Questionfailed emissions test-NOx was 2023 standard is 937 HC was 150 standard 124 replaced plugs-wires-rotor-distrib cap transmition seems to downshift hard into low gear when slowing to come to a stop and sometimes stalls when you shift into gear when engine is first started ( cold) checked for engine codes NONE
AnswerHi Joe,
Thanks for the details of your problem. I assume that when you checked for codes that you did get a 55 which says "end of code readout" and thus verifies that you actually accessed the memory for the codes. If not, then try it again, making sure that you do the key switching in less than 5 seconds.
The causes of high NOX are mainly due to high temperatures in the combustion chambers. Is the engine running hot based on the temp gauge on the dash? If so, then checking out the thrermostat for opening at too high a temp would be in order. Also too heavy carbon deposits in the combustion chambers can cause high temps, so consider a fuel additive that claims to help remove those deposits. The timing may be too far advanced: it should be 12 BTDC with the engine idling, warmed up and the wire to the temp sensor next to the thermostat housing disconnected temporarily. The Catalytic Converter may be compromised is a possibility as regards its chemical "bed" being worn out.
The final possibility is that there is a leak around the exhaust manifold gasket or from the connections between the exhaust manifold and the cat. Also checking the intake manifold vacuum lines for a leak would be good to eliminate if you find one as that would raise the HC.
If your vehicle was originally sold in California it will have an egr valve and that system is designed to reduce the temperature of the combustion chambers and thus reduce NOX. So if you have the egr you would want to check whether the stem is free to move in and out and if not then lubricating the stem where is enters the valve body. Otherwise you won't have such a system.
Those are the causes of high NOX and HC.
Good luck on curing the problem.
Roland
