I'm STUMPED! Wierd one, Any takers?
07-22-2007, 12:17 PM
#1
I'm STUMPED! Wierd one, Any takers? Long post, so be prepared. Okay, so heres the deal...
First the problem...
My LT1 Z has a very annoying little problem. On a cold start the idle is a bit on the low side, but fine. Once it gets to closed loop though, it slowly climbs to 1600 rpms in park...not very nice for the neighbors with the loud 3" TDs! Sounds simple enough? Its got both me and my speedshop stumped. The kicker is that the idle is only jumping up once it hits closed loop. Open loop is fine, except for a bit of cam surge. Since its only in closed loop, I suspected the o2s right away. Bought and installed the 4-wire heated o2 conversion kit, and they respond much faster then the single wire 93 o2s. Smoothed out a bit of the higher rpms too, but problem still there...but now it goes into closed loop much faster(aka idle jumps up faster!)
Some useful information about the car...
The car is a MAFless Speeddensity LT1.
TPS is brand new, adjusted to .55v at idle.
IAC is new, working properly.
O2s are brand new 4-wire heated.
Unhooking brake booster vacuum line raises idle like it should(not vacuum leak).
TB is fairly recent 58mm, with IAC bleed hole drilled same size as stocker(11/64?) for help on cold starts. Was on the car previously when everything was fine.
What its NOT(AFAIK)...A lot should theoretically have nothing to do with it, but just makin sure everything is covered.
- Not o2s. Brand new 4-wire heated.
- TPS is brand new, verified with DMM, ~.55v idle.
- TB is fairly new, same one that was previously working fine(doesnt neccesarily rule that out though...)
- IAC is fairly new. Counts start at 160(fully open) at cold open loop, and once it warms up and goes into closed loop the idle goes up and IAC counts drop steadily to 0(fully closed) in an attempt to bring rpms down but the idle stays high.
- No vacuum leaks. Removing brake booster line makes the idle jump up quite a bit like it should. Also, all emissions/evap/pcv crap has been deleted and removed and/or plugged off. No real spots for vacuum leaks!
- Not the tune - my dyno tuner and speed shop were pretty stumped to. Car runs great, a little bit on the rich side when cruising but nothing bad at all.
- Not the Opti. New AC-Delco OEM unit with MSD cap and rotor. No missing, everything smooth.
- Not the wires. Fairly new Taylor Thundervolt wires, routed carefully with no arcing or visible damage.
- New Blaster coil.
- Plugs look good, taken out and cleaned.
- All cylinders firing properly - not a fuel or ignition issue!
As you can see, I've just about hit all the bases I can think of here. Typing it out and thinking about it makes me lean towards the Throttle body, even though it was working fine before. I just dont want to drop over $300 on a new TB and the problem still be there! I have searched countless forums and the internet and have seen two or three threads explaining my exact problem to the T, but none ever finish with any solutions!
LT1s are pissin me off! I've never been stumped like this...
First the problem...
My LT1 Z has a very annoying little problem. On a cold start the idle is a bit on the low side, but fine. Once it gets to closed loop though, it slowly climbs to 1600 rpms in park...not very nice for the neighbors with the loud 3" TDs! Sounds simple enough? Its got both me and my speedshop stumped. The kicker is that the idle is only jumping up once it hits closed loop. Open loop is fine, except for a bit of cam surge. Since its only in closed loop, I suspected the o2s right away. Bought and installed the 4-wire heated o2 conversion kit, and they respond much faster then the single wire 93 o2s. Smoothed out a bit of the higher rpms too, but problem still there...but now it goes into closed loop much faster(aka idle jumps up faster!)
Some useful information about the car...
The car is a MAFless Speeddensity LT1.
TPS is brand new, adjusted to .55v at idle.
IAC is new, working properly.
O2s are brand new 4-wire heated.
Unhooking brake booster vacuum line raises idle like it should(not vacuum leak).
TB is fairly recent 58mm, with IAC bleed hole drilled same size as stocker(11/64?) for help on cold starts. Was on the car previously when everything was fine.
What its NOT(AFAIK)...A lot should theoretically have nothing to do with it, but just makin sure everything is covered.
- Not o2s. Brand new 4-wire heated.
- TPS is brand new, verified with DMM, ~.55v idle.
- TB is fairly new, same one that was previously working fine(doesnt neccesarily rule that out though...)
- IAC is fairly new. Counts start at 160(fully open) at cold open loop, and once it warms up and goes into closed loop the idle goes up and IAC counts drop steadily to 0(fully closed) in an attempt to bring rpms down but the idle stays high.
- No vacuum leaks. Removing brake booster line makes the idle jump up quite a bit like it should. Also, all emissions/evap/pcv crap has been deleted and removed and/or plugged off. No real spots for vacuum leaks!
- Not the tune - my dyno tuner and speed shop were pretty stumped to. Car runs great, a little bit on the rich side when cruising but nothing bad at all.
- Not the Opti. New AC-Delco OEM unit with MSD cap and rotor. No missing, everything smooth.
- Not the wires. Fairly new Taylor Thundervolt wires, routed carefully with no arcing or visible damage.
- New Blaster coil.
- Plugs look good, taken out and cleaned.
- All cylinders firing properly - not a fuel or ignition issue!
As you can see, I've just about hit all the bases I can think of here. Typing it out and thinking about it makes me lean towards the Throttle body, even though it was working fine before. I just dont want to drop over $300 on a new TB and the problem still be there! I have searched countless forums and the internet and have seen two or three threads explaining my exact problem to the T, but none ever finish with any solutions!
LT1s are pissin me off! I've never been stumped like this...
07-22-2007, 02:11 PM
#3
Originally Posted by SincalT/A
borrow a stock t.b. from a friend.
None of my crew still ride with LT1s. I would love to try a different TB for free though!!!
Now that I think about it, maybe my speed shop has one lying around...
07-22-2007, 05:48 PM
#5
Originally Posted by SincalT/A
i can do that let me know where you live.
Thanks though!
07-22-2007, 07:52 PM
#7
Originally Posted by Zralou
Check the tune, may have made a mistake in one of the blocks that only shows up at idle in closed loop.
Sara Lou
Sara Lou
Its happening at all closed loop cells, regardless of IAT/ECT/o2 temperature. Since its closed loop only, it would leave me to believe its a sensor - but what would casue that????
I'm beginning to think it is a sly mix of Throttle Body Malfunctioning along with Possesed LT1 Trickery.
