Cylinder compression check Ford F150

1. A compression check will tell you what mechanical condition the upper end of your engine (pistons, rings, valves, head gaskets) is in. Specifically, it can tell you if the compression is down due to leakage caused by worn piston rings, defective valves and seats or a blown head gasket.

Note: The engine must be at normal operating temperature and the battery must be fully charged for this check.

2. Begin by cleaning the area around the spark plugs before you remove them (compressed air should be used, if available). The idea is to prevent dirt from getting into the cylinders as the compression check is being done.

3. Disable the fuel pump by disconnecting the electrical connector from the FPDM (see Fuel pressure relief procedure).

4. Remove the ignition coils (see Engine electrical systems) then remove all of the spark plugs from the engine (see Tune-up and routine maintenance).

Note: On 3.3L and 3.5L Duratec (non-turbocharged) engine, the upper intake manifold must be removed to access the ignition coils and spark plugs (see V6 engines ).

Ford F150 General engine overhaul procedures_Cylinder compression check _Use a compression gauge with a threaded fitting for the spark plug hole, not the type that requires hand pressure to maintain the seal - typical installation shown

Use a compression gauge with a threaded fitting for the spark plug hole, not the type that requires hand pressure to maintain the seal — typical installation shown

5. Install a compression gauge in the spark plug hole (see illustration).

6. Have an assistant depress the accelerator pedal to the floor and crank the engine over at least seven compression strokes while you watch the gauge. The compression should build up quickly in a healthy engine. Low compression on the first stroke, followed by gradually increasing pressure on successive strokes, indicates worn piston rings. A low compression reading on the first stroke, which doesn’t build up during successive strokes, indicates leaking valves or a blown head gasket (a cracked head could also be the cause). Deposits on the undersides of the valve heads can also cause low compression. Record the highest gauge reading obtained.

7. Repeat the procedure for the remaining cylinders and compare the results to this Chapter’s Specifications.

8. Add some engine oil (about three squirts from a plunger-type oil can) to each cylinder, through the spark plug hole, and repeat the test.

9. If the compression increases after the oil is added, the piston rings are definitely worn. If the compression doesn’t increase significantly, the leakage is occurring at the valves or head gasket. Leakage past the valves may be caused by burned valve seats and/or faces or warped, cracked or bent valves.

10. If two adjacent cylinders have equally low compression, there’s a strong possibility that the head gasket between them is blown. The appearance of coolant in the combustion chambers or the crankcase would verify this condition.

11. If one cylinder is slightly lower than the others, and the engine has a slightly rough idle, a worn lobe on the camshaft could be the cause.

12. If the compression is unusually high, the combustion chambers are probably coated with carbon deposits. If that’s the case, the cylinder head(s) should be removed and decarbonized.

13. If compression is way down or varies greatly between cylinders, it would be a good idea to have a leak-down test performed by an automotive repair shop. This test will pinpoint exactly where the leakage is occurring and how severe it is.

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