High HC due to rich mixture?


Old 12-12-02, 10:38 AM
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Question High HC due to rich mixture?

I'm having difficulty getting a 1991 Toyo Camry to pass the state tailpipe idle emissions screening. The car consistently blows high hydrocarbons. This is the 2.0 liter 4 cyl with auto trans. Mine has 230k (!) and runs like a top other than the emissions deal. We've changed plugs, air filter, cleaned the intake manifold and fuel injector rail, checked the timing, and run some "Guaranteed to Pass" through the fuel tank. The only result was a decrease in HC from 390 to 350. The state standard is 220. According to the owner of a muffler shop (I was checking into a cat converter replacement) the black soot in the tailpipe indicates an overly rich mixture. On these computer controlled engines what can I do to lean the mixture?

Thanks in advance!!
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Old 12-12-02, 10:56 AM
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A rich mixture would cause high CO readings. High HC generally indicates a misfire. Check your secondary ignition. You already replaced the plugs, but did you replace the plug wires, cap & rotor?

If the ignition checks out ok, do a compression test or a cyl. leakdown test. A slightly burned exhaust valve can cause a nearly imperceptible misfire, yet still load the exhaust system w/ unburned fuel (HCs).

BTW, what was the CO reading on each test?
Old 12-12-02, 11:04 AM
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I agree with Knuckles.
Old 12-12-02, 11:00 PM
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Thumbs up

Try changing the oil, I was told some years ago that dirty oil will raise HC levels. Whenever I take my vehicles in for the emissions test I do the plugs, cap and rotor etc, plus a fresh oil change. Also make sure the engine is good and warmed up, leave it running while waiting for the test. Good luck.
just my $.02.
Old 12-13-02, 03:29 AM
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The oil has nothing to do with passing the test, unless the vehicle consumes it .

The problem is elsewhere as Knuckles suggested.
Old 12-13-02, 04:36 AM
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Totally disagree with you on that one Joe.

Oil that has been diluted by cold starts and short trips will cause HC failures.

I have had inspections that had HC failures and after diagnosis found no problems. Changed the oil and no more HC failure.

The NYS Inspection chart for HC failures lists inspect oil condition as one of the first checks.

Old 12-13-02, 05:57 AM
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Never had that in 15+ years of inspecting vehicles.

The problem has ALWAYS been elsewhere. Tune ups and maintenance are what have caused non-compliant vehicles to pass.

Sometimes simply bringing the vehicle warmed up will help. A lot of times, the vehicle is put on the machine before the converter "lights off" and this will cause failure---then you precondition it (rev it up to warm it up) to pass.
Old 12-13-02, 07:07 AM
Join Date: Jun 2002
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Oil thats diluted with gas will cause high HC.

Have been an inspector for 35 years and emission testing for about 20 years. It's not until recently that this has shown up due to lowering of testing cut points

Old 12-13-02, 11:09 PM
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Oil diluted with gas like that will not lubricate very well and you may not get to the inspection station because it will knock to beat the band .

I always change the oil before going on the dyno---due to the stress it puts on the car.

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