failed emission test

[kevin]

Hi, I have a '91 S-10 That I have had since new. It's in really good shape. It failed the California emission test. I just moved from out of state and really don't want to spend a ton of money trying to fix it. It passed every category on the 25 mph test, but failed only in one category on the 15 mph test. That category was the NO (PPM). The max allowed is 1037, I got a 2715. Can anyone tell me what part cause it the fail the 15 mph test and not the 25 mph? How can you test each of the following - The CAT, EGR valve, EGR solenoid, and the oxygen sensor. Any expert advice would really help me out alot!! If the truck was yours want would you do? Thanks for your time!! Kevin

[DanD]

Kevin
There are so many variables to cause emissions readings to go high that it can be very difficult to say without having equipment like a gas analyzer, scan tool, temperature gauges and something as simple as a vacuum gauge connected to the vehicle.
Knowing what the different causes for a gas to be produced in higher amounts and a good understanding of how the emissions control devices work independently and together to lower these gases.
NOx is a byproduct of heat and load.
Lean fuel mixtures can cause excessive combustion chamber heat, a transmission not in the proper gear due to throttle valve cable being out of adjustment or the lock-up torque converter locked at the wrong time will cause excessive load.
The only reason I mention these is that neither one is an emissions control device but could be the cause of the high readings.
So again you have to know how the systems are suppose to work and how they react to each other. Yes you may get lucky by throwing a couple of parts at the truck and get it to pass but it may be more cost effective to take it in and have someone diagnose it for you.
Here are a couple of charts to show you what a diagnostic procedure looks like.
Dan.