CO/HC

This check is used only to determine whether or not the idle CO/HC complies with regulations.

Initial conditions
1. Engine at normal operating temperature
2. Air cleaner installed
3. All pipes and hoses of air induction system connected
4. All accessories switched OFF
5. All vacuum lines properly connected
6. SFI system wiring connectors fully plugged
7. Ignition timing checked correctly
8. Transmission in neutral position
9. Tachometer and CO/HC meter calibrated by hand
Start engine
Race engine AT 2,500 RPM for APPROX. 180 seconds
Insert CO/HC meter testing probe AT least 40 cm (1.3 ft) into tailpipe during idling
Immediately check CO/HC concentration at IDLE and/or 2,500 RPM
When doing the 2 mode (2,500 rpm and idle) test, follow the measurement order prescribed by the applicable local regulations.

If the CO/HC concentration does not comply with regulations, troubleshoot in the order given below.

Check heated oxygen sensors operation. (See page SF-72 )
See the table below for possible causes, and then inspect and correct the applicable causes if necessary.

CO	HC	Phenomenon	Causes
Normal	High	Rough idle	Faulty ignitions: Incorrect timing Fouled, shorted or improperly gapped plugs Open or crossed high-tension cords Cracked distributor cap Incorrect valve clearance Leaky EGR valve Leaky intake and exhaust valves Leaky cylinder
Low	High	Rough idle (Fluctuating HC reading)	Vacuum leaks: PCV hose EGR valve Intake manifold Air intake chamber Throttle body IAC valve Brake booster line Lean mixture causing misfire
High	High	Rough idle (Black smoke from exhaust)	Restricted air filter Faulty SFI system: Faulty pressure regulator Clogged fuel return line Defective ECT sensor Faulty ECM Faulty injector Faulty throttle position sensor Faulty mass air flow meter

This guide is based on the book edition Toyota (RM502U, 1997)

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