Failed emmissions - The 1947 - Present Chevrolet & GMC Truck Message Board Network

mikeoj · 06-16-2005, 02:09 PM

Was high on HC,read 12.0256 and the limit is 7.0.CO was 52.1595,Limit is 120.0.NOx was 4.9239,Limit is 7.0.Any suggestions.What's the main cause for high HC.This is all on a 70 1/2t 250 6cyl w/electronic ignition, .060 plug gap,timing at 10 atc.If the timing was to far advanced,would that be the cause?Was at 14,It did ping a little when accelerating. Checked after the test and set at 10.Thanks for any advise,Mike

**pjmoreland** · 06-16-2005, 02:30 PM

High HC means unburned gas is being blown out your tailpipe. Could be caused by weak ignition. Timing that is too advanced can cause this. Is .060 the correct plug gap? Seems kind of large to me. Shouldn't it be down around .045 for electronic ignition?

Tim240Z · 06-16-2005, 03:25 PM

Lean it out a bit and get the CO up a little.That will drop the NO2 and the HC.
Back the timing off a couple of degrees and like above, reduce the plug gap. You could also add a 10% mix of denatured alcohol to the fuel tank to pass for sure.......

Jason915 · 06-16-2005, 04:06 PM

I would set your plug gap down a bit, drain the oil put fresh oil in that'll usually reduce HC's 10* initial shouldn't be a problem.

**kwmech** · 06-16-2005, 09:05 PM

Here we go again--- OK CO is fuel delivery (rich or lean) HC is the engines ability to burn the delivered amount of fuel. So --if the CO is high then more than likely the HC will be high. If CO is relatively low say 2% and lower and the HCs are high then most likely an ignition problem. If your CO is 7% then way too much fuel is being put into the engine. You did not state whether these were idle or high speed readings. And by cali. standards those readings do not make much sense.

Russell · 06-16-2005, 09:14 PM

.060 is too big of a gap for an HEI, set it down to .045 and you'll probally be way better off

mikeoj · 06-16-2005, 09:37 PM

OK.Let me ask this.For the people on here that added hei to there 6 cyl:What plugs do you have and at what gap.Also what is the timing set at.This is a stock motor.These were high speed readings!

86swb · 06-16-2005, 09:43 PM

Man, I am sure glad we do not have those test here in Kentucky. I know they do in Tennessee. If it starts and drives then you are good to go. But we do have to wear our seat belts.

06-16-2005, 02:09 PM	#1
mikeoj Senior Member Join Date: Nov 2000 Location: WI,USA Posts: 495	Failed emmissions Was high on HC,read 12.0256 and the limit is 7.0.CO was 52.1595,Limit is 120.0.NOx was 4.9239,Limit is 7.0.Any suggestions.What's the main cause for high HC.This is all on a 70 1/2t 250 6cyl w/electronic ignition, .060 plug gap,timing at 10 atc.If the timing was to far advanced,would that be the cause?Was at 14,It did ping a little when accelerating. Checked after the test and set at 10.Thanks for any advise,Mike __________________ 69 Lwb C-10 402/700R4 70 Lwb C-10 250/3pd Wisc.

06-16-2005, 03:25 PM	#3
Tim240Z Senior Member Join Date: Jan 2005 Location: LAX area, So Cal Posts: 499	Lean it out a bit and get the CO up a little.That will drop the NO2 and the HC. Back the timing off a couple of degrees and like above, reduce the plug gap. You could also add a 10% mix of denatured alcohol to the fuel tank to pass for sure....... __________________ 1972 Suburban C20 454/TH700R4, eaton-corporate 4.10 (looking for 3.7) Sold: 71 Cheyenne 20 Installed a 99 5.7L L31/4L60E from Suburban 1500 (2005)

06-16-2005, 04:06 PM	#4
Jason915 Registered User Join Date: Jan 2005 Location: Colorado Springs Posts: 693	I would set your plug gap down a bit, drain the oil put fresh oil in that'll usually reduce HC's 10* initial shouldn't be a problem. __________________ 1971 GMC K1500 SWB Fleetside, 4" skyjacker softride and, 14 Bolt FF, HD Dana 44 4.10's 1970 K5 TBI conversion lifted etc SOLD 1968 C20 camper truck SOLD “To ban guns because criminals use them is to tell the innocent and law-abiding that their rights and liberties depend not on their own conduct, but on the conduct of the guilty and the lawless.” Jeff Snyder

06-16-2005, 09:05 PM	#5
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,630	Here we go again--- OK CO is fuel delivery (rich or lean) HC is the engines ability to burn the delivered amount of fuel. So --if the CO is high then more than likely the HC will be high. If CO is relatively low say 2% and lower and the HCs are high then most likely an ignition problem. If your CO is 7% then way too much fuel is being put into the engine. You did not state whether these were idle or high speed readings. And by cali. standards those readings do not make much sense. Last edited by kwmech; 06-16-2005 at 09:06 PM.

06-16-2005, 09:14 PM	#6
Russell Professional Grade Join Date: May 2003 Location: Fort McMurray, Alberta Posts: 7,915	.060 is too big of a gap for an HEI, set it down to .045 and you'll probally be way better off __________________ 1995 Chevrolet 2 Door Tahoe (6.6L LBZ Duramax / ZF6 / NP241 with 1 ton solid axle swap)

06-16-2005, 02:30 PM	#2
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 5,982	High HC means unburned gas is being blown out your tailpipe. Could be caused by weak ignition. Timing that is too advanced can cause this. Is .060 the correct plug gap? Seems kind of large to me. Shouldn't it be down around .045 for electronic ignition?

06-16-2005, 09:37 PM	#7
mikeoj Senior Member Join Date: Nov 2000 Location: WI,USA Posts: 495	OK.Let me ask this.For the people on here that added hei to there 6 cyl:What plugs do you have and at what gap.Also what is the timing set at.This is a stock motor.These were high speed readings! __________________ 69 Lwb C-10 402/700R4 70 Lwb C-10 250/3pd Wisc. Last edited by mikeoj; 06-16-2005 at 09:46 PM.

06-16-2005, 09:43 PM	#8
86swb Born a Chevy Man Join Date: Apr 2005 Location: Navarre, Florida Posts: 10,221	Man, I am sure glad we do not have those test here in Kentucky. I know they do in Tennessee. If it starts and drives then you are good to go. But we do have to wear our seat belts. __________________ John or 86 (Viet Nam combat Vet-BIG RED ONE) '65-'66 1986 Chevy SWB Silverado 1984 Chevy Longbed Silverado New Kia Sportage (Wife's car and she loves it) CHEVY, American made w/pride! If you can read this, YOU NEED TO THANK A TEACHER! If you can read this in ENGLISH, PLEASE THANK A SOLDIER! GOD BLESS AMERICA AND BLESS OUR FIGHTING TROOPS



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.