Generac GT-990 — Cylinder Leak Down Test
Testing & diagnosis · Engine
For the Generac GT-990, from the Generac service manual.
Applies to: Generac GT-990 engine
Procedure
- Remove a spark plug.
- Gain access to the flywheel. Remove the valve cover.
- Rotate the engine crankshaft until the piston reaches top dead center (TDC). Both valves should be closed.
- Lock the flywheel at top dead center.
- Attach cylinder leak down tester adapter to spark plug hole.
- Connect an air source of at least 90 psi to the leak down tester.
- Adjust the regulated pressure on the gauge to 80 psi.
- Read the right hand gauge on the tester for cylinder pressure. 20 percent leakage is normally acceptable.
- Listen for air escaping at the carburetor, the exhaust, and the crankcase breather to determine where the fault lies.
- Repeat the procedure for the remaining cylinder.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- 20 percent leakage normally acceptable
- Regulated pressure 80 psi
Source: Generac service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.