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LSX454 idle challenge - Failing to catch RPM when returning to idle

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Gentlemen,

I have an LSX454 installed in a 2002 Pontiac Trans Am. I am running an LS7 intake manifold with factory LS7 injectors. The throttle body is a 90MM Holley with a mechanical throttle plate. The car has a manual transmission in it as well.

As for the issue I am facing, it mainly only occurs in the summer when the ambient temperature is 80+ degrees Fahrenheit and is usually difficult to replicate when the car is parked. The engine RPM will drop past the target idle and will sometimes stall the engine. It may drop to 300 RPM and recover, stop at the target idle without issue, or die.

It does appear to me that the computer does not recognize the base idle air flow needed and what the IAC should need to achieve this. I have seen the IAC go to 0 as the idle falls and not go up until the engine is so far past it can't recover. Other times, the IAC will stay close to the 60 or so steps that is needed.

I have it all set to open loop. Currently both long term and short term fuel trims are turned off.

I have attached a couple of log files. The one with the stumble/stall is where I was tapping the throttle while parked and I managed to replicate the issue. The other I am just driving the car. Not sure if there is much value in the second one. The AFR error is showing high on the second one in most areas, but it will normally run within 2 percent once the intake air temp goes down.

Any thoughts or ideas to help get this idle issue resolved would be greatly appreciated! It has been an issue ever since this engine was installed many years ago.

It would be great if there was a way to set the minimum IAC position, but that would be too easy!

Thanks guys!

Attached Files

Hi Bradley, the two tables areas that you need to focus on are your throttle follower and throttle cracker. These affect the IAC opening as you move the throttle (throttle follower), and when you're moving (throttle cracker). From your logs I'd suggest you need to start by increasing the numbers in your throttle follower Airflow table. Make an across the board change to the values of 20% and test to see if you notice an improvement. You can always back this down if you find the rpm tends to hang.

Thanks Andre. I will give it a try and report back with the results.

Just an update. I adjusted throttle by small increments and didn’t notice any help. I then decided to go BIG to see what would happen. I now have it set in the 7+ grams a second range and it does not stall anymore. It did have the RPM hang at first, but I increased the decay rate.

Doesn't appear to have a negative effect on drive ability at this level No cruise control effect or idle hanging due to these changes. The idle cracker was causing the idle to hang some when above the enable threshold and would cause a cruise control effect, but I reduced it and that went away.

I will be adjusting the throttle cracker and refining the throttle follower to get the decay just right.

Overall, between your advice and the courses I have completed, my car runs amazing compared to what it has since it was first tuned by a so-called professional. I found and corrected a huge amount of issues over the years, but the last few months after taking your courses have made that difference I needed to be confident and to maximize its potential.

Thank you Andre and HP Academy!

P.S. I had messed with every table at some point fighting the idle issue, including the ones you recommended. That being said, I didn’t have near the understanding needed to fix or figure it out. After your recommendation, I scratched my head some and the light bulb came on. Thanks again!