[Gmecm] Vortec L31 oscillating at idle

Mon Mar 19 00:01:15 UTC 2007

  Hi, Jake.

 If I were checking this problem, I wouldn't suspect the IAC first. Usually changes in spark timing and fuel are responsible for idle hunting. Often the engine is slightly lean at one rpm or load and is not so lean at another. As the MAP signal changes, then engine is less lean, then more lean, then back which causes RPM swings. Additionally the timing will change as load and rpm change which can aggravate the problem. Finally, the IAC will play catch up trying to follow and correct the engine rpm and eventually the fuel / spark / IAC values will match up just right to produce a stall.

 I'd try to make sure there are no vacuum leaks and I'd adjust the minimum air (the throttle stop) on the throttle body. Sometimes it helps to temporarily force the idle plate open slightly to get the truck to idle at a steady rpm. Then you can back the idle down and watch your scan software for something to go wrong.

 With a '75 block it's safe to say you're not using the Vortec cam. One thing which may help is to adjust the timing tables so there is less change between 20 kPa and 100 kPa at 400 - 800 rpm. Once you get idle stabilized then you can start looking at O2 values to try and judge lean / rich operation.

 hth
 Zaphod

 -----Original Message-----
 From: jktucker at usamedia.tv
 To: gmecm at diy-efi.org
 Sent: Sun, 18 Mar 2007 4:53 PM
 Subject: RE: [Gmecm] Vortec L31 oscillating at idle 

  The software I'm using is called car-code for the scan tool.  I also have

tunercat's OBDII tuner software.  Do you have a proceedure on adjusting the

IAC?

-----Original Message-----

From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On Behalf

Of Edwin leahy

Sent: Saturday, March 17, 2007 9:17 PM

To: gmecm at diy-efi.org
Subject: [Gmecm] Vortec L31 oscillating at idle 

Sound's like your IAC can't compensate enough to keep
it idling.This is either from to much fuel or more
than likely the IAC count's are off.Try adjusting them
in small increment's.What software are you using and
what ECM?It is possible to use the 2000+ LS1/LS6 ECM
with that injection/motor.

> Date: Fri, 16 Mar 2007 06:55:03 -0700
> From: "Jake Tucker" <jktucker at usamedia.tv>

> Subject: [Gmecm] Vortec L31 oscillating at idle

> To: <gmecm at diy-efi.org>

> Message-ID:

> <000401c767d2$b3c713f0$0200a8c0 at tuckeroffice>

> Content-Type: text/plain; charset="us-ascii"

> 

>   Well, we're finally getting close to having a

> vortec injection

> transplant running in an '84 Suburban.  We started

> with a fresh 350 with

> vortec heads, and installed a vortec injection setup

> out of a '97 onto the

> motor to replace the TBI system that had been

> running the motor.  There were

> a few setbacks along the way, but now the motor is

> running and appears to

> need just a little tuning.  The main problem now is

> that it is hunting at

> idle.  Engine speed oscillates from 300 to 1000

> RPM's for several seconds

> before the engine dies.  This is especially a

> problem when it is cold, and

> the engine won't stay running at all without opening

> the throttle a little.

> In general, the engine runs, but doesn't give the

> power we'd expect at this

> point.  It is a '75 block that was rebuilt with

> Vortec heads.  To do the

> vortec conversion, all the sensors, intake,

> distributer, crank sensor,

> harmonic balancer, and accessories were transplanted

> from a '97 vortec motor

> with a cracked block.

>   Any ideas what would cause the oscillating problem?

>  I would like to

> watch the IAC motor counts, but I can't seem to find

> that data on the

> software I'm running.  Right now, I'm using car-code

> 2 which is sold at

> www.obd-2.com.  I also have the OBDII tuner by

> tunercat.  He has since sold

> his design to Jet performance.

> 

> Jake

> 

> 

> ------------------------------

> 

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> 
> End of Gmecm Digest, Vol 25, Issue 7
> ************************************
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