1996 Z28 LT1 - 411 PCM Fuel Trim Split on Coast (Cell 21)

[jahajazz]

Hello,

I have a 383 LT1 which I have converted over to run off of the 411 PCM (24x reluctor conversion, 8 coil packs, etc). By starting tune is a 2001 M6 LS1 Camaro, and I have been using a few other tables from a 01 5.7 Express Van. I have a small fuel trim split at idle, within 2-5%, however this is down from the baseline split which was around 20%. The LT1 Intake Manifold and Idle Air Circuit lead to poor cylinder balance, especially at idle with aftermarket camshafts. Due to this, I am fairly happy with the current idle fuel trim split. The biggest issue currently is that the fuel trims have a huge split on cost down. Ie 25mph+, 1500rpm+, and 0% throttle. During this condition, the fuel trim cell is 21, and the split is huge, often maxing out both banks in stft (+25/-25). You can see this in my attached log, around frames 445, 875, 1645, and 2200. What is the recommend approach to work this issue out?

See below for test performed:
Installed new spark plugs (NGK TR6)
Used an HEI spark tester to test each coil and plug wire
Switched coils from bank to bank
Switched injectors from bank to bank
Used a Noid light to test the Injector harness
Tested for voltage, ground, low reference, and coil control signal at each coil.
Double checked, using a digital volt meter, the PCM continuity for the coils, injectors, and O2 sensors.
Switched in the stock O2 sensors
Performed a compression test on each cylinder
Tested fuel pressure regulator for leaks and for proper pressure at vacuum
Ohmed each spark plug wire
Checked for vaccum leaks / exhaust leaks using brake cleaner and propane
Installed new exhaust clamps at the header collectors, along with exhaust sealer

None of the above has helped resolve the situation.

Thank you for the help,
Jahajazz

[flame]

What brand throttle body? Does it use the idle passage in the intake?

[jahajazz]

I found some idle improvement by fixing
the issue described in this link:
http://www.hashmarks.com/techtips/throttle_body_mod.htm

My throttle body had the incorrect idle air passages, but I have now corrected them.

[flame]

Yeah that's what I was refering to. It makes a world of difference. I'll have to fire up the laptop to have a look at your log.

[ElecTech]

Yep, if the trims even out under steady speed then 9 times out of 10 its a TB issue. Ive only ever seen it once from a bad vac leak on that side.

[flame]

Took a quick look and a few things stick out. At idle your tps is showing 1% and the voltage is too high. You need to close the throttle blade and drill out the iac hole bigger. Set the tps to .67 volts and make sure the % reads zero. Then get your iac counts between 30-50 hot idle by drilling out the iac hole. LT1's like a lot more timing then you have. Did you tune with a wideband? The VE table is pretty high and your MAF is turned off in case you didn't know that.

[MikeOD]

Took a quick look and a few things stick out. At idle your tps is showing 1% and the voltage is too high. You need to close the throttle blade and drill out the iac hole bigger. Set the tps to .67 volts and make sure the % reads zero. Then get your iac counts between 30-50 hot idle by drilling out the iac hole. LT1's like a lot more timing then you have. Did you tune with a wideband? The VE table is pretty high and your MAF is turned off in case you didn't know that.

I agree about the timing...every N/A LT1 (350's, 383's, 396's) I've ever tuned wants 34-36 degrees at full throttle, and in turn significantly more at part throttle. This might make a big difference in trims too, I've often seen that with the LT1.

[jahajazz]

Thanks for the replies.

There seems to be split opinions on whether or not to crack the throttle at idle, I will go back to throttle 100% closed and try drilling the IAC Bleed hole to see what improvements it makes.

Regarding the timing, I have found idle quality to be best at the current timing. Especially on 1st gear starts, it is much smoother with the current timing. I have a wideband, but I need to install it, which I planned to do after I sorted out the idle splits issue.

Will report back with my findings.

[jahajazz]

I set my TPS back to fully closed, with .69v displaying. Just for curiosities sake, I turned off closed loop, and I noticed that only bank 2 spikes lean (50mV) after I blip the throttle (frame 350, 415, 480). It seems also to happen after the throttle has closed, not actually during the throttle blip. I also notice that when I turn open loop on, I see bank two staying low voltage (frame 0-100).

Is this significant enough that there is other investigation I should do before drilling the throttle? After all the previous testing I did, I do not know what else I could test? Is it possible the IAC bleed hole issue could be significant enough to cause this issue, only on one bank?

[jahajazz]

Thinking my previous result may have been some fluke tip in lean spike, or DFCO related, I did some more data logging. At around 1750, bank 2 O2 jumps all over the graph, while bank 1 02 is fairly constant. See attached log.

[flame]

Put the wideband in and tune the VE and MAF tables, spark tables then worry about idle. Unless fuel and timing are correct you will be chasing your tail.

[jahajazz]

Is 1750rpm low enough to be considered idle? I assumed the erratic o2 activity at 1750 means there is a problem i need to resolve before wideband tuning.

[flame]

Just went back to the first post and see you have NGK plugs. If you are rich at idle it will foul them out pretty quick. Pull them and have a look. The O2 signal will spike lean if there is a misfire because there will be a high oxygen content from incomplete burn.