Question about compression test

[blkfordsedan]

Typical procedure is to do a compression check with the engine at normal operating temperature. I assume most published figures reflect readings at this temp (i.e. 125psi min to 185psi max). Does anyone know if there's a rule of thumb for pressure readings on a cold engine (like 15% or 20% less for a "cold" engine)?

Two reasons for asking: 1) I really don't like the idea of removing plugs from an aluminum head when it's at operating temp. 2) I found a replacement B6 at local junk yard with low miles. They claim the "cold" compression is 130-145 and I don't know how that translates to most normal published specs.

[WmWatt]

You could remove the spark plugs when the engine is cold, replace them with little torque, warm up the engine, and remove them for the test. I've done compression tests at home on a warm Festiva engine with no problems.

[blkfordsedan]

You could remove the spark plugs when the engine is cold, replace them with little torque, warm up the engine, and remove them for the test. I've done compression tests at home on a warm Festiva engine with no problems.

Very true. I still need to get an idea of how "cold" pressure readings correlate to "hot" readings. I know it depends on the engine construction, i.e. piston silicone content, block material, cubic inches, etc..
A lot of the junkyards do a "cold" test. I guess I could do a cold & hot test on my B3, but I know I have leak-down issues so the results may not be completely reliable.

[kyleeeee]

I personally have removed thousands of spark plugs from hot aluminum heads without any problems ever and ford recommends performing compression test on warm engines only
Compression Test



Before performing the Engine Compression Test, check the battery (10655). The battery must be fully charged, in good condition and properly connected. If the battery is not fully charged, recharge or replace it. Compression Gauge Check

1. Run the engine (6007) until normal operating temperature is reached.

1. Turn the engine OFF.

1. Remove the spark plugs (12405) (if necessary, refer to Section 03-07 for the removal and installation procedure).

1. Disconnect the distributor connectors to eliminate fuel flow and spark.

1. Install Rotunda Compression Tester 164-R0250 or equivalent, tightly in the spark plug hole.
   1. Depress and hold the accelerator to Wide Open Throttle (WOT) and crank the engine.
   1. Normally, the first compression stroke will run the gauge indicator needle a considerable distance up the scale. Succeeding strokes will raise it more until the highest level is shown (this will require at least four or five compression strokes). Record the highest reading.
   Test Results
   
   Repeat Steps 5-7 above for each cylinder and compare the readings with the following specifications.
   • Compression should be 1412 kPa (204 psi) with a minimum of 1030 kPa (149 psi).
   
   Remember that the engine must be at normal operating temperature and the throttle valve must be fully open. Compression Readings — Interpreting
   Examine the readings for all of the cylinders. Pressure variation between the highest and lowest cylinders should be within 75 percent of one another. See Example Readings in this section. The following chart shows maximum compression readings and their corresponding 75 percent minimums.
   
   
   Variations between cylinders will have a greater effect on engine performance than overall readings that are even but slightly below specifications.
   When taking compression readings, watch the action of the gauge needle. When it raises only a small amount on the first stroke, a little more on succeeding strokes, and ends up with a very LOW reading; burned, warped, or sticky valves are indicated.
   A moderate reading with a low buildup on the first stroke and a gradual buildup on succeeding strokes, can mean worn, stuck, or scored piston rings (6149).
   If two adjacent cylinders are low, a blown head gasket (6051) or warped cylinder head to cylinder block surface could be responsible.
   Add 4ml (1 teaspoon) of heavy (30W minimum) engine oil to a cylinder with a low reading. Insert the compression gauge and recheck the cylinder. Crank the engine for a few extra compression strokes and watch the gauge. If the compression goes up 10 percent or more, worn piston rings are indicated. If the addition of the oil produces no significant change, valve trouble, a broken piston (6108), or a blown head gasket is probably causing the low reading.
   If the compression pressure exceeds specifications, there is a build-up of carbon on the head of the piston and on the combustion chamber walls.
   Another sign of excessive carbon is "dieseling" (the engine continues to run after the ignition is turned OFF). Dieseling can be caused by glowing bits of carbon. Hard cranking can also indicate excessive compression from carbon buildup. Example Readings
   If the highest cylinder reads 924 kPa (134 psi) and the lowest cylinder reads 696 kPa (101 psi), then the 696 kPa (101 psi) reading is within 75 percent of the 924 kPa (134 psi) reading. See compression chart for maximum and minimum compression readings.