92 flatbed

About a month ago, I went to emissions for bi-annual test, and Failed. Unburned hydrocarbons were 190PPM, allowed 160PPM.The Monoxide and Nitrogen sulfide were both extremely low (CO read .62%, allowed 4.47%, NOx read 1081PPM, allowed 5793PPM.)
I changed plugs ( Denzo TT's) And put in a new O2 sensor (also Denzo) Wires, cap and rotor had been changed last summer, so I figure they are good. Truck ran noticeably better.
I went back for the retest, it failed , with about the same numbers.
I then retarded the timing, to 0* BTC, cleaned the K&N filter and changed oil/filter.
Retested again, about same numbers...... (187 PPM hydrocarbons)

Any Ideas?

Terrys87

iTrader: (1)

Sounds like you need to talk to ChefYota4x4.. He just passed his and has been thru quite a bit with his. He has a thread on here about it. Alot of good info on it and what all he went thru. He is very helpful.

92 flatbed

I read through that thread, he actually had a different emissions issue.... He was High on NOx.... Not Hydrocarbons.

scope103

I've never owned a vehicle in CT, but I think need to look at your numbers again. CO max should be around .5-1%, and NOx max around 500-1000ppm (NOx is oxides of nitrogen; no sulfides are tested). http://www.autoshop101.com/

But your problem is HC, and your numbers sound correct. The most common cause is ignition misfire (every time a plug misses, all that unburned gas goes out the tailpipe.) The link above (follow to OBD-1, Emission Analysis) gives a whole stack of other possible causes.

Since you're relatively close, I'd also check the operation of the O2 sensor. You don't even need to get under the truck. http://www.ncttora.com/fsm/1990-1995...e/heatedox.pdf (You'll need either a digital voltmeter with an "analog bar" to see the 6-times-in-10-seconds change, or an old-fashioned needle meter. Alas, most cheap needle meters don't have a low enough scale.)

92 flatbed

When putting Timing back to normal, I re-checked for vacuum leaks, and found one at the Cruise control vacuum motor. after fixing that, with timing at normal setting, It passed at 160 PPM hydrocarbons.