See Figure 1
The PAIR system is used on some 1980 and later 250 6-cylinder engines. The system utilizes exhaust system pulses to siphon fresh air into the exhaust manifold. The injected air supports continued combustion of the hot exhaust gases in the exhaust manifold, reducing exhaust emissions.
Air is drawn into the PAIR plenums through a hose connected to the air cleaner case. There are two plenums, mounted on the rocker arm cover. The air passes through a check valve (there are four check valves-two at each plenum), then through a manifold pipe to the exhaust manifold. All manifold pipes are the same length, to prevent uneven pulsation. The check valves open during pulses of negative exhaust back pressure, admitting air into the manifold pipe and the exhaust manifold. During pulses of positive exhaust back pressure, the check valves close, preventing backfiring into the plenums and air cleaner.
- Visually check all system valves, pipes, hoses and grommets for any leaks or cracks. Repair or replace components, as necessary.
- To check the operation of the pulse air valves, connect a handheld vacuum pump at the grommet end of the valve.
- Apply a vacuum of 5 kPa (17 in. Hg) to the valve. Observe the gauge for an allowable drop in vacuum of 1.7 kPa (6 in. Hg) in two seconds. If vacuum loss is greater than specified than replace valve or hose.
REMOVAL & INSTALLATION
- Remove the air cleaner. Disconnect the rubber hose from the plenum connecting pipe.
- Disconnect the four manifold pipes at the exhaust manifold. Remove the check valves from the plenum grommets.
- Unbolt the check valve from the manifold pipe, if necessary.
- To install, assemble the check valves to the pipes before the pipes are installed on the exhaust manifold.
- Install the manifold pipe fillings to the exhaust manifold, but tighten the fittings only finger tight.
- Use a 1 in. open end wrench, or something similar, as a lever to align the check valve on the A pipe assemblies (see the illustration) with the plenum grommet. Use the palm of your hand to press the check valve into the grommet. A rubber lubricant can be used to ease assembly.
- Repeat this operation on the B pipe assembly.
- After all the check valves have been installed in the rubber grommets, tighten the manifold pipe-to-exhaust manifold fittings to 28 ft. lbs. Connect the rubber hose to the plenum pipe and install the air cleaner.