'95 LT1 tip-in Stumble in SD mode?

[bobdec]

Updated my profile to Atlanta, GA area. Moved here 26 years ago from NY, Westchester County. Locations surrounding Atlanta require emission testing. One other thing I forgot to mention your IAC count at idle is low for an LT1, your TPS voltage (set screw) 0.61 is good, but IAC counts 6-13 at idle will work but may cause slight stumble when hitting the throttle. The LT1 IAC valve feeds the intake air plenums, and the plenums distribute idle air evenly to all cylinders. The engine likes that much more than idle air entering the TB blades up front. Your IAC before the stumble was at 6, saying you are getting a lot of idle air from the TB blades. Cracking the screw back a bit will drop voltage and raise IAC counts. It will move the TB/IAC balance of air towards the IAC side. Voltage specs. of closed TBS is 0.45-0.75, going to 0.55 will be in spec and may help off throttle hesitation.
Back to EGR.. it's purpose is to cool the combustion temperature (adding co2) thus reducing NO a by-product of hot combustion. The hot combustion occurs at the lower RPM load conditions (1000-2100), higher RPM starts flowing a lot or air into the cylinder cooling the combustion, so EGR cuts off at that point. And lower RPM below 1000 does not result in a hot cylinder situation. Key is to activate EGR so you don't feel it or know it's working, in a stock setup 1000 turn on and 2100 turnoff s/b fine and will keep some poisons out of the air. It has high MAP cutoff so when you hit the throttle for performance it should be out of the picture as far as performance. It's really only on during DD street cruising.

[sherlock9c1]

Thanks for the advice Bob - I did notice the low IAC counts on this motor, even at low RPMs. Questions:
1. Am I right to think that I should first verify no other vacuum leaks are present before tweaking the throttle body set screw?
2. I have not cleaned the IAC on this throttle body in awhile. A cruddy IAC should result in higher IAC counts, not lower, correct?
3. Is it worth trying to clean the idle air spider plenum (without removing the intake manifold, preferably, or even possible)?

[bobdec]

1. Correct any other air entering the intake will increase idle speed and PCM will close IAC (lower counts) to compensate.
2. The setup is a DC stepper motor and worm gear that moves a plunger valve. If valve sticks in open or allows air to bypass the valve seat then the PCM will continue to try and close the valve using -steps that will lower the count.
3. Been a few years since I installed my cam, was pretty busy at that time I hosed water through the idle air plenums, nothing came out and did not really look into the actual plenum design. I have heard of using a pressure washer, etc. But nothing when intake is installed. Maybe something like a super long pipe cleaner. But in reality if plenums were obstructed then air would be starved and the PCM would open the IAC and counts would increase to compensate. In modded engines incorrect idle air distribution causes split BLM's at idle and low TB position conditions.

A little more trivia PCM controls idle two ways 1-fast action milliseconds by using idle underspeed/overspeed timing tables (adds or pulls 2-3* to speed/slow idle) 2- slower action seconds, it starts incrementing/decrementing IAC stepper motor (IAC counts) and that could take seconds. When PCM sees ign-on it sets IAC according to park position table (15-125 depending on coolant temp) after startup IAC steps from park position setting to obtain desired RPM per temp and that can take seconds.

[sherlock9c1]

If I can hijack my own thread...

Cold-start tip-in stumble in MAF mode is the subject of this post.

I'm working on an LT1/T56 setup for a friend. This is an "interim" setup so yes, the parts are a little mismatched - long story. Stock 5.7L motor with high-quality fully-characterized injectors. Supercharger on low boost with an intercooler. Headers and huge exhaust. 3.5" F-body LT1 MAF is between intercooler and throttle body downstream of blowoff valve, in a nice straight section of pipe. I've got the MAF dialed in perfectly. Car runs awesome when fully warmed up at part throttle and WOT. BUT - Before the car gets fully warmed up, there is a tip-in stumble from idle up to 2000rpm. I've added 5% more fuel to the VE tables in those areas thinking it was a transitional fueling problem. He says the cold stumble has not improved. What am I missing here? Do I need to adjust the open-loop AFR vs. coolant vs. MAP table?