I've been busy tuning the new cams in my LT-5. We put in larger exhaust cams 227/.425" to complement the larger intake cams 237/.425" installed last year. Really like the combo. throttle is very responsive and yet the car drives like a stock motor. Really has responded to the tuning and just about there now in terms of BLMs through the rpm/kPa range. The LT-5 has 4 VE tables.
0-2000rpm/2000-7500rpm and for both Open/Closed secondary throttle.
The issue I am chasing right now, and it doesn't seem to be all that of a bother, is that oveer the course of a 1 hour drive, the idle IAC counts will continue to drop until it gets to 0. When at 0 the motor still appears to idle as it did before with no surging. The idle rpms will vary up/down +- 25 rpm around a commanded 850idle(maybe too low?). I have tried setting min air, motor running, and starting out w a bit higher IAC count, but eventually it gets to 0. IIRC, Grumpy said he liked 0 IAC counts for idle. I would like to have them at somewhere between 8-12. Other than that, the motor behaves fine, doesn't stall blah blah blah. I cleaned out the IAC channel in the TB yesterday and that seemed to stabilize the steps.
I thought that it might be due to higher engine temps, but on the last drive, the temps were lower than before and the IAC still sat at 0 for idle.
Thoughts?

 

set your min air after a long drive? or close your throttle blades a little while hot.

 

Actually I have tried that. Both motor running hot and coming back after long drive, leaving motor idling and adjusting from there.

 

It's looking more like a vacuum leak. Review of the datalog is showing the Left O2 as consistently lean at idle. Pulled the plugs, and #7 looked lean. Porcelain was white, not washed however.
The other plugs had a dark tan color. I checked injector resistance, and all were within spec.
Also, getting a Lean O2 (Left bank) code on occasion while sitting at idle.
Looks like I'll probably be pulling the plenum and left side injector housing.

 

Dug into the motor and redid the intake tract. Everything torqued. I moved the #7 cyl Injectors over to #2 and vice-versa to eliminate a bad injector as a reason for Lean O2 on left bank. Also installed new spark plug in #7 to see if that cylinders continues to run lean. Spark Plug from #7 went over to #2 cylinder. Got the motor buttoned up and started. However, same results.
I get a Code 44, Left Lean O2 at idle. The O2 sensor just is not crossing although the BLM seems ok. I've tied playing w the O2 windows and Prop Gains realizing that this should affect both sides but the right side appears to be working fine. Motor idles fine but I have not had an opportunity to take car out and see if IAC steps drop over time. so I have eliminated a few possible explanations. Could it be an exhaust leak?

 

One other note, is that the O2 sensor will register rich when in O/L. IOW, I can see the O2 sensor voltage as a .7 or .8 when in Open Loop, but once in C/L, the voltage doesn't get above .3-.4 max. I can rev the motor, and voltage will go up but then drop down at idle. Eventually Code 44 SES comes on which goes away with engine rpms. Haven't had the opportunity of taking car out and see if IAC count still drops to 0.

 

Could the cam allow reversion/overlap? Seems mild at 227 @ .05. Was it installed straight up? Possibly retarded or advanced in error? I am uncertain which way would allow more ext gas into intake....

 

Ronny,

Well w a DOHC motor, the cam timing could be a bit diff but my friend has done these for many years. And two of them did the cam
timing. Pretty sure it's been checked. Again, the condition only shows up at idle and in C/L.
Reading the FSM, doesn't matter what the O2 windows are set at in cal. As long as O2 is below .2v for 50sec., that will set code. You can see right side O2 tracking up and down, while left stays down. Once out of idle, it will cycle although still not as quickly as right side.
When I put on scan tool w Ignition ON,
Both O2s register .45v.

 

Well the update is that we swapped O2 on the left side. Now it's fine, no more lean O2 code. I think I have finally tracked my disappearing IAC count to loose fuel rails. Air entering from around injectors. In process of a fix now.

 

Took car out yesterday for datalog. Damn IAC dropped to 0 again!!?? However the car idles well with no sign of a vacuum leak. I moved my MAT sensor to the filter housing in the air box. Looking at the datalog it appears that the IAC counts at idle in particular drop as the Intake Air temps go up. I have a
MAT v decreasing IAC steps table which may be the issue. Not exactly sure what it does, but I am leaning towards just zeroing it out. As I drive the car, the IAC counts decrease with successive idle events. I'll drive the car into the garage but leave it running. At this point IAC steps are at 0. I'll then adjust Min Air so as to get an IAC count. Yesterday adjusted Min Air and got 3-8 steps. Also wondering if the larger exhaust cams are acting as if they are a large enough vacuum leak.
Motor idles at 52-55kPa.

 

Hey Dominic, have you upgraded to the "bird cage" style IAT? Faster response time compared to the MAT (same as CTS sensor). Delco p/n 213-190.

 

Yes it's the plastic one and its positioned in the air tract before the air bridge.

 

After addressing the loose fuel rails and reading a bit more on this forum, I started playing with timing at idle. Also continued to lower the IAC counts in the
MAT table. Taking the car out for a drive in the lovely Chicago weather we are having my idle IAC counts were at 15. What's really nice is that the motor idles more smoothly and at ~ 850. I initially had idle timing set at 21d, now it's at 17.
So I am thinking that my IAC counts were disappearing because the timing was keeping the motor from idling down any further and as motor got hotter it was more difficult to bring down the idle. Another lesson learned.Thanks again RBob.