Lrod

1996 LT-1 Automatic NO (PPM) meas. 787 - MAX 484, AVE 67
Passed last year no problems? Only changes are removed mufflers and had the battery disconnected for about a week. I'm trying to run it on the freeway to have it re-learn, but it's kind of difficult. I checked codes and I got a "C-12" do I need to reinstall the mufflers?

leesvet

Maybe so...

High NO is from excess combustion heat where the toxic gas is a byproduct of a system thats burns too hot...we want hot to burn away excess hydrocarbons but not TOO hot to create this toxic side effect.
Check your EGR, test it and clean it. If its NOT working right that will cause NO to rise. If your HC was inline, or low, thats your problem. The hotter combustion the lower the HC..the Lower the HC the higher the NO...

Smog is a balancing act between HC and NO and a little CO thrown in just so its not easy...

Sidney004

What was your HC?

Lrod

@ 15 mph, HC Meas @ 4
@ 25 mph HC Meas @ 2

PLRX

No EGRs on 96 Vettes.

LRod, drive the Vette for about 100 miles and take it back. My 96 is not stock and passed if yours is working properly, it should pass.

lkl2005

Wow, I don't envy you fine folks out west. No emissions test here in WI. As a matter of fact the collector plate was a one time fee . No stickers or future payment. Dangit how am I going to resist driving in the snow though?

Lrod

Hey! Good to hear from you. I caught your post after I had removed the muffler eliminators and started installing the mufflers. If all goes well tomorrow I'll run it up to Stoddard Wells and back to try and get five driving cycles in before I take it back to the smog shop. I replaced the EGR valve about 2 years ago, it's between the intake and firewall, actually I broke the oil press. sending unit when I replaced it. I read a post that stated the EGR was necessary to pass emission?

RUU

LT4s don't have them but I think the LT1s do. 1996 FSM has a section on removal & installation of it.

cv67

Lt1s have egr
gotta be working to pass.

Sux about Ca they lowered the allowable emissions which makes it even harder. They want us all driving prius one more reason to leave.

kimmer

I still have some emission charts from my smog days in the 90's and the max amounts for the same car has not changed.

John A. Marker

I am going thru some issues with my 95 trying to get it to pass the NOx part. Basically what I have found out is that the lack of sufficient back pressure is causing the EGR valve to bleed off the vacuum early so it is not opening and doing what it is supposed to do. You can lift the diaphragm manually and the NOx drops to zero. Working on a couple of different ideas to get the valve to work properly.

PLRX

Check, didn't know the OBD II LT1's had an EGR.

LRod, yes the EGR needs to stay put for the SMOG check.

LRod, these are the numbers used for my zip code. Are they the same for NoCal?



I wish I live in Imperial County, no SMOG test there.

desertmike1

I would think that the "Cats" create the needed Delta P and anything Aft is just reducing noise. There is a procedure called out in the FSM for checking the exhaust back pressure but it is quite involved...but something to consider possibly.

With this said; I would first Manually lift the EGR and make sure that the engine stumbles badly, or just simply dies. If this does not happen the EGR porting could be carboned up. Also you can install a vacuum gauge on to the EGR Vacuum line and while softly reving the engine observe for a Vacuum signal. FSM has a very good procedure on how to perform the needed checks.

leesvet

This is why you relocate the EGR solenoid so you can access the hose end anytime. You apply a vac of ANY kind on the egr and it should move enough to change the engine tone..or kill it at idle. If the egr is indeed functional and causes the engine to change that proves the egr is not at fault and the problem is elsewhere. I had issues when I ran muff-eliminators and changed the min exhaust back-pressure...its near nothing to start with...less than 3 psi. Look up the test for plugged cats. 2 or 3 psi back pressure indicates a plugged cat. So normal free flowing ain;t much. But it is sensitive to the engine vac and egr operation. Thats fairly delicate from what I know. These are Negative back-pressure EGR valves in L98s and LTs as far as I know. They are subject to bleed down thus shutting the passage early IF the vac is not constant and ex pressure stable. The Neg back pressure egrs are an odd animal....

The different zip codes/counties have different levels. Orange county will be different than LA country. San Berdu will be even tougher...(dessert dwellers pay the price for the hot climate with smogs) as do coastal communities that are backed up to mountains where the ocean breeze traps the smog against the mountain. The LA basin is like a soup bowl for smog...tough standards.

mtwoolford

The 1996 LT4's (manual transmission cars) with their longer duration cams with more overlap did not need and had no EGR ; all other 96's have EGR

Lrod

the MAX's are the same, although your RPM'S @ the %CO2 are double what mine are? My NO(PPM'S) are @ 15mph 787 and @ 25mph 515