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Failed NJ Emissions, High HC and CO at Idle


1993 Chevrolet Caprice

5.0 V8 (L03) TBI

76.4k miles, driven 944 miles since 1/1/10

New AC Delco plugs installed at 74141 miles, 9/17/09.

Original O2 sensor and catalytic converter, no check engine light (OBD-I)

Took the Caprice to the NJ Inspection station.

HC (Standard - 220 ppm)

Idle reading: 3735

High Idle reading: 79

CO (Standard - 1.20%)

Idle reading: 4.36%

High Idle reading: 0.36%

Any thoughts why there is such a large difference between the idle and high readings. In 2008, the HC was 25 and CO was .15%. The 2008 Emissions did list separate values for Idle and High Idle.

Normally I take the car in the middle of the month when lines are short, this time I had a 20 minute wait and was thinking that the Cat cooled down and was marginal at idle. Any suggestions would be appreciated.


Ed B.

Did you take it for a good run on the interstate just prior to the test? This is very low miles car and if it is primarily used around town that could be part of the problem.

If it were mine I go on a good 120 mile round trip on a highway at 70 mph. Along the way get on the gas, full power Scotti! See if you can blast some crud out of the engine and go back for a retest.

A small thing like a leaky, rotted, or disconnect vacuum line could knock off all your emission test results.

No short trips, it usually gets driven 40-50 miles when it’s taken out. I took an 80 mile round trip last weekend (50-55 mph). It was driven about 50 miles yesterday, but the interstate is a good suggestion. I’ll check under the hood for a vacuum leak. I bought a 2010 Cobalt in February, so the Caprice has been driven very little.

On thing I forgot to add. The Caprice has a homemade CAI consisting of a Caddy TBI hat, some PVC pipe, and a K&N cone filter. It passed in 2006 and 2008 with this setup, but I can put it back to the stock intake if that will help.

Ed B.

The homemade stuff could be the problem, but it passed before. Just make sure all the connections are still tight. How clean is the K&N filter? A dirty air filter won’t help.

Perhaps the original set up with a known good (as in new) air filter would help. If you aren’t driving the car much you aren’t getting the claimed benefits of the K&N filter anyway.

I’m not a fan of the K&N filters. I don’t really believe they do much of anything even if they are installed properly and cleaned regularly. If the K&N filter isn’t cleaned periodically and reoiled properly they can restrict airflow instead of increase it.

Idle is when a car runs least efficiently, with a little load (high idle) it is much more efficient. That’s the difference there. Vacuum leak is a good idea

I agree with Uncle Turbo about K&N filters, and would add that if they’re overoiled they can ruin the mass airflow sensor.

However, I’m going to suggest that you check out your Exhaust Gas Reirculation (EGR) system also. The EGR system is designed to allow a bit of exhaust gas to be drawn back into the engine along with the air/fuel mix when the engine is under load to prevent the cylinders from becoming too hot and producing pinging and NOx. Because exhaust gasses pass through the EGR system, the valve can become crudded up and stay open inappropriately at low idle, allowing exhaust gasses to be drawn in when they need not be and affecting the burning in the cylinder. That can cause poor emissions at low idle by displacing some of the free oxygen, causing high CO and HC (which come from too little oxygen), the problem disappearing as volume increases (high idle).

I hope I’ve managed to simplify the explanation sufficient to understand. Bottom line, check out the EGR system. The valve can often be cleaned if it’s sooted up.

Thanks for the suggestions. The weekend is coming up so I should have time to tinker with it. I plan on doing the following and I have a GM service manual.

  1. Reinstall the OEM air box and ductwork with a new filter. Replace the PCV valve.
  2. Check all the vacuum lines and connections.
  3. Check and clean the EGR valve if necessary.
  4. Add some Chevron Techron or Seafoam and put some miles on it.

Thanks for all the suggestions.

Ed B.


Reinstalled the OEM air box and duct work. Noticed oil level was a little low (1/2 instead of at the Full mark), pulled PCV valve, it was stuck closed from varnish and gunk. The PCV and sparkplugs were changed in Sept. 2009. No obvious vacuum leaks.

Installed new PCV and added 20 oz of Chevron Techron to the gas tank. Plan on driving until most of the tank is gone, refilling with fresh gas and taking another shot at emissions inspection.

If that fails, the next step is cleaning the EGR valve and replacing the O2 sensor.

I am wondering if you have a leak in the fuel pressure regulator diaphragm. Since the regulator is part of the fuel cover body assembly there is no way to determine if fuel is leaking visually. One way to check is to put a pressure gauge on the fuel line; cycle the key to get the pressure up; shut off the key; and see if the pressure holds. Alternately, you could scan the ECM for the fuel trim numbers at idle and see if the ECM is commanding lean mixture.

Have you monitored the activity of the O2 sensor at curb idle and high idle?

Just a quick update. The Caprice passed emissions on the second attempt. There’s no paperwork for a passed test, so I don’t know what the new numbers are for HC and CO.

I did the following since the original post.

  1. Added a 20 oz bottle of Chevron to the gas tank. Put 1/2 bottle of Seafoam into the crankcase and poured the remainder into a vacuum line with the engine running. Changed the oil and filter after 200 miles. The oil was pretty black, so I’m assuming the Seafoam in the crankcase dissolved any deposits in the engine.

  2. Replaced the PCV valve and the O2 sennsor. An interesting note, the factory service manual states the engine will go into open loop mode if it’s left idling for an extended period of time.

  3. Reinstalled the factory airbox and ductwork.

  4. Filled up with Top Tier gas (Sunoco Ultra) and drove about 40 miles before going to the Inspection Station.

Again, thanks for all the suggestions from everybody.

Ed B.

That’s good news. Thank you so much for the update. We hear the results far too rarely.

Happy Motoring