How do I test the Air Control Valve (ACV) on a Mazda RX-7 FB after failing a smog test?

Difficulty: Moderate

The Air Control Valve (ACV) on your FB RX-7 is bolted to the side of the intake manifold and directs air from the air pump to either the exhaust ports ("port air"), the catalytic converter ("split air"), or dumps it into the air filter housing during deceleration. When these fail, they usually dump air into the air filter continuously, causing a rich condition and a failed smog test.

Here is how to test it, inspect it, and a common community workaround if you just need to get it through a smog check.

1. The Running Airflow Test

You can verify if the valve is routing air correctly based on engine RPM.

1. Warm the engine up to operating temperature.
2. Disconnect the hose going from the ACV to the air filter box, and the hose going to the catalytic converter.
3. At Idle: You should feel no air coming from either hose ¹.
4. At 2100 RPM (Smog Test Speed): Air should be blowing from the cat hose at about the velocity of a hairdryer on low, and absolutely no air should be coming out of the line to the air filter housing ¹.

If the ACV is pushing air to the air filter housing at 2100 RPM, the valve has failed.

2. Vacuum and Solenoid Test

The ACV is controlled by the second solenoid valve closest to the firewall ². To isolate whether the ACV itself is bad or if it's just not getting the vacuum signal:

1. Disconnect the bottom hose on that solenoid.
2. Connect a longer piece of vacuum tubing to it.
3. With the engine running, apply manual suction to the tube.
4. Check the hose running from the ACV to the air cleaner. If it is still flowing air while you apply vacuum, the ACV is bad. If the airflow stops, your ACV is likely fine, and you have a bad solenoid or a vacuum leak ².

3. Internal Diaphragm Inspection

The ACV contains two rubber vacuum diaphragms that harden, crack, and fail from under-hood heat and age.

1. Unbolt the first round diaphragm chamber on the ACV (the one closest to the air pump, held on by four bolts). Caution: There is a spring and some oil inside, so open it carefully.
2. Inspect the rubber diaphragm. If it is hard, inflexible, or cracked, it needs to be replaced ².

I don't have the verified torque specs for the ACV mounting hardware or the diaphragm cover bolts, so don't take a number from me on it—check the factory service manual. The general procedure is to clean the mating surfaces, ensure the gasket is intact, and snug the bolts evenly in a criss-cross pattern so you don't warp the housing.

Community Workarounds for Smog

If your ACV is dead and you cannot source a replacement or rebuild kit (part number FDXU-13-991 for FC/FD kits can sometimes be adapted but is hard to find), FB owners commonly force the valve to route air to the exhaust to pass the test:

• Vacuum Bypass: You can run a vacuum line directly from a known, constant vacuum source to where the solenoid would normally connect. This forces the ACV into "port-air" mode, sending air to the exhaust ports instead of the air cleaner ³.
• Mechanical Wedging: Remove the ACV, take the cover off the underside valve, and use a low-profile machine screw to physically wedge the internal valve open against its flange. This permanently blocks air to the air filter and forces it to the cat ¹.

A quick warning on the wedging trick: Forcing air to the cat at all times will cool the catalytic converter down significantly at idle. If you do this, you must take the car for a 30-minute freeway drive to get the cats blistering hot right before pulling onto the smog dyno, otherwise it won't burn off the hydrocarbons effectively ¹.