Pegged 25% LTRIM - o2's seem ok?
01-17-2013, 09:05 PM
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Pegged 25% LTRIM - o2's seem ok? I have a 1997 z28 LT1 with ~120k miles. Stock everything, only moroso CAI and GMMG catback. I've been getting like 14mpg, but other than that the car seems to run OK. In the data log, I'm seeing my idle LTRIM stuck at 23% on bank1 and 25% on bank2. I was told by a friend that the O2's voltages jumping around means they're ok? I get that the engine is basically pouring gas out the tailpipe, but why if its running alright?
datalog here:Google Docs spreadsheet
datalog here:Google Docs spreadsheet
01-18-2013, 01:45 AM
#3
if its running correctly, and it doesnt burn your eyes from being rich, then theres a good possibility that you just need a tune...
I've seen a few cars that have some whacky LTFT's that Just needed a tune
the other possibility is a leak somewhere...maybe at the intake, possibly one of the vacuum lines in the rear came loose or somewhere else in the car.
I've seen a few cars that have some whacky LTFT's that Just needed a tune
the other possibility is a leak somewhere...maybe at the intake, possibly one of the vacuum lines in the rear came loose or somewhere else in the car.
01-18-2013, 10:05 AM
#5
Ahh forgot about that one, Good thinking on the fuel pressure regulator. but wouldn't be the issue. With the fpr leaking, the car would show negative trims as it was getting fuel from another source so the pcm would be pulling fuel not adding. My guess is still bad or REALLY DIRTY mass air meter or a leak in that area..
01-18-2013, 12:41 PM
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01-18-2013, 12:56 PM
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I've already replace the fuel pressure regulator and while I was there I cleaned the mass air flow sensor at 90k... I'll check the rear of the intake (uh oh)
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01-18-2013, 05:41 PM
#8
When a stock GM vehicle comes into my shop with large positive fuel trims on both banks, the first thing we do is unplug the Maf.
90% of the time the fuel trims then fall in line indicating a bad Maf. A lot of the times cleaning a Maf doesn't help as it can fail electronically.
A quick test to rule out vacuum leaks is to unplug the Maf then drive the car under medium load at ~2000 RPM while scanning the fuel trims. A vacuum leak always affects the positive fuel trims worse at idle. So if trims are still high, the Maf is the problem.
We have replaced a lot of the 78mm Mafs (F-Body's, early C5's, all C4's, and V6 & V8 vortec trucks) See a lot less bad 85mm Mafs.
Russ Kemp
90% of the time the fuel trims then fall in line indicating a bad Maf. A lot of the times cleaning a Maf doesn't help as it can fail electronically.
A quick test to rule out vacuum leaks is to unplug the Maf then drive the car under medium load at ~2000 RPM while scanning the fuel trims. A vacuum leak always affects the positive fuel trims worse at idle. So if trims are still high, the Maf is the problem.
We have replaced a lot of the 78mm Mafs (F-Body's, early C5's, all C4's, and V6 & V8 vortec trucks) See a lot less bad 85mm Mafs.
Russ Kemp
03-15-2013, 12:01 PM
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I know this is an old thread, but I just wanted to post a follow-up in case someone comes across this searching.
I found a large (2-3 inch) tear in my intake elbow. No idea how it got there. Found it *after* having my mechanic friend "who is a pro, been doing this for 20 years, builds awesome engines from nothing". Replaced the elbow with a trick flow metal one, and things are much better now. This just goes to show you that experience doesn't equal knowledge.
I was also having low oil PSI / check gauges come on at high temp when the engine was at low RPM. This is likely to my oil being diluted with gas. I knew something was up, so I was changing really often, like every 2500 miles or so until I could find what the hell was diluting it. Hopefully that saved me from destroying the engine.
Unfortunately, I'd been driving it like that for who knows how long. Maybe years? The TB was really gunked up with blow-by (I assume from the extra pressure), so I went ahead and replaced that. Also replaced the vapor pressure solenoid since it was old / possible vac leak. Both parts were super cheap from a friend at a junkyard so everybody just relax before yelling at me about throwing money at my problems!
Long Term Fuel trims averages went from 9.71% | 12.44% to 0.3% | 0.6%. They're not perfect, but they're a hell of a lot better than before.
Logs:
Before (aggressive run on highway): https://docs.google.com/spreadsheet/...3c&usp=sharing
After (casual run during commute, with stops): https://docs.google.com/spreadsheet/ccc?key=0AoB7q6j02Ca5dDJ1SWRKOUZ4aFBfZS1JZUpiQzdLS Hc&usp=sharing
I found a large (2-3 inch) tear in my intake elbow. No idea how it got there. Found it *after* having my mechanic friend "who is a pro, been doing this for 20 years, builds awesome engines from nothing". Replaced the elbow with a trick flow metal one, and things are much better now. This just goes to show you that experience doesn't equal knowledge.
I was also having low oil PSI / check gauges come on at high temp when the engine was at low RPM. This is likely to my oil being diluted with gas. I knew something was up, so I was changing really often, like every 2500 miles or so until I could find what the hell was diluting it. Hopefully that saved me from destroying the engine.
Unfortunately, I'd been driving it like that for who knows how long. Maybe years? The TB was really gunked up with blow-by (I assume from the extra pressure), so I went ahead and replaced that. Also replaced the vapor pressure solenoid since it was old / possible vac leak. Both parts were super cheap from a friend at a junkyard so everybody just relax before yelling at me about throwing money at my problems!
Long Term Fuel trims averages went from 9.71% | 12.44% to 0.3% | 0.6%. They're not perfect, but they're a hell of a lot better than before.
Logs:
Before (aggressive run on highway): https://docs.google.com/spreadsheet/...3c&usp=sharing
After (casual run during commute, with stops): https://docs.google.com/spreadsheet/ccc?key=0AoB7q6j02Ca5dDJ1SWRKOUZ4aFBfZS1JZUpiQzdLS Hc&usp=sharing
03-15-2013, 12:24 PM
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For future reference, a good way to identify a vacuum leak or unmetered air entering the system is to look at your MAF reading ( in grams per sec) @idle and @operating temp with A/C or defrost off. It should read close to 1 gram per sec per liter. So a 5.7L should flow approximately 5.4 - 6.0 grams per sec at idle. If its more than 15% above or below the 1 liter per gram then you have a problem.
Another thing you can check if you have a graphing datalog is MAF efficiency through the RPM range using a VE calculator. (http://www.d-tips.com/Tools/toolBox/ONlineMafCalc.aspx) That's the one I use, you can do a search for Volumetric Efficiency calculator and find others. Readings MUST be taken at WOT so be careful doing it, but if it fails the idle grams/sec test and passes the VE test you have a vacuum leak, if it fails both youhave a bad/dirty MAF or an air leak between the MAF and Throttle body.
Another thing you can check if you have a graphing datalog is MAF efficiency through the RPM range using a VE calculator. (http://www.d-tips.com/Tools/toolBox/ONlineMafCalc.aspx) That's the one I use, you can do a search for Volumetric Efficiency calculator and find others. Readings MUST be taken at WOT so be careful doing it, but if it fails the idle grams/sec test and passes the VE test you have a vacuum leak, if it fails both youhave a bad/dirty MAF or an air leak between the MAF and Throttle body.