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High HC(PPM) reading at idle during emissions testing

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Old 05-23-2008, 06:34 PM   #1
93lt1fun
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Default High HC(PPM) reading at idle during emissions testing

I just failed my emissions test in Texas with a high HC(PPM) reading at idle compared to the HC(PPM) reading at speed. The standard is 220. Below are my results:
HC(PPM) Idle (847 RPM) = 454
HC(PPM) Speed (2545 RPM) = 83
Any ideas on why the different and high HC(PPM) at idle?



All other emissions reading passed easily compared to the standard.
Co% Idle = .59
Co% Speed = .06
C02% Idle = 14.5
C02% Speed = 14.8
02% Idle = 0.1
02% Speed = 0.0
NOx(ppm) Speed = 0.0
NOx(ppm) Speed = 0.0
Dilution(%) Idle = 15.1
Dilution(%) Speed = 14.9
I know that it smells rich at idle and probably need to get the chip reprogrammed but up to this time there has been no issue. I plan on getting Ed Wright at Fastchips to update my PROM which he did originally about 10 years ago. But I'd like to get a dyno run and will get some scan data before he reprograms.

Without going into all the details I have 396 stroker motor which all emission equipment / sensors installed. I run Random Technology CATS and heated O2 sensors with headers. As you can see from my signature when it's tuned and running well it performs well!

Some additional info is that the Vette sat for about a year garaged without driving it. I started it a few times and drove it once when we moved. It fired right up recently when started it to clean it up for the road again. However, as I mentioned it seemed to burn rich at idle. It does have the year old gas still in it which I need to drain but don't know if this would cause the rich burning at idle.

Any help/suggestions are appreciated!

'93 Corvette LT1 Coupe
Polo II Grn/Tan, Auto,
NA 396 (No Power Adder!!)
7.31@92+ 1/8
11.63@118.98 1/4
1.54 60ft

Last edited by 93lt1fun; 05-23-2008 at 06:38 PM. Reason: Add signature
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Old 05-24-2008, 12:09 AM   #2
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HC are unburned fuel. Old gas doesn't burn well. So it may be just old gas. I would get rid of the the old fuel and run through a couple of tanks of fresh gas before retesting it.
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Old 05-24-2008, 01:02 AM   #3
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Thanks for the confirmation fo the old gas theory. That will be the first and simple step in attempting to resolve the problem.
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Old 05-24-2008, 11:54 AM   #4
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Another thing you might try is to run it and replace with new gas...but only fill to 6-7 gallons. Add 1 gallon of denatured alcohol to the tank. Will not hurt the car. You can find this at a hardware store/paint store ($14-15/gal). This is a VERY clean burning fuel used on boats, should help. Then after going thru smog, fill the tank.
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Old 05-29-2008, 02:42 AM   #5
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Amazing what new gas and plugs will do!

I drained the old gas and decided to replace the Bosch +4 with NGK TR6 (possible NOS system in future).

What a difference! No more WOT miss! Smooth, fast acceleration! Just like it used to run before it was parked for a year!

I also replaced the gas gap which didn't meet the pressure test. Now I'm ready to go pass emissions! I got a tip to drive it for 20-30 min to get the CATS heated up which should help get the high-lift CAM to pass the HC standard.

I'm going to also get it dynoed on Friday and I've ordered an ALDL cable to go with the DataMaster software to get some scan numbers. I'll then send this to Ed Wright at Fastchips to get my PROM updated. And finally go to the track and see if I can still run high-11's in the heat or possible mid-11's.
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