Need help passing emissions
Thread Starter
Registered User
Joined: Dec 2001
Posts: 88
From: Miss.Ont.Canada
Need help passing emissions
Had the car tested twice and it failed both times.It passes easily on co`s
and hc`s but was over on nox.The car is a 1995 a4 z28 with a cai and bbk headers and a high flow cat and a catback.The car is virtually unchanged from two years when it passed(close on nox but it did pass)I was talking to one
mechanic who said putting the stock 192 thermostat back would help and also
having the test done in drive instead of overdrive would help.The rpms were
only 1491 during the test.Are our engines more efficient at higher rpms.
HC ppm limit 55 reading 19
co% limit .31 reading .01
NO ppm limit 408 reading 753
and hc`s but was over on nox.The car is a 1995 a4 z28 with a cai and bbk headers and a high flow cat and a catback.The car is virtually unchanged from two years when it passed(close on nox but it did pass)I was talking to one
mechanic who said putting the stock 192 thermostat back would help and also
having the test done in drive instead of overdrive would help.The rpms were
only 1491 during the test.Are our engines more efficient at higher rpms.
HC ppm limit 55 reading 19
co% limit .31 reading .01
NO ppm limit 408 reading 753
Registered User
Joined: Nov 2003
Posts: 1,770
From: Mishawaka, IN
Either the EGR system is not working/plugged or that high flow cat isn't cutting it. Based on your CO/HC readings I would tend to think the cat is ok.
You can test the egr by pulling a vacuum on the egr valve at idle. The engine should run very rough (may stall). Usually egr systems will plug up w/ carbon and the egr valve may be sticking as well, which requires valve replacement and cleaning out the egr passages. If there's no change in idle quality then it's definitely not working at all.
Generally speaking an engine produces the most nox when it's hot and under load. So I doubt the thermostat will do much, but putting it in drive may ease things up a bit if it's allowed. Running the test as soon as the engine goes into closed loop (just warmed up) will keep nox lower as well, but will obviously push hc and co up.
You can test the egr by pulling a vacuum on the egr valve at idle. The engine should run very rough (may stall). Usually egr systems will plug up w/ carbon and the egr valve may be sticking as well, which requires valve replacement and cleaning out the egr passages. If there's no change in idle quality then it's definitely not working at all.
Generally speaking an engine produces the most nox when it's hot and under load. So I doubt the thermostat will do much, but putting it in drive may ease things up a bit if it's allowed. Running the test as soon as the engine goes into closed loop (just warmed up) will keep nox lower as well, but will obviously push hc and co up.
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Joined: Nov 1998
Posts: 71,110
From: Hell was full so they sent me to NJ
Had the car tested twice and it failed both times.It passes easily on co`s
and hc`s but was over on nox.The car is a 1995 a4 z28 with a cai and bbk headers and a high flow cat and a catback.The car is virtually unchanged from two years when it passed(close on nox but it did pass)I was talking to one
mechanic who said putting the stock 192 thermostat back would help and also
having the test done in drive instead of overdrive would help.The rpms were
only 1491 during the test.Are our engines more efficient at higher rpms.
HC ppm limit 55 reading 19
co% limit .31 reading .01
NO ppm limit 408 reading 753
and hc`s but was over on nox.The car is a 1995 a4 z28 with a cai and bbk headers and a high flow cat and a catback.The car is virtually unchanged from two years when it passed(close on nox but it did pass)I was talking to one
mechanic who said putting the stock 192 thermostat back would help and also
having the test done in drive instead of overdrive would help.The rpms were
only 1491 during the test.Are our engines more efficient at higher rpms.
HC ppm limit 55 reading 19
co% limit .31 reading .01
NO ppm limit 408 reading 753
Stock t'stat in an LT1 is 180*F. It is a different thermostat than the Gen 1 SBC uses. If your mechanic thinks 192* is stock, he probably doesn't realize that the LT1 uses a unique thermostat either.
And the mechanic's logic is backwards.... raising the coolant temp lowers HC and CO, and raises NOx. NOx is produced by high combustion chamber temperatures, and lean A/F ratio.
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