Really need help with the last small issue with my 90

[Ccmano]

Originally Posted by grahambehan

Prev post said 150 iac, park position prior to start, 50 counts at start, dropping to single digits/0 counts.

Thanks Graham, missed that. That tells me the IAC is not controlling the idle speed (zero count) at the point it's stalling. We have found before that the base mechanical idle adjustment of the throttle body sometimes has to be redone when the intake has been ported. To me the next step would be to readjust that. I have to do a search but it's out there in the tech forum.
H

[Corvette95]

Hi, to clarify, the IAC is at 150 on start then comes down. When the first start happens and it dies, the IAC count is still around 80-100. I just finished replacing both fuel pumps again...they were new Airtex, I went with Delco pumps this time, no significant change, I have maybe three more psi at all ranges with the new pumps...no change in leakdown. What I wonder about is the requested RPM before start that shows 3000K plus requested but first revolution in crank sequence brings requested RPM immediately down to 875. Car runs rough in back up fuel and I wonder if the back up fuel controls the initial start up air and spark until the ECM sees 400+ RPM? I have asked Haibeck about this and he says he has never encountered this issue before . He initially wondered about the spark retard on start up, but pulled the knock sensor clip off so that wasnt the issue/. He also asked if I had another ECM to swap..I don't, he also asked if my Tech1 was giving accurate readings..well pulled out the old Tech1 with a different Mass Storage Cartridge and I got the same readings ( I found an old Vetronix rep that was selling out and I bought all his extra Tech1 stuff).

[grahambehan]

Ok so the iac is at 80 - 100 at the stall? So as stated adjust the throttle blades further open, I normally shoot for 20- 30 iac counts on a hot idle, some may disagree but thats that. With kr on start up, common lt5 is mechanically noisey, that normally goes away fast. Since you said it is the first start only the car light off reduction is probably active, between 20 and 32 c coolant temp from memory, so it is certainly reducing engine efficiency, a simple change to that will help out with no more sustained negative spark. Outside of those temp ranges there is no spark reduction from the light of feature. After having the engine ported, why no tune, just as a matter of interest.

Graham

[XfireZ51]

Graham,

I believe you meant he should CLOSE throttle blades so more bypass is needed to run motor. At 0 IAC counts, the IAC is fully closed.
By closing blades the ECM would have better control over idle speed. I shoot for 12- 20 steps, again on a hot idle.

[grahambehan]

Well 1 says it goes to zero and one says it is at 80 so both maybe true. Thats why I said a warm idle iac value

Graham

[XfireZ51]

Usually, for me, if the IAC at operating idle is at 0, either the blades are open too wide or there?s a vacuum leak. With a ported top end and rebuilt TB, I suspect Chris? motor is moving more air thru the TB, so closing the blades would bring up the IAC counts.

[Corvette95]

Thank you SO much for the feedback. I will data record from the IAC tomorrow and post results. I will record the live IAC data from cold start in the morning and post! Thanks again .

[Corvette95]

FYI IAC drops from 150 to under 100 after first start then close to near zero after successful start after first. I think this a pre program open loop issue. Can this be tuned out. Will update Saturday morning. Thanks to all!

[XfireZ51]

I think you should do the Min Air adjustment before doing anything else IMO.

[Corvette95]

Originally Posted by grahambehan

Ok so the iac is at 80 - 100 at the stall? So as stated adjust the throttle blades further open, I normally shoot for 20- 30 iac counts on a hot idle, some may disagree but thats that. With kr on start up, common lt5 is mechanically noisey, that normally goes away fast. Since you said it is the first start only the car light off reduction is probably active, between 20 and 32 c coolant temp from memory, so it is certainly reducing engine efficiency, a simple change to that will help out with no more sustained negative spark. Outside of those temp ranges there is no spark reduction from the light of feature. After having the engine ported, why no tune, just as a matter of interest.

Graham

So to clarify,adjust the throttle blades to open further? Would that be by adjusting the throttle body idle adjustment screw? Currently TPS is at .54 or are we talking about doing an IAC relearn (or both)? Coolant temp has been steady on start up as I have it inside a climate controlled garage so engine temp is +/- 68F and MAT is +/- the same 68F reading in the morning. Also map is reading correct barometric outside pressure. Also, when warmed up for a few minutes I have a zero IAC count. warm idle is dead on 650 to 675 via Tech1. Also, a new calibration is definitely getting done, this was the last gremlin to tackle before getting a calibration. I will have to order a standard chip for the light mods on my car until I can get it to a ZR-1 specialist. I am in Alabama, last work was done on my 95 10+ years ago by South Georgia Corvette with is an 8 hours drive with a car hauler behind me. I am mostly concerned about the die in case there may be a mechanical issue that I don't want to mask such as a coil, plug, or intake leak. I have done my best to rule it out but I do have a home made "smoke" machine I can test for intake leaks but again, secondary pump runs a few seconds and holds for 4-5 minutes before a very short cycle. I don't seem to notice a difference on start up with normal or full power.