and it would idle @ 800 the whole way. I used to chuckle at getting 85 instant mpg for the trip.
This whole thing started right about the time I was plagued by that other issue, which was also right after I got headers, 52mm, plenum porting, and chipped.
Sometimes the idle does not settle unless I rev the engine for a couple seconds.
This whole thing started right about the time I was plagued by that other issue, which was also right after I got headers, 52mm, plenum porting, and chipped.
Sometimes the idle does not settle unless I rev the engine for a couple seconds.
Le Mans Master
Joined: Dec 2000
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From: San Diego, Ca
The IAC acts as a Dashpot meaning that when you let off the throttle, it doesn't really let it slam shut because that will stall it. It doesn't happen so gradually though that you should feel it or necessarily see it. A quick test is to start it and note idle speed. Than restart and that idle speed should be higher, but quickly return to the previous number. I think for your problem though, you will want to hook up a scanner so that you can see actual IAC counts, Targeted Idle, Actual Idle & TPS. That will tell you whether there's a fault in IAC circuit or a throttle cable hang up (and the latter really sounds like what the problem is).
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St. Jude Donor '07
Yeah. When it idles high, I check the cables. CC & Throttle are loose and the TB is closed. This issue has happenned thru 2 different TBs and 2 IACs.
Interesting add'l info:
When it idles high like this, I can stomp on the gas pedal and it will often return to a normal idle. This symptom has me thinking canister purge valve, which only opens at high rpm. (I have no EGR or AIR.) Maybe the computer (new) and MAF module (new) just need to see a surge of air?
Interesting add'l info:
When it idles high like this, I can stomp on the gas pedal and it will often return to a normal idle. This symptom has me thinking canister purge valve, which only opens at high rpm. (I have no EGR or AIR.) Maybe the computer (new) and MAF module (new) just need to see a surge of air?
Melting Slicks
Joined: Aug 2003
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From: Lakeville MI
When rolling above 1.8 mph (pid vehicle speed threshold) and when off throttle, the IAC position is defined by the warm park position vs coolant temp table plus a 10 count offset.
If the idle is hanging too high during these situations rolling with clutch disengaged or in neutral, then your min air setting is too high for the pre-programmed warm park table + 10 count offset setting.
If the idle is hanging too high during these situations rolling with clutch disengaged or in neutral, then your min air setting is too high for the pre-programmed warm park table + 10 count offset setting.
Team Owner
Joined: Dec 2000
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From: SE NY
Cruise-In II Veteran
Is there a sensor that can cause this?
Perhaps your IAC is dirty and/or needs service??
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Safety Car
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From: :45 minutes from everywhere / E-I-E-I-O
WmMc's is still too high tho' when he coasts in neutral. It should only be 350 - 450 revs over normal idle speed. That's about what my GM's have always done...
Melting Slicks
Joined: Aug 2003
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From: Lakeville MI
Desired idle speed is not dependent upon vehicle speed. It is a function of coolant temperature. If you see an increase in the desired idle speed while moving, it is due to decreasing coolant temperature with normal tuning.
There are also idle speed additions for park/neutral (50 rpm for auto trans cars) and for and A/C on (set to 0 rpm by default).
When the car is moving above 1.8 mph, the idle speed is not controlled.
The IAC position will vary however, according to the warm park position table plus offset (for manual transmission cars) and the throttle position. Fan anticipation and A/C on will also add additional offsets to the IAC position.
There are also idle speed additions for park/neutral (50 rpm for auto trans cars) and for and A/C on (set to 0 rpm by default).
When the car is moving above 1.8 mph, the idle speed is not controlled.
The IAC position will vary however, according to the warm park position table plus offset (for manual transmission cars) and the throttle position. Fan anticipation and A/C on will also add additional offsets to the IAC position.
Last edited by tequilaboy; May 25, 2008 at 05:24 PM.
Team Owner
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From: SE NY
Cruise-In II Veteran
The IAC position will vary however, according to the warm park position table plus offset (for manual transmission cars) and the throttle position.
Thanks for the correction.
Last edited by 65Z01; May 25, 2008 at 05:36 PM.
Thread Starter
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St. Jude Donor '07
It's sounding like the Hypertech chip might have a hand in this.
In addition to a larger intake and headers, I'm running a 160 stat and Hypertech chip. When the fan comes on, the idle has behaved erratically. The old MAF may have been weak to this situation and added to the faults.
I think it's time for a PCMforless chip. Worth a try?
In addition to a larger intake and headers, I'm running a 160 stat and Hypertech chip. When the fan comes on, the idle has behaved erratically. The old MAF may have been weak to this situation and added to the faults.
I think it's time for a PCMforless chip. Worth a try?
Safety Car
Joined: Oct 2007
Posts: 3,816
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From: :45 minutes from everywhere / E-I-E-I-O
It's sounding like the Hypertech chip might have a hand in this.
In addition to a larger intake and headers, I'm running a 160 stat and Hypertech chip. When the fan comes on, the idle has behaved erratically. The old MAF may have been weak to this situation and added to the faults.
I think it's time for a PCMforless chip. Worth a try?
In addition to a larger intake and headers, I'm running a 160 stat and Hypertech chip. When the fan comes on, the idle has behaved erratically. The old MAF may have been weak to this situation and added to the faults.
I think it's time for a PCMforless chip. Worth a try?
For the price of a chip you can buy a hardwire link for your laptop - ALDL linkup.
AND, you can see everything, change everything, etc., ...
Thread Starter
Race Director
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St. Jude Donor '07
Advanced
Joined: Dec 2006
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From: Newton KS
My 85 with M/T idled just like described - it drove me nuts! Here is how I fixed it: updated to a 165 ecm, bought a flash e-prom, bought a laptop, downloaded Tuner Pro, downloaded several Bin Files, studied tuning for 3 months (tequilaboy was a big help - THANK YOU!), edited and loaded a bin with the laptop. Cost a little over $1,000 but it idles how I want and I can make changes and load them anytime I want!
Le Mans Master
Joined: Apr 2005
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From: The Old Dominion
I always just assumed this is because the car is still moving. The vehicle-sitting idle speed is quite low, usually 600-650rpm. If you are coasting and then things change and you speed up, I don't think you'd want to put it in gear at a leisurely 650rpm. If you picked the right gear, the motor should be faster than that, and if you picked too tall gear, it would probably be a crummy experience to have the clutch slow the engine down further.
At 1,000 rpm, the engine is a bit more ready to go should you stop slowing down and start speeding up. So like I said, it seemed like a drivability thing to me. Both Corvettes I've owned have done that, and I never thought anything was wrong with them.
At 1,000 rpm, the engine is a bit more ready to go should you stop slowing down and start speeding up. So like I said, it seemed like a drivability thing to me. Both Corvettes I've owned have done that, and I never thought anything was wrong with them.
