Exhaust Gas Sniffer Facts

THIS INFORMATION IS PRESENTED FOR YOUR KNOWLEDGE AND LEARNING ONLY - TAMPERING WITH EMMISSIONS RELATED EQUIPTMENT IS ILLEGAL AND COULD RESULT IN FINES AND/OR JAIL TIME!!!

I. Passing the Test
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A common problem many have that live in a city like myself is passing the exhaust gas test "sniffer" at the local test station. If you have the typical performance car that is carbureted, had many modifications done, or just plain high mileage, then this article is for you.

The way a sniffer test works is taking a sample of the exhaust pipe discharge and measuring the amount of hydrocarbons (HC) in parts per million (ppm) and the carbon monoxide (CO) in percent. If the measuring device is fooled into thinking that the percentage of each product is lower than the total amount of measured gas then you get a lower reading.

So how is it done? Easy, just introduce more air and less combustion products into the exhaust stream that the "sniffer" will measure. This is done by not having a perfectly sealed exhaust system. Either partially unhook your tail pipes or cut about a 1" hole in the top of your tail pipes at the highest point where it goes over the axle. On my MAC system I just loosen the tailpipes right after the rear axle. Don't unhook too much, just enough to pass, most sniffer tests must detect a certain level of combustion products or your are failed for a leaky exhaust system. My experimentation show I only need to loosen both sides to expose about a 1/4" gap at the flange to pass. See picture below.

Results:

Pollutant	Standard	Before	After
Hydrocarbons (HC) in ppm	220	405	171
Carbon Monoxide (Co) in percent	1.20	2.16	0.55

II. Other Methods for Lowering Emissions
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1) Pour a pint of 100% pure rubbing alcohol in a full tank of 93 octane. This will lower the combustion flame temperature which in turn will lower the Co produced.

2) Pull the PCV and let it hang in the air. This will keep the oil / "blow by" emissions out of your exhaust stream and lean the air/fuel ratio.

3) Locate a factory H-pipe with good converters the you can put on your car whenever you have to go to the emissions test.

4) Turn the air/fuel ratio mixture screws in as far as possible to lean the mixture. Carbureted cars smoothest when the mixture is on the rich side (about 12:1 A/F ratio), but this is not the optimum place for emissions.

5) Move timing back to the factory setting, or about 6° BTC will produce lower emissions than the typical 14°+ that most people use.

III. Air/Fuel Ratios and CO Emissions
passing the test* other methods* A/F ratio* diagnostic tips* top of page

Determining the air/fuel ratio you are running can be beneficial to not only tuning your vehicle but also to passing the sniffer test. The following chart shows the correlation between the A/F ratio and Co pollutants. Based on your first sniffer test you can change the mixture screws to adjust your Co reading. Note: 15:1 A/F ratio is what is considered ideal for perfect combustion but your engine will run smoother between 13:1 - 12:1.

%CO	A/F	%CO	A/F	%CO	A/F	%CO	A/F	%CO	A/F
0.1	14.71	2.1	13.72	4.1	12.95	6.1	12.21	8.1	11.49
0.2	14.53	2.2	13.68	4.2	12.92	6.2	12.17	8.2	11.45
0.3	14.41	2.3	13.62	4.3	12.89	6.3	12.12	8.3	11.42
0.4	14.33	2.4	13.58	4.4	12.85	6.4	12.09	8.4	11.39
0.5	14.27	2.5	13.55	4.5	12.82	6.5	12.06	8.5	11.35
0.6	14.22	2.6	13.53	4.6	12.79	6.6	12.02	8.6	11.31
0.7	14.20	2.7	13.48	4.7	12.74	6.7	11.99	8.7	11.27
0.8	14.16	2.8	13.44	4.8	12.69	6.8	11.95	8.8	11.24
0.9	14.14	2.9	13.40	4.9	12.66	6.9	11.92	8.9	11.20
1.0	14.10	3.0	13.37	5.0	12.63	7.0	11.88	9.0	11.15
1.1	14.08	3.1	13.33	5.1	12.58	7.1	11.85	9.1	11.11
1.2	14.03	3.2	13.30	5.2	12.53	7.2	11.81	9.2	11.07
1.3	14.00	3.3	13.26	5.3	12.50	7.3	11.78	9.3	11.04
1.4	13.97	3.4	13.23	5.4	12.45	7.4	11.75	9.4	11.00
1.5	13.93	3.5	13.19	5.5	12.42	7.5	11.71	9.5	10.96
1.6	13.89	3.6	13.14	5.6	12.39	7.6	11.68	9.6	10.93
1.7	13.85	3.7	13.11	5.7	12.36	7.7	11.64	9.7	10.89
1.8	13.81	3.8	13.07	5.8	12.32	7.8	11.60	9.8	10.85
1.9	13.79	3.9	13.02	5.9	12.29	7.9	11.57	9.9	10.81
2.0	13.76	4.0	12.99	6.0	12.24	8.0	11.53	10.0	10.78

IV. Diagnostic Tips
passing the test* other methods* A/F ratio* diagnostic tips* top of page

Problem	Causes	Check for:
High Hydrocarbons (HC)	1. Idle speed adjustment (usually too low)	1. Defective points/electronic ignition
	2. Excessive high CO at idle can cause moderately high HC at idle	2. Open plug wire or fouled spark plug
	3. Idle circuits on 2- and 4-barrel carburetors imbalanced or adjusted improperly	3. Vacuum leak
	4. Improper basic ignition timing	4. Incorrect air/fuel mixture
	5. Excessively lean idle mixture or vacuum leak causing subsequent misfire in some cylinders	5. Inoperative air pump
	6. Compression leak through one or more valves	6. Incorrect ignition timing
		7. Inoperative EGR system
		8. Vacuum Source is misrouted
		9. Engine condition is worn
High Carbon Monoxide (Co)	Improper carburetor air/fuel mixture adjustment and/or idle speed	1. Incorrect air/fuel mixture
		2. Dirty air cleaner, choke or carburetor/fuel injection
		3. Carburetor float level improperly adjusted
		4. Inoperative air pump
		5. Power valve actuating too early
		6. Carburetor main jetting too rich

So go ahead, give it a try. It beats having your car retested until your so frustrated at your hot rod that your ready to give up and sell it. GOOD LUCK!!

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