Low RPM sputter, failed emissions test
07-26-2007, 05:14 PM
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Car: '90 Trans Am GTA
Engine: LB9
Transmission: 700R4
Axle/Gears: 2.73's
Low RPM sputter, failed emissions test
A spectacular failure really. At idle it scored a 1302 HC ppm (limit 200) and a 6.46 CO% (limit 1.00). The high idle test came out with 1597 HC ppm (limit 60), 3.09 CO% (limit 0.33), and passed the NO ppm.
To add to the symptoms, it has a rather nasty sputter/shake between 1k and 2k. It idles okay if a little rough, and above 2K it runs great.
Now the backstory. Over the spring I tore the intake down to replace the gaskets. While it was apart I also installed my Holly AFPR (set close to factory pressure, although the **** is close to maxed out...), replaced the valve stem seals and valve springs, and upgraded to 1.6 roller rockers (GMPP).
So, any suggestions? If I had set the lash too tight on the rockers, would that cause these symptoms? I'm thinking about switching back to the stock FPR just for giggles, I know the Holley has mixed opinions at best. Either that or pull a spark plug and see what she's doing...
To add to the symptoms, it has a rather nasty sputter/shake between 1k and 2k. It idles okay if a little rough, and above 2K it runs great.
Now the backstory. Over the spring I tore the intake down to replace the gaskets. While it was apart I also installed my Holly AFPR (set close to factory pressure, although the **** is close to maxed out...), replaced the valve stem seals and valve springs, and upgraded to 1.6 roller rockers (GMPP).
So, any suggestions? If I had set the lash too tight on the rockers, would that cause these symptoms? I'm thinking about switching back to the stock FPR just for giggles, I know the Holley has mixed opinions at best. Either that or pull a spark plug and see what she's doing...
07-27-2007, 06:31 PM
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Car: '90 Trans Am GTA
Engine: LB9
Transmission: 700R4
Axle/Gears: 2.73's
Re: Low RPM sputter, failed emissions test
I rechecked the fuel pressure and I have it set to 48 psi without the vacuum line hooked up, and it drops to around 38 psi at idle with the line hooked up. The diaphram doesn't seem to be leaking (a common complaint with the Holley). Could too-tight rockers cause my problems? I also replaced the oxygen sensor late last year and it hasn't been run very much since. A bad sensor perhaps? *shrug* Grasping at straws...
07-28-2007, 02:36 PM
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Car: '90 Trans Am GTA
Engine: LB9
Transmission: 700R4
Axle/Gears: 2.73's
Re: Low RPM sputter, failed emissions test
She's throwing a code 45 now, rich exhaust. She wasn't doing this last time I checked, before a fairly long run. Anyone?
07-29-2007, 12:17 AM
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Car: 89 TTA
Re: Low RPM sputter, failed emissions test
Yo,my stock 89 GTA is doing this same thing in the same rpm range,i can hear it in the exhuast note,and can smell unburnt fuel,Im gona go pull a few plugs and see what turns up.
07-31-2007, 10:33 AM
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Car: '90 Trans Am GTA
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Axle/Gears: 2.73's
Re: Low RPM sputter, failed emissions test
I just got back from passing my e-test. 
I went back and carefully re-lashed my valves. She's running way smoother now, and she passed with an 129 HC ppm (limit 200) and a .63 CO% (limit 1.00) at idle, and high idle test spat me out 59 HC ppm (limit 60), .22 CO% (limit 0.33). Kinda close on the HC but considering I haven't stuck the new cat on yet I think it did pretty well.
Looks like 1/4 turn beyond zero lash really is the best way to go. I'll remember that.
I went back and carefully re-lashed my valves. She's running way smoother now, and she passed with an 129 HC ppm (limit 200) and a .63 CO% (limit 1.00) at idle, and high idle test spat me out 59 HC ppm (limit 60), .22 CO% (limit 0.33). Kinda close on the HC but considering I haven't stuck the new cat on yet I think it did pretty well.Looks like 1/4 turn beyond zero lash really is the best way to go. I'll remember that.
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