What are the signs of a blown motor in a Mazda RX-7 FD?

In a Mazda RX-7 FD, a "blown motor" usually refers to a failed or broken apex seal inside the rotary engine. When an apex seal fails, the affected rotor loses compression, leading to a very distinct set of symptoms.

Here are the most common signs that your FD's motor may be blown:

• Sudden Loss of Power: This often happens under heavy load or at high RPMs, followed immediately by the car struggling to stay running ³.
• Running on One Rotor: The car will have an extremely rough idle and may feel like the spark plugs aren't firing. This is because the engine is essentially dragging a dead rotor ¹.
• Heavy Fuel Smell & Fouled Plugs: Because the damaged rotor is no longer compressing and burning the air/fuel mixture, raw gas gets dumped straight into the exhaust. If you pull the spark plugs on the dead rotor, they will likely be heavily fouled with fuel ³.

How to Verify (Difficulty: Easy)

Before assuming the worst, verify that you haven't simply popped off the MAP sensor vacuum hose. This hose is located on the rear firewall to the right of the Upper Intake Manifold (UIM). If it blows off, the car will idle terribly and mimic the signs of a blown engine ¹.

If your vacuum lines are intact, you need to perform a compression test. You have two options:

1. The "Poor Man's" Compression Check: Remove the trailing spark plug from the suspected bad rotor. Disable the fuel and ignition, then crank the engine over and listen. A healthy rotor will make three distinct, evenly spaced sounds ("woosh, woosh, woosh"). A rotor with a blown apex seal will have gaps in compression, sounding more like "woosh ...... ...... woosh" ³.

2. Standard Compression Tester: You don't necessarily need a specialized rotary tester. Using a standard compression gauge with the schrader valve removed, crank the engine and watch the needle. You are looking for three even pulses per revolution. If you see one strong pulse followed by two weak or missing pulses, an apex seal is broken ³. For reference, a healthy FD motor will show compression around 110-120 PSI, while a blown motor will show zero or drastically lower compression on the affected rotor ¹.