I've got a very light miss/buck under light throttle and the STFTs max out in opposite directions. The car has 65k miles on it with the only engine modifications being a CAI. The intake manifold has been off twice for an oil pressure sending unit and another time to have the intake painted. However, the intake manifold hasn't been R&I'd in at least 6 months. This is a 2001 Z06.
Does anyone have a suggestion on where to start? Is this as simple as plugs/wires?
Can you post a run log?
Sounds like bad o2 sensors, but post a log.
Sulski Performance Tuning
2000 WS6 M6 - LS6 (long block, refreshed top end), 10.8:1 CR, 90 mm ported FAST, Exo-Skel, 227/232 cam, QTP HVMC, EWP, GMMG, 9" w/4.11s
2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB
You can see the split STFTs at around frame 475. I also have a dead spot at WOT around 5k. If I ease into the throttle, the car pulls strong to redline. However, if I go WOT the car falls flat around 5k rpms (haven't logged this yet) and picks back up after a few hundred RPMs.
http://www.durhamcons.com/HPT/slightmiss-splitSTFT.hpl
Log that, car is OL at WOT so unless it is adding some weird LTFTs, sounds like something else.
Sulski Performance Tuning
2000 WS6 M6 - LS6 (long block, refreshed top end), 10.8:1 CR, 90 mm ported FAST, Exo-Skel, 227/232 cam, QTP HVMC, EWP, GMMG, 9" w/4.11s
2018 Sierra SLT 5.3L A8 - Airaid intake tube, GM Borla catback, L86 Intake/Ported TB
Here is a scan I made last night on the way home from work.
http://www.durhamcons.com/HPT/slight...otbreaksup.hpl
Light throttle miss (frame 417)
===================
The STFTs max in opposite directions. It was just after me coasting (5% tps) and applying very light throttle. B1S1 flat lines during this period.
WOT breakup/miss/deadspot (frame 1246)
===================
I'm WOT but the MAF looks jagged. The 02 sensors dip perhaps related to the MAF. If you follow the injector DC, you can see what I'm talking about. The car takes off fine... but there is a dip/lull/deadspot but picks back up for a period before redline.
The car was tuned without the MAF to 0 to -3 LTFTs. When I plugged the MAF back in... the LTFTs were around 0 at partial throttle. This is my personal car that I don't race so I didn't bother tweaking the MAF table (IIRC). I suppose the next step might be to unplug the MAF and test both
scenarios?
Looks like some sort of torque management maybe. The TPS is being closed and opened? What is the exact frame of the miss? Were you rolling at 23% TPS/70 mph and then tromp it? Didn't enter WOT untill frame 1248. Any DTC's like misfire?Originally Posted by tech
Last edited by mowton; 02-21-2009 at 04:08 PM.
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The issue surfaced months after the last time the car was flashed. I can put the stock tune back in it if the consensus agrees.
I can't tell you exactly what frame the missing occurs... other than the scanner usually shows maxed/split STFTs.
I can try to get a smaller log that isolates when the problem is prevalent.
Looks like the upstream 02 sensors are crossed. B1S1 plugged into B2 connector and vise versa. When they split, the FT that goes lean leans out the opposite 02.
FYI... I've put at least 20k miles on the car and the 02 sensors haven't been removed. The split STFTs are new. Thanks for the suggestion though.
I realize that. Only stating what the log info looks like. I've seen it before, many times.
you might have a bad valve spring. Looks like it is misfiring under load on one bank.
the lean dips on the Bank 2 O2 sensor at WOT are typical of valve float or similar. Could be a plug but usually a plug misfire will degrade to a random or consistent misfire and part throttle as well.
I count sheep in hex...
FYI, I drove the car today and lugged it in high gear at 1k-2k rpm and the car pulled smoothly. It certainly seems related to RPM. I don't get traction in 1st or 2nd but when I try to go WOT, my tires break loose around 5k rpm (easing up to WOT just before 5k). I don't notice the miss when the tires are spinning. I realize that might not mean much other than I've only noticed the WOT problem in 3rd gear (haven't tried 4th).
Could the MAF explain both problems?
BTW, I forgot to mention that I have an ASP pulley. I bought it used and it came with belts. The belts weren't in great shape but I opted to use them thinking that I'd swap them out if I had any problems. The WOT breakup seemed to begin just after the ASP install. Could it be belt slip? Could my pulley be spinning on the crank? Crank bolt does appear to have walked out any but I haven't had the car in the air to verify.
The suggestions in this thread have pointed at quite a few different possible causes. Given all of the information I've provided, what do you guys/gals suggest I do to rectify the problem? Can any of the suggestions be ruled out?
- Replace 02 sensors
- Check spring pressures
- Replace plugs (and I presume wires)
I can't put my finger on a common denominator in that list.
The repair that would involve the most work is the valve springs.The weak springs would explain a lean bank, but would it also explain a rich opposite bank? The weak springs might also explain the 5k miss/lull/whatever but would the car pick back up before redline? Would a weak/damaged spring cause the slight miss at light throttle?
I certainly see one of the 02 sensors flat lining while the STFTs split. I also see the MAF looking a little peaky during the same period. That seems to indicate the problem exists before the air enters the engine (unless the spring fluttering is causing air turbulence that reaches back to the MAF).
It seems that many ignition-related issues on LSx vehicles eventually result in a DTC. I'm not getting anything (but that doesn't mean it's not ignition).
So where do I start? While I don't want to throw money at it, I'd rather do that then spend a week diagnosing it.
i think the high rpm issue you have is valve springs (maybe this will also fix the part throttle issue).
here's how i would diagnose the other part throttle thing.
1. note which way the STFT's swing, if one consistently goes high and the other low then do this.
2. swap your plug leads to the other bank. see if the problem has switched. ie the STFT's are swinging the other way now.
3. do same for plugs
4. do same for injectors (be careful not to drop dirt/dust into your intake when removing injectors).
if any of these result in the STFT's swinging the other way then that is probably the cause. If not then you may have a vacuum or exhaust leak of the worst kind.
Chris...
I count sheep in hex...
Thanks for the additional reply, Chris. Unfortunately, the valve springs are the biggest pain in my @%&. I made a thread on Corvette Forum, not because I don't believe you, but because I don't want you to be right. (see comment about them being a PITA)
the valve spring misfire (float) will usually go away again after you get past the 5000-5500rpm harmonic. Seen it many times on the dyno, one or two springs just lose enough seat pressure to float at the harmonic.
I count sheep in hex...
Come to think of it... valve springs aren't such a pain in the butt in C5s. And the LS6 replacement valve springs are only about $60. That makes it the least expensive thing to try.
Last edited by tech; 02-23-2009 at 08:13 PM.
if you are going to the effort, buy some that are slightly more seat pressure.
I count sheep in hex...
Can you post a log of it flat lining when the STFTs split? What does it flat line at?
The cheapest quickest thing to check to see if the 02 sensor connections are swapped side to side. What you're failing to realize is when the FTs split, the one that goes rich is driving the opposite bank FT rich, and the one that goes lean, is driving the opposite bank lean.
How can you not think the 02s are swapped when you can clearly see one side driving the other sides FTs?
The most common cars I see this on is are Corvettes. I bet you have headers, and the 02s are pointing inward, and the natural thing to do is to plug the DS 02 into the pass side and vice versa, because that's the way they're pointing, so it looks right. Might wanna just crawl under there and look. That's the only thing that makes sense. The 02s are working just fine. When bank 2 FTs go all the way lean, bank 1 02 sensor goes lean, that in turn drives bank 1 FTs rich, which in turn drives bank 2 02 sensor rich.
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