1999 GST compression test

[GST781]

So Im just about done with my new turbo project and thought it would be a good idea to do a compression test before i got too excited. i have driven the car and she runs very strong but i just wanted to check for peace of mind.

Please keep in mind, from what i was told, i did the test wrong.

on a cold motor my car ran a clean 150 across all four. i know it is supposed to be at 178, but is this an ok number for a cold motor? cranked the motor about 4 seconds per test for consistancy.

the car should be back together tonight so i can do a hot test but im freaking out about the low numbers. if anyone can give me some peace of mind that would be a huge help. thank you!

 

[daren_p]

Sounds about right for a cold motor, but a cold test doesn't mean much. Do a hot test & then worry about the numbers.

Make sure the battery is fully charged, crank until the gauge stops increasing & make sure throttle is wide open (main engine/ecu fuse removed)

 

[GST781]

again, noob question...is that fuse in the main box by my air filter?

 

[805DSM]

again, noob question...is that fuse in the main box by my air filter?

Yes, it's in the center of the box, pull out the fuse and run another test with the throttle wide open.

 

[XiKeiyaZI]

Keep in mind that the reason a cold test doesn't really tell a whole lot is because when metals are cold, they are at a certain size. When you add heat, things expand and contract. This can expose weak areas such as cracks or stress areas that are only apparent when warmed up (in the range where your engine usually works). It also helps things expand to seal and close gaps that are only there when the engine is idle(shut down) and cold.

You only run tests on components/equipment under similar conditions in which they are usually ran. Otherwise your results are void.

 

[eatsleepboostin]

Compression Pressure Check


Before inspection, check that the engine oil, starter and battery are normal. Also, set the vehicle to the following condition:
Engine coolant temperature: 80 - 95°C (176 - 203°F)
Lights, electric cooling fan and all accessories: OFF
Transaxle: Neutral (P range on vehicles with A/T)
Disconnect the spark plug cables.
Remove all of the spark plugs.








Disconnect the crankshaft position sensor connector.
NOTE: Doing this will prevent the engine control unit from carrying out ignition and fuel injection.



Cover the spark plug hole with a shop towel etc., and after the engine has been cranked, check that no foreign material is adhering to the shop towel.
WARNING:


Keep away from the spark plug hole when cranking.
If compression is measured with water, oil, fuel, etc., that has come from cracks inside the cylinder, these materials will become heated and will gush out from the spark plug hole, which is dangerous.








Set compression gauge to one of the spark plug holes.
Crank the engine with the throttle valve fully open and measure the compression pressure. Standard value (at engine speed of 250 - 400 r/min): 1,250 kPa (178 psi)
Limit (at engine speed of 250 - 400 r/min): min. 935 kPa (133 psi)


Measure the compression pressure for all the cylinders, and check that the pressure differences of the cylinders are below the limit. Limit: max. 100 kPa (14 psi)
If there is a cylinder with compression or a compression difference that is outside the limit, pour a small amount of engine oil through the spark plug hole, and repeat the operations in steps 7 and 8.
If the compression increases after oil is added, the cause of the malfunction is a worn or damaged piston ring and/or cylinder inner surface.
If the compression does not rise after oil is added, the cause is a burnt or defective valve seat, or pressure is leaking from the gasket.
Connect the crankshaft position sensor connector.
Install the spark plugs and spark plug cables.
Use the scan tool to erase the diagnostic trouble codes, or disconnect the negative battery cable for more than 10 seconds and reconnect it.
NOTE: This will erase the diagnostic trouble code resulting from the crankshaft position sensor connector being disconnected.

 

[travs92vr4]

+1

If the cylinders carry the same compression across the board, and its only 150 cold, you have nothing to worry about. I also agree with making sure you either have someone to watch the gauge till it stops jumping or propping it up so you can see yourself. It will jump little by little at the end and may get a little more out of it. Not bad numbers at all. Don't worry.