So I'm in lovely CO, where the emissions laws are almost as restrictive as CA, and I failed emissions for high HC this week. Here are the results (actual/lmit). Note the stinkin' 4-minute dyno test for older vehicles (http://aircarecolorado.com/).

First test:
HC 1.6182 / 1.2
CO 2.5781 / 15
CO2 433.8257
NOx 0.6145 / 1.5

I tried to get through on the cheap: dumped a bottle of Techron cleaner into 1/2 tank and drove it to 1/4 tank. Then I made sure to heat the cats up better (15 min @ 70-80MPH). Oddly, the results were even worse:

Second test:
HC 2.137 / 1.2
CO 3.2463 / 15
CO2 438.3613
NOx 0.6995 / 1.5

This is an 04Z, bought 2 years ago. I can see a Halltech intake and Dynatech headers, but the exhaust (and everything else including cats) looks stock. The PCM doesn't throw any diagnostic codes and the car runs fine, as far as I can tell (smooth idle, no misfires/choppiness, gas smell, etc).

I'm thinking one of these:
- Injectors gummed up (car only has 36K on it, so it must've sat around a lot with gas in it)
- O2 sensors
- Cats
- Vacuum leak
- Car was tuned (the timing advance in the 35+ range seems high, but I'm not sure what it should be)

If it was O2 or cats, wouldn't I probably get a CEL/code and/or rough running? I almost wish there was a code so I could diagnose the problem.

Any suggestions? Thanks!

 

try using seafoam or water and do a top end cleaning and see if that clears things up... how is the car normally driven?... if you don't open it up very much there's a good chance you may have excessive carbon built up in the combustion chambers and a good cleaning may clear that up and help you pass

 

Do you know if there were any internal modifications or an aftermarket tune? Sounds like you bought the car used and aren't sure of what modifications were done to it. Has it ever passed in prior years while you have owned it?

Before throwing a lot of money at this, I would first try to figure out what you have in the car if you don't already know.

 

When I lived in CA, there were times nothing short of new cats would get me through the legalized theft.

 

I open it up little, but probably not as much as the average Z06 driver.

I have no idea what was done except the intake and headers. No documents whatsoever. With as little as it was driven, I'm assuming not much was done, but who knows.

It passed once... I smogged it immediately the day I bought it from the dealer 2 years ago so I could get it registered. But who knows what he did to get it in shape well enough to pass just that once.The numbers then were:
HC: 0.441
CO 5.7583
CO2 599.8386
NOx 1.153

So CO and NOx were roughly double what they are now but HC was way lower. That seems strange, now that I look at it.

Does that give anyone a hint? Is there some temporary trickery that could be done to lower HC even if it raises CO and NOx?

 

High octane gas (even using supplements like isopropanol in some cases), and driving the **** out of it for a while (get it to redline, have some fun, get the engine hot and the exhaust hot.)

Also, your state may ban buying used cats, but corvette exhausts are very commonly sold used, because people love to replace them. That includes the pipe with the cats. Consider buying one used for like $200. Beats new for $2000...

 

Unless they have been tuned out, the rear O2's should alert on a bad cat. If they are tuned out then there is no guarantee that someone didn't guy the cats.

 

I live I CA and I'm well versed when it comes to smog. I have an 04Z.

Seafoam through the brake booster line should do the trick.

New plugs and wires with Seafoam...even better.

35 degrees is waaaaaaaay too much if it's in the WOT areas. I'd have to see the tune though...

The reason it probably got worse? It's possible your runs to make it hot pulled timing and those values are learned and is using the low octane table.

Do Seafoam, put in good gas, drive it for 100 miles, then go back to the smog station. Report back. Good luck.

 

Does your A/C work correctly? I'm wondering if the vacuum feed to the AIR pump is intact?

 

there is an easy trick to get rid of hc: plug the pcv line to the manifold and filter (or whereever else they might go).

 

As has been suggested perform a top end clean. Feed Techron or SeaFoam (slowly at first) in through the booster hose while running @ 1500-2000. You may need a helper to keep it running. When the bottle is nearly empty let it drink full speed & let the engine stall (or shot off). LET IT SIT 1 HOUR. After it cools down start it up & rev @ 1500-2500 for a bit (it will smoke). Take it for a rip (it may smoke) to finish clearing it out.

CHANGE THE OIL & CLEAR ANY CODES

Before the re-inspection hold the RPMS @ 2000 for 2 min to fully warm the cats.

 

If you feed seafoam to the engine through the brake booster hose, you only get cleaner in cylinders 7 & 8.

Go to the other end of the manifold. Remove the PCV hose vacuum line on the passengers side of the manifold and feed seafoam into the manifold from there. That will get cleaner to ALL the cylinders.

I have an ACTRON Scanner and it can monitor LIVE SENSOR DATA. If you monitor the O2 sensor output, the output voltage should be swinging rapidly from 200 mv. to 850 mv. The more it hangs around 450 mv, the worse the condition of the O2 sensor is. Monitor your O2 sensor outputs to make sure that they are switching properly.

Clean your MAF and TB throttle blade and yes, temporarily restrict the PCV from entering the intake manifold to prevent oil mist from entering the intake.

 

I've recently seen Seafoam in an aerosol can and I think they have straws that flatten and you can sneak it in through the rubber intake connectors as long as your past the MAF. Might even help for it to be atomized?

 

Thanks, all! Nice to know there are some things I can try on the cheap and easy before plunking down real money. Will report back.

 

Ok I am here in smog hell in CALIF------After many years of helping guys pass smog these 2 things are what I would recc for a car that fails only for the HC
1st of all a car that fails for Knox is generally due to a failing cat
A car that fails due to HC's is a fueling issue OR a cat that is NOT lighting off
IF you have LT headers on your car or aftermarket cats this would cause the cats not to light off
1. Get your tank 1/2 full of gas----(say 8-10 gallons) then find a station that sells E85 Ethanol------Put in about 3-5 gallons of E85
Ethanol has "0" hydro carbons----It will immediately show lower H/C's Now don't drive it WOT or hard as it will be very lean---But it will run--often poorly but good enough to drive to the smog shop
2nd-----IF you have access to a tuner with HP or EFILIVE--have them remove timing in your tune--In both the idle timing tables and the normal timing table (hi octane) We often will remove 5-10* but only in the idle and P/T fueling areas
This timing reduction will only help IF you have a car with LT's aftermarket cats or a cammed car with lots of overlap or a very small LSA--- (109-112) The lower timing will burn up all the unburnt fuel in your cats and allow them to "light off" and work---
Most often simply the E85 will work----I have done this 100's of times without fail---
SHHHH do say a word to the smog shop----tell them you had a mechanic work on it !!! LOL