(84 Avanti failed Emissions inspection) If it failed on the HC side (high content of unburned fuel) then changing the oil, a new air filter and plugs can help. But if the HC is very high it is a misfire caused by a bad spark plug/ plug wire, a lean condition, vacuum leak or stuck open EGR valve. A lazy/dead catalytic converter or inoperative air injection can cause high HC too. Changing the oil, plugs etc. will do nothing to make it pass if one of these problems exist. High CO (rich condition) can be caused by a bad O2 sensor, high float level in the carb, lazy/ dead catalytic converter, bad coolant sensor. Occasionally on old cars with high miles a saturated carbon canister can cause high CO readings. If the HC and CO were both high but not way off then it may not have been warmed up enough or the catalytic converter is lazy. Paul Katson (paulkatson@inspectacar.com) March 2001 And don't forget my Carburetor saga. It started because my foam carburetor float was getting saturated and causing a high fuel level in the bowl. chuck Paras (pcparas@erols.com) March 2001 ============================================ Even in an old car with no emission devices CO would drop at a 1500 rpm test. If your car does not you have been wasting fuel and rather than look at it as a bad thing try to find the problem and be rewarded with a more economical ride. Popular wisdom says "blow the car out" before the test. This will actually increase readings sometimes. Bringing the high speed circuits in the carb into play, dumping fuel from the secondaries into the manifold. What should be done is to bring the car up to about 1500 rpm and hold it there for a few seconds. That will clean out unburned fuel without bringing any more fuel than necessary into the manifold. Anything that would change the relationship of air to fuel will throw the CO reading off. So any restrictions in air intake will cause the same reading as too much fuel. Plugging the pcv will increase CO not decrease it. And of course if the car isn't fully warmed up the choke isn't open and that restricts air. A high enough CO to cause an increase in Hydrocarbons is a serious CO problem. It means that the A/F ratio is so bad the engine is misfiring. If your car has a computer controlled Quadrajet it's possible that the ECM is telling it to do things it shouldn't so there are a whole bunch of scenarios that a good GM dealer should explore for you. ========================================= A P.S. to the ramblings of a madman. If your car has a vacuum leak and the computer decides there is too much air it will compensate by telling the carb to add fuel. As I've said before I'm not an expert on anything built after 1977 so forgive me. But if your car has an Oxygen sensor then it is monitoring A/F ratio and does have a way to change the amount of fuel going in. That high hydrocarbon reading makes me think there may be a vacuum leak because an A/F ratio that is bad enough to cause a car to mis-fire would manifest itself in horrendous gas mileage and other idle and driveability problems. However it doesn't take a big vacuum leak to throw hydrocarbons off.And with the carb compensating with more fuel you may not even notice the problem in day to day use. (erizzolo@gateway.net) March 2001 Ernie Rizzolo ================================================= Shortly before my 84 was due for it's biannual emissions inspection the darn "check engine" light insisted on staying lit up. Not having time to investigate then I put a bit of electrical tape over the light so the inspectors wouldn't notice it and hoped for the best, as the inspection had to be done before the end of Sept. Well it flunked emissions with flying colors but the strange thing was it flunked on carbon monoxide, being over twice what is allowed here in Mass.. the inspectors commented they had never seen anyone flunk on carbon monoxide before.. as the HC and NOx were well within limits. Of course I "forgot" to tell them it was running in the limp home mode. At least this "bought me" 60 days to make repairs.. My 84 touring coupe has pretty much a 82 or so about's 305 Camaro engine and computer setup, so out came the Camaro books. The computer kept complaining about a coolant sensor (code 15) and throttle position sensor (code 21). The coolant sensor was about 100 ohms high in resistance with the engine hot, so I replace that, but alas the computer is still complaining about it. I started checking the wiring and noticed that the 84 and the Camaro book wiring sensor colors matched. Both the coolant sensor and the throttle position sensor inside the carb show they return to ground with a black wire, Ha, guess what, no ground connection...! A quick jury rigged ground connection to the coolant and TPS ground wires and presto, no "check engine" light. Chasing the black ground wires leads me to the main wiring harness heading back to the computer I guess, so I cheated and spliced in a new ground, problem solved. According to my GM emissions book by Haynes, it tells me that when the check engine light is on, the computer puts the entire 305 in the "limp home" mode, with no emissions control functions what so ever. The high carbon monoxide level was due to the air pump dumping into the air cleaner instead of the cat converter with a hot engine.. And guess where they do the emissions test here, on a dyno running about 30 MPH.. Over the past couple of years the check engine light would randomly come on, changing the computer sometimes cured this, sometimes not. On those occasions I was never able to determine why this was happening, intermittent connections can get to be a bit nerve wracking.. I hope that this can be added to the "for what it's worth" department, as that is what I found to be the trouble on my 305 computer controlled 84 touring coupe. Merle RQB 3884 (The black "ground" wires were intermittent, mostly open to where ever they made the ground connection. I chased them back into the main harness heading toward the firewall in the direction of the computer. As I was running out of daylight and getting a bit chilly, I did a "quick fix" by adding a separate ground to solve the immediate problem. I don't know if anything else depended on this ground, guess time will tell..) =============================