: Failed Emission - high HC

01-18-04, 02:14 AM
My 94 STS failed emissions test as "Gross Polluter" with the HC at idle (685 rpm) at 1003 ppm. (At 2500 rpm, no problem at 2ppm) - all other measurements are well within spec.
I had been advised as a "general" call that the FPR is a common failure that would give this type of symptom with high HC reading; however, although it didn't appear to be leaking fuel out of the vacuum nipple, I replaced it anyway but I believe I still have the issue.
When I connect a GM Tech II scan tool, I find while monitoring the O2 sensors that the front bank is oscillating almost in a complete sine wave, saturating from hi to lo extremes & at much lower frequency compared to the rear bank O2 (it has almost 3-4 times the frequency of excursions and rarely goes to the extremes)
It looks like the rear bank is controlling with the trims, whereas the front is trying to, but can't correct adequately & is hunting between extremes.
I changed both sensors thinking the front one was bad & since car has 150K miles, decided just to do them both.
I still have the same signatures from the O2 analyzers.
One other point of note, is that the idle seems to hunt somewhat at idle, not really maintaining a constant idle speed.

I am wondering about an injector problem? If ignition, each coil is common to one cylinder in both front & rear bank, so I would have expected a cylinder misfire due to ignition to perhaps show up in both banks O2's? An injector obviously would be common to just one? A cylinder isolation test, disabling each injector, doesn't really show up too much difference cylinder to cylinder however.

What to check next???? (I have scan tool available through Sunday, so other things to check would be appreciated along with any suggestions to resolve)

Thanks in anticipation of assistance.

01-18-04, 08:17 PM
U know i dont know exactly what it is....but if you are barely failing a can of bg44k has been known to lower this.....we should really get them as a sponsor

01-19-04, 01:56 AM
U know i dont know exactly what it is....but if you are barely failing a can of bg44k has been known to lower this.....we should really get them as a sponsorWell, "Gross Polluter" is hardly barely failing unfortunately - I'd actually really like to fix this.

06-09-04, 12:23 AM
I know this sounds counter-intuitive, but it may be partly the fault of the Cat. I have a 97 STS that would not pass with 60K miles. Idle HC's 288, 2500 RPM HC's 79. I swapped out the plugs to bosch, still wouldn't pass. dealer said changing back to new AC's would make it pass, same result. Kept thinking it was an incomplete burn in the chamber. The dealer finally determined that the "O2 sensor, bank 1 sensor 3" was indicating that the cat wasfailing to oxidize the exhaust. The Cat was replaced (free) and the went to Idle HC's 1, 2500 RPM HC's 5. DRAMATIC drop. I thought they were FOS, and faked some results to make me go away for a year, but it has kept passing since then. Have the plugs and wires been replaced? It may go without saying, but if you replaced the plugs with AC's from NAPA, be advised, they aren't properly gapped from the factory, and NAP may give you the wrong gap spec. granted your HC's are significantly higher than mine were at idle, so this may not have helped. I went so far as trying to open up the manual portion of the idle setting to 800-1000 to squeek by