IAC effecting start time

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n01tk
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IAC effecting start time

Post by n01tk »

Engine cranks 5 - 6 times when cold. If IAC electrically disconnected starts with 1/4 turn hot or cold. This engine has a rebuilt TB and new IAC motor. What could be the problem? Checked for vacuum leaks, can't find any. Vacuum measured when running, 20 "hg and steady. Any ideas? :roll:
1995 Chevy 3500 deully dump truck
ECU 7427
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JP86SS
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Post by JP86SS »

try increasing the "Park position' of the IAC (hot or cold)
There should be something for that.
I've gotten much quicker starts by opening the IAC a bit more during cranking. Motor seems to turn over easier too.
Jp
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n01tk
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Post by n01tk »

I tested the IAC input with a noid light. The noid light flashes with engine running. Lights turning red and green if throttle is varied. Noid light does not flash when key is turned to on position. Also no flashes when key is turned to off position. Noid lights stay lite but don't flash either in "on" position or " off " position. Looks to me the ECM is not running the software routine to park the IAC for cranking? Comments please. :roll:
1995 Chevy 3500 deully dump truck
ECU 7427
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1227730 IROC
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Post by 1227730 IROC »

JP86SS wrote:try increasing the "Park position' of the IAC (hot or cold)
There should be something for that.
I've gotten much quicker starts by opening the IAC a bit more during cranking. Motor seems to turn over easier too.
Jp
HOW DO I CHANGE THE PARK POSITION OF THE IAC VALVE OR COMMAND IT FOR DIAGNOSTICS ? ?
THE QUEST FOR SPEED CONTINUES...
billgraves
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CHANGING THE "PARKED IAC" Position

Post by billgraves »

You asked how do I change the Parked IAC position? The PARKED POSITION is a CALIBRATION. A Standard Parked IAC position is 140 counts.
In the 1980's and 90's TRUCKS were VERY SLOW to Decrement the IAC counts and so were SOME car lines. Like 3.8L and 2.8/3.1L because of all the Stalling complaints.

In the 730 ECM you can change the parked position from 140 to about 160. You can also change the MAX Position from 140 or 160 to 255 max (limited by the software in the 730 case. Most IAC bores do not increase the RPM much past 160. In some only a small increase is noted from 140 to 160. So in some 140 is about the highest the car will go.
You MAY increase the hole size but you again don't gain much because the hole to IAC pintle is about as large as it can be with the hardware limits.

I would be careful if you do increase the size as you may have a run away or sailing effect with the idle on lift off of the gas pedal IF A FAILURE OCCURS.
t
billgraves
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Location: Memphis, TN

Post by billgraves »

n01tk wrote:I tested the IAC input with a noid light. The noid light flashes with engine running. Lights turning red and green if throttle is varied. Noid light does not flash when key is turned to on position. Also no flashes when key is turned to off position. Noid lights stay lite but don't flash either in "on" position or " off " position. Looks to me the ECM is not running the software routine to park the IAC for cranking? Comments please. :roll:
I Think the ECM needs a RUN FLAG to run it's "HOME" position part of it's routine.
If you had the engine running with the noid lite, the lites flash (for movement ) as the throttle follower runs as the tps is changed. When rev pulses stop, the IAC zero's itself out and then Parks at it's calibrated position like 140.

I don't think you had the engine running is why you didn't see any IAC activity command after you shut off the key.
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