Hey all,

I just checked my compression on my 87 GL D/R SPFI. The car has 220k and no rebuild that I know of. I wanted to check up on my results to see what you think. What is the compression range supposed to be?

 

Passenger side

Front .... back

170 178

Drivers

158 170

 

I added oil to the lowest one and got 175 PSI afterwards. What I realized after this test is that 8 cycles of the engine get much steadier and higher readings so perhaps this is a false low. I turned it over only 4 times but I added the oil before I thought of this so I couldn't exactly retest it dry.I suppose this could be rings, is this an issue, worth rebuilding an engine for? what are good readings for this engine?

 

 

 

 

 

My procedure:

1. warmed up car

2. disconnected wires to fuel pump and let the engine run out of fuel

3. Remove all sparkplugs

4. Prop throttle wide open (depress pedal all the way as alternative

5. Disconnected wires to ignition coil, disconnect wire from coil to distributor cap.

6. Put battery charger on battery

7. installed compression reader on each cylinder and cranked engine ~8 times, 4 was too few.

8. dropped alittle oil into low compression cylinder and retested to diagnose bad rings.

 

Hope this is helpful to those about to do a compression test.

Edited September 21, 200916 yr by nathan.chase

The numbers aren't important - what is important is that all the cylinders at within 10% to 15% of each other. For a variety of reasons, no two engines/testers/people will end up with reading that can effectively be compared. The test is comparitive - you are checking the cylinder's against each other and against the engine as a whole - not other people's engines.

 

The "wet" test trick with oil doesn't really work on Subaru engines - the cylinder being horizontal causes the oil to seal poor valve seats just as easily as it will seal the rings. You need to do a proper leak-down test if you want to know the source of the low number.

 

GD

what is a leak down test?

A leak-down tester threads into the cylinder like a comp. tester. But instead of cranking the engine, you apply compressed air to the cylinder with both valves closed. The rate of leakage and where the air is escaping tells you for certain what is going on - air from the exhaust = exhaust valve(s), air from the intake = intake valve(s), and air from the crankcase = rings. You can also move up and down in the cylinder with the valve closed (power stroke) and see if there are any bad spots in the cylinder walls.

 

WAY more effective than a comp. tester - which really only tells you that you do/do not need to run a leak-down test

 

Harbor Frieght has a LDT kit for like $30.

 

GD