Ford — Compression Test
Testing & diagnosis · Engine
From a Ford service manual — check it applies to your exact model before use.
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Procedure
- Ensure crankcase oil is at proper level and battery is properly charged.
- Operate engine minimum 30 minutes at 1200 rpm or until normal operating temperature; turn ignition off and remove all spark plugs.
- Set carburetor throttle plate and choke plate in the wide open position.
- Install a compression gauge in the No. 1 cylinder.
- Install an auxiliary starter switch; crank the engine at least five compression strokes and record the highest reading.
- Repeat the test on each cylinder.
- Lowest reading cylinder must be not less than 75 percent of the highest.
- Squirt approximately one tablespoon of engine oil on top of pistons in low cylinders and repeat the check.
- If compression improves considerably, piston rings are at fault; if it does not improve, valves are sticking or seating poorly.
- If two adjacent cylinders are low and oil does not increase compression, suspect a cylinder head gasket leak.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Lowest cylinder not less than 75% of highest
Source: Ford 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.