Classic Car Exhaust Gas Inspection: Inspection Methods for Simultaneously Lowering HC and CO

If both HC (hydrocarbons) and CO (carbon monoxide) are high during an exhaust gas test for a classic car, the shortcut is to methodically identify the causes of incomplete combustion, rather than resorting to superficial, last-minute fixes right before the inspection.

Although HC and CO are different components, they can increase simultaneously due to shared causes such as ignition failure, an overly rich air-fuel mixture, or insufficient air intake. This article organizes the items you should check for classic cars in the order of maintenance.

What do HC and CO indicate?

HC: Unburned fuel components

HC refers to hydrocarbons that did not combust or did not burn completely. Levels tend to rise due to misfiring, weak sparks, low compression, or an unbalanced air-fuel mixture.

CO: Components that increase with oxygen-deficient combustion

CO levels tend to rise with an overly rich air-fuel mixture (too much fuel relative to air) or during incomplete combustion. For carbureted vehicles, check the choke and mixture adjustments; for fuel-injected vehicles, check the water temperature sensor, intake sensor, and oxygen sensor.

What to suspect when both HC and CO are high

If both values are high simultaneously, check to see if the following causes are occurring together:

• Deterioration of spark plugs, plug wires, distributor, or ignition coil
• Choke not returning properly or an overly rich carburetor mixture
• Insufficient air intake due to a clogged air cleaner
• Poor injector spray pattern or abnormal fuel pressure
• Malfunction of water temperature, intake, or oxygen sensors
• Incorrect valve clearance, compression pressure, or ignition timing
• Catalytic converter not sufficiently warmed up or degraded
• Exhaust leaks or faults in the secondary air injection system

Inspection points vary depending on the model year and control system. Do not use the same procedure for carbureted and electronically controlled vehicles; instead, follow the service manual for that specific vehicle.

Check in this order before chasing specific numbers

1. Check current symptoms
   Record issues such as starting difficulty, idling instability, black smoke, fuel odor, misfiring, or warning lights.
2. Check basic maintenance
   Inspect engine oil, air cleaner, spark plugs, ignition system, fuel leaks, and vacuum hoses.
3. Measure after complete warm-up
   The mixture tends to be rich when cold, and the catalytic converter does not function properly. Measure only after warming up to the specified conditions.
4. Check air-fuel mixture and ignition timing
   Compare carburetor adjustments, choke operation, water temperature signal, fuel pressure, and ignition timing against standard maintenance values.
5. Check compression and catalytic converter
   If basic maintenance does not improve the results, expand the scope to include compression pressure, valves, and the state of the catalytic converter.
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When only HC or only CO is high

This table is just an entry point for diagnostics. Do not decide to replace parts based on measurement values alone; perform additional testing at a maintenance facility.

Things to confirm before the exhaust gas test

• Is the engine completely warmed up?
• Is there any old fuel remaining from long-term storage?
• Are the idle speed and ignition timing within standards?
• Is the choke fully returned?
• Was the battery disconnected recently, causing the learned values to be initialized?
• Are there any warning lights, abnormal noises, or fuel odors?

Inspection standards vary depending on the vehicle's initial registration date, fuel type, and structure. Check the pass/fail criteria and measurement conditions for your specific vehicle at the inspection station or maintenance facility where you will be testing.

Fuel additives are not a substitute for repairs

Fuel additives do not repair ignition system failures, low compression, sensor malfunctions, or catalytic converter damage. If HC or CO levels are high, you must first identify and repair the underlying cause.

Once repaired, you may consider GERHON Fuel Conditioning for gasoline as an option for routine fuel maintenance. When using it, verify your fuel type and adhere to the prescribed dosage of 0.5mL per 1L of fuel. This product does not guarantee passing exhaust gas inspections or a reduction in values for all vehicles.

See Gerhon Fuel Conditioning for Gasoline

Related Information

• Past 10-mode gasoline vehicle test and CO/HC measurement records
• Types of fuel additives, how to choose them, and the concept behind Gerhon
• Research & Development: Our approach to testing, measurement, and comparison
• Inquire about product compatibility and usage

Summary

There is no "magic bullet" to lower both HC and CO at the same time. The fundamental approach is to isolate the cause in the order of: full warm-up, ignition, intake, air-fuel mixture, compression, and catalytic converter, and to return the vehicle to its original condition. For classic cars, it is especially important to distinguish between areas that can be fixed by adjustment and areas where part degradation has occurred.

Catchphrase image: Free stock image from Pexels, edited into a square format.