Hi all, I just bought this truck and failed the CO part of the emissions test. The high speed CO barely passed and the idle CO was about 4 times the max allowed.
Before taking it in I replaced the TPS (fixed the CEL on), and the PCV.

Since failing the test I have replaced the spark plugs, plug wires, cap, & rotor. It has a K&N filter that is clean.
When I pull the cable off the IAC the engine dies, when I pull the vacuum off the MAP the engine dies, & removing the cable from the air charge sensor in the manifold causes the RPM to drop. I won't say these are definitive tests but they indicate the sensors do something and are not completely failed.

The truck probably has a bajillion miles on it and the lifters rattle crazy loud but there is no smoke and the engine seems to run good.

Any help on what to check next would be appreciated.

 

Rings due to the bajillion miles would be my guess.

 

Running with too much fuel ups the HC output, but CO means there wasn't enough combustion going on, more fuel by a bit than was able to burn.
Check your fuel pressure & regulator function. Check that the thermostat is closing. Do you get a good high idle when first starting the engine? If not, the computer likely doesn't know the coolant temperature. The CTS -coolant temp sensor- may not be functional.

Failure at high speed would make me think an overall problem exists that is shared by all cylinders, so be sure to check the fuel pressure. I think you have replaced the proper parts so far, but check that there is not excessive fuel diluted into the oil in the crankcase. Pull the dipstick and take a whiff, and if it is odiferous, do an oil change also.
Take it on a 10 mile run out of town and back on a good highway, and then show up at the test facility.
tom

 

Well it turns out it was the HC levels that were too high.
After replacing the tune up parts I went and bought a timing light. (Can you imagine?) The timing was way advanced so I dropped it down to +6 degrees. Took it down for a retest and dropped HC from 697 to 81 at idle. I passed the test and am overjoyed.

Edit: I did put the timing back to +10 when I got back home.

 

Low compression could cause high CO counts, like dvlsmstr has said, do a squeeze test and make sure what you got is worth dumping the money in, if all else fails force a common vacuum leak and that will lower your CO. Just a small one, not enough to cause a miss or a high idle.

 

I have a 88 with the same engine, twice it failed about 10 years apart, both times it was the burned out o2 sensor!

 

here in florida, when they did have emmision testing in hillsboro county the main reason so many people would fail a test is they would not admit how many miles were on their car,,,,if it is 15 years old it in all reality should have 150000 miles but when they bring it to test they tell the testers 50000,,,,,i had a 89 chevette at the time ,the engine was set up like a 80 model,,, the catalythic converter was gutted in such a way that they couldnt tell that it was,,when pull up to test told them 300000 plus miles it nevered failed..... i am glad they stopped testing when they figured out the government wasnt making money and too many people figured out how to cheat the test.......