I have a 92 Z28 with TPI. After reading all the posts about emission failures and not getting anywhere I took my car to a emissions repair facility. Turns out my diverter (not sure on spelling) valve was not working. It was stuck. He was able to get it unstuck and he drove my car over to get it inspected nd it passed with no problems. I was failing on hydrocarbons. The standard is 0.80 and I was 2.70. After unsticking the valve I was 0.30. Hope this helps others having the same problem.

 

How do they test in your locality? Idle only, dyno run, etc.? Any visual?

 

They run your car on the dyno here, its called the IM93 test. They vary the speed up to 30mph or 35mph I think. The shop I took it to to find out my it failed said my AIR MANAGEMENT VALVE/DIVERTER VALVE was stuck. I guess it helps send oxygen to your cat to help with emissions.

 

The diverter valve sends air to the manifolds when in open loop operation and under certain conditions in closed loop. Otherwise, in closed loop, it goes to the cat.

If air is going to the manifolds when the ECM thinks it's going to the cat, the O2 sensor is going to get extra oxygen, and the ECM is therefore going to enrichen the mixture - but it doesn't really need it for the combustion process, so it will actually be too "rich".

 

How did he determine the diverter valve was "stuck", and how did he then "un-stick" it?

 

Not sure how he discovered that it was not working. He said he unplugged the connection to it and then plugged it back in. He was testing the emissions coming from the tailpipe, and he said when the valve started working he saw my hydrocarbons go down. He also said he has a hard time finding new valves, and he usually does some kind of bypass to get the car through emission testing.