Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
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Hydrocarbon (HC) emissions and Carbon Monoxide (CO) emissions at the same time means the engine is being given more gasoline than it needs. If HC is high enough it will damage the catalytic converter.
Several causes for CO emissions due to overrich mixture should be checked out:
MAP sensor supply hose broken, rotted or leaking.
Coolant temperature sensor reading cold when actually warm.
Fuel pressure regulator jammed or injector(s) leaking or bad pattern. Note that the truck has two injectors.
Evap canister saturated, in-tank check valve leaking.
An overrich condition does NOT cause a stall, but if your spark plugs are fouled because of the rich mixture, then it can stall. Since your spark plugs are new, the likely cause of stalling on decel may be low rpm due to the rich mixture.
Rapid decel like that often turns out to be deposits built up around the inside of the throttle body. The computer should be able to offsett a stall by opening an idle bypass, but if it is slow to open you can catch it with its pants down.
Emissions tests are harder and harder to pass as a car ages. Sometimes a car will need more work to clean up than it's worth, but the issue is never about how well it runs, rather how much pollution your state is tolerant of.
Some solvents actually attack the seals inside the idle control motor and cause it to stick - don't use too much of the solvent! Tune-up in a can has a spotty record for curing worn parts.
Since the idle speed is computer controlled, the computer should have reacted quickly enough to keep the engine from stalling. If the motor is sluggish (they get that way) the engine will quit before the idle motor opens enough to save it.
A service tool for diagnosing this is an idle passage (IAC) port plug (<-item368 here). Putting it in will force your engine to run as if the valve were completely closed. Your truck should idle smoothly with the plug installed at speeds as low as 500 rpm and if it won't, then major tune work is needed even if the IAC motor is functioning. This tool is used to establish whether the truck will idle properly at minimum - never in gear or under any loads. If it doesn't, it's major tune time.
The idle control motor is a strong possible cause for stalling on deceleration, but it does NOT by itself cause high HC or CO emission at cruise speed. 2500 RPM isn't idle.
AET tests are not diagnosis any more than telling someone they have a flat tire is diagnosis. Find someone you trust to fix the tire.
Driveability testing by qualified personnel is the first thing you should think about. Stall on decel is a symptom that might be a clue to why you are failing the emissions test. The AET guy can't do anything more than tell you to have it looked at by a professional who is willing to do the proper tests and interpret the results, and that's my advice - don't throw parts at it unless you have a serviceman there to point at it. A good serviceman will be able to use all the info above to assist. So far I have suggested possible causes for both and also several tests that any qualified person in the field can perform.