LT1 "fine" tuning PCM
07-09-2008, 10:15 AM
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LT1 "fine" tuning PCM OK, I have been tuning the LT1 PCM for probably 10 years. I actually did a few dozen "mail order" tunes for other people back in the day. Anyway, I have an annoyance that I can't seem to get rid of on my personal car. Maybe with my engine it is just not possible to fix...but I think it is. So, here goes.
First off the engine is a 12:1 396 with HOGAN sheetmetal intake and AS&M monoblade throttle body. Cam is a CAM MOTION 250/260 .634"/.637" on a 114 centerline. So, it is a little on the "radical" side. My problem is this.
The car is a six speed car now and it will not idle down to a reasonable RPM when coming to a stop (out of gear). If I start the car in the garage, it will idle say 900-1100...with this motor that is OK with me. But as soon as I take off down the road and come to a stop it idles as high a 2K! If I get stuck at a light for a while it will eventually idle back down...but it take 45 seconds or so...thats too long.
The PCM has a 900 RPM idle target programmed in. I have tried adjusting the TB blade, I have watched the "counts" of the IAC and adjusted the blade position to "center" IAC counts (at close 900 rpm). I know tha IAC works fine. I have looked for vacuum leaks, etc. The motor make monster power...over 600HP. But, I just can't seem to get the idle quite right. Am I asking too much of this engine to have a predictable idle? Any suggestions?
Thanks in advance!
First off the engine is a 12:1 396 with HOGAN sheetmetal intake and AS&M monoblade throttle body. Cam is a CAM MOTION 250/260 .634"/.637" on a 114 centerline. So, it is a little on the "radical" side. My problem is this.
The car is a six speed car now and it will not idle down to a reasonable RPM when coming to a stop (out of gear). If I start the car in the garage, it will idle say 900-1100...with this motor that is OK with me. But as soon as I take off down the road and come to a stop it idles as high a 2K! If I get stuck at a light for a while it will eventually idle back down...but it take 45 seconds or so...thats too long.
The PCM has a 900 RPM idle target programmed in. I have tried adjusting the TB blade, I have watched the "counts" of the IAC and adjusted the blade position to "center" IAC counts (at close 900 rpm). I know tha IAC works fine. I have looked for vacuum leaks, etc. The motor make monster power...over 600HP. But, I just can't seem to get the idle quite right. Am I asking too much of this engine to have a predictable idle? Any suggestions?
Thanks in advance!
07-09-2008, 11:04 AM
#2
Two options that I think may help the most.
1. Adjust the "throttle follower" in the tune. Take out maybe 50% in the throttle follower table.
2. The throttle body linkage (blades) are loose in the housing and sometimes does not close all the way. I have seen this in numerous t-bodies and the fix is to weld the linkage to the blade shaft.
1. Adjust the "throttle follower" in the tune. Take out maybe 50% in the throttle follower table.
2. The throttle body linkage (blades) are loose in the housing and sometimes does not close all the way. I have seen this in numerous t-bodies and the fix is to weld the linkage to the blade shaft.
07-09-2008, 11:23 AM
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Two options that I think may help the most.
1. Adjust the "throttle follower" in the tune. Take out maybe 50% in the throttle follower table.
2. The throttle body linkage (blades) are loose in the housing and sometimes does not close all the way. I have seen this in numerous t-bodies and the fix is to weld the linkage to the blade shaft.
1. Adjust the "throttle follower" in the tune. Take out maybe 50% in the throttle follower table.
2. The throttle body linkage (blades) are loose in the housing and sometimes does not close all the way. I have seen this in numerous t-bodies and the fix is to weld the linkage to the blade shaft.
I have checked the throttle body pretty closely...but I will certainly look again. It only makes sense it is getting more air from SOMEWHERE!!
Thanks again!
07-09-2008, 12:07 PM
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I second the throttle follower option......
My car if I rapped on it lightly to bring the revs up it would stumble and cough and almost stall..... wasn't a real big deal until I tried to put my car on my trailer..... trying to creep forward slowly i'd have to give it light gas....
tuner went into the throttle follower and tuned it out.... And I noticed that my car before would care less about the idle speed I had programmed in. once he fixed up the TF it went right to the target based on coolant temp....
My car if I rapped on it lightly to bring the revs up it would stumble and cough and almost stall..... wasn't a real big deal until I tried to put my car on my trailer..... trying to creep forward slowly i'd have to give it light gas....
tuner went into the throttle follower and tuned it out.... And I noticed that my car before would care less about the idle speed I had programmed in. once he fixed up the TF it went right to the target based on coolant temp....
