No idle at lower altitude

[madprof01]

This is a 00 c5 with AFR heads and a mild cam. The people who installed the parts gave the car back wiht crappy idle quality, and he said mi-range Rpm didn't seem to be strong enough.
He brought it to us on one of our dyno days and the car made 405 hp and 413 ftlbs. (tought it was a z06!) He decided to have us check out his idle before race season after he saw that we used EFILive and not LS1 edit. I worked with it for a while and got it to idle and not surge except a small amount when cold and definately not stall at all. I the n went on to the VE table and cleaned up what looked like an EKG readout. Timing wasn't too far off but it was blocked out as RPM increased instead of being a smooth increase. When I was done it made 438 hp and 446 ftlbs. overall the customer was happy. He did return acouple days before his first race and wanted the idle lowered so it would lope. I warned him it hunt more and not seem quite so smooth in drivebility. From Texas I get a call, The car will not idle but is awesome off idle, he was very happy with the results of his racees nad come back in after he had returned. It idles now he said everything semms fine after returning to altitude.
The question is, Why would the car idle at 5000 ft and not at sea level? Doesn't the PCM compensate for the change in altitude? What can I do to fix this?
Thanks in advance,
madprof01

[Bruce Melton]

This is a 00 c5 with AFR heads and a mild cam. The people who installed the parts gave the car back wiht crappy idle quality, and he said mi-range Rpm didn't seem to be strong enough.
He brought it to us on one of our dyno days and the car made 405 hp and 413 ftlbs. (tought it was a z06!) He decided to have us check out his idle before race season after he saw that we used EFILive and not LS1 edit. I worked with it for a while and got it to idle and not surge except a small amount when cold and definately not stall at all. I the n went on to the VE table and cleaned up what looked like an EKG readout. Timing wasn't too far off but it was blocked out as RPM increased instead of being a smooth increase. When I was done it made 438 hp and 446 ftlbs. overall the customer was happy. He did return acouple days before his first race and wanted the idle lowered so it would lope. I warned him it hunt more and not seem quite so smooth in drivebility. From Texas I get a call, The car will not idle but is awesome off idle, he was very happy with the results of his racees nad come back in after he had returned. It idles now he said everything semms fine after returning to altitude.
The question is, Why would the car idle at 5000 ft and not at sea level? Doesn't the PCM compensate for the change in altitude? What can I do to fix this?
Thanks in advance,
madprof01

My guess is that it goes rich at lower alt. B4307 will likely fix it but simulating the lower alt will be the trick.
I had one that would die on hot restart and by bumping up the air on the hot end of the table it completly fixed it.

[TAQuickness]

My guess is that it goes rich at lower alt. B4307 will likely fix it but simulating the lower alt will be the trick. I had one that would die on hot restart and by bumping up the air on the hot end of the table it completly fixed it.


I'm with Bruce on this one. Idle is very particular to timing and AFR. You may want to use the BIDI to determine the optimum timing and AFR for idle, from there, adjust the VE and spark tables, then dial in the B4307.

[madprof01]

thanks for the iput guys. I tried to lean out the idle but there was some other table of modifier interfearing and keeping idle afr in the 13's. I ran out of time and had to return the car ( no appt. don't ya know!) but I will definately look into this.
madprof01

[TAQuickness]

you may also consider adjusting the injector offset table to improve idle and drivability quality

[madprof01]

will do!

[caver]

Dropping altitude like that will lean the idle out considerably. I used to have to go up a size on my sidedraught idle jets when dropping form 5500ft to sea level.
I dont trust the altitude compensation completely after tuning cars up here and then seeing what happens to them at the coast.

I would tweak the values in B0304 as I am sure the whole range is leaning out.

Also with a hot cam it will want the mixtures at around 13.5:1 or richer to maintain a decent idle.
I would also verify my wideband readings with a 4 gas as big cams tend to cause lots of oxygen in the mixtures at idle and low rpm which will make the wideband read plenty leaner than the mixtures are.