How to do a proper compression test

[Ryan]

A motor with low compression can show various symptoms:

-low power
-oil consumption
-exhaust smoke on hard accel/decel
-exhaust smoke on cold start only
-difficulty starting
-running poorly
-high fuel consumption
-ruined O2 sensors/catalytic converters


Tools needed:

-Spark plug socket and extensions
-Compression gauge. (looks like this)
-Pen(cil) and paper
-Some clean engine oil


Procedure:

1. Warm up the car to full operating temperature (By definition, the cooling fan comes on)

2. Remove all of the spark plugs.

3. Unplug the injector plugs (or remove the FUEL INJ fuse, the purpose is to stop the injectors from firing)

4. Remove the 3 pin connector on the disty (both doesn't hurt, the purpose is to stop the disty from sparking)

3. Screw the compression gauge into sparkplug hole #1.

4. Prop up the gauge so you can see it from the driver's seat.

5. Crank the car with the clutch AND THE THROTTLE on the floor (100% throttle) until the gauge stops moving. (This is so the cylinder can have as much air as it wants)

6. Record the cylinder and the reading. Organize them on the paper like they are on the motor, this will help you see relationships.

7. Repeat for all cylinders.


Numbers to know:

All compression test numbers are assumed to be taken at 300 RPM.
Maximum "good" difference between cylinders is 30 PSI.
Minimum "good" compression is 140 PSI.


Maximum compressions:

B6 SOHC: 186 PSI (9:1 compression ratio)

B6 DOHC: 185 PSI (9:1 compression ratio)

K8: 193 PSI (9.2:1 compression ratio)

KLDE: 203 PSI (9.2:1 compression ratio)

KLG4: 210 PSI (9.5:1 compression ratio)

KLZE: 225 PSI (10.1:1 compression ratio)

ZE pistons with DE heads with Colt's .363"/218º cams: 175 PSI


A few scenarios you can find yourself in are:

1. One or more lone low cylinders
2. Two or more low adjacent cylinders

1. Add 1 TBS of oil down the spark plug hole and test it again.
1a. If the compression goes up significantly(more than 5-10 PSI), you have a piston ring issue. The ring is either caked in carbon, stuck in the groove, or just worn.
1b. If the compression does not change, you may have a valve sealing issue (Burnt or improper seating) or a headgasket issue.

2. Do the 1. test to one of the cylinders. Check the compression in the low adjacent one without adding any oil to it.
2a. If the compression in the oil-less cylinder increases, you may have a headgasket issue.
2b. If it remains low, it may have a valve issue (burnt or improper seating). It may be just coincidence, or if it is the entire bank, the bank may be running lean causing the valves to burn.


Notes:

-If one or more cylinders are below 140 PSI, consider them dead cylinders. A motor with 140 PSI will have difficulty starting.

-The most important thing though is that they are EVEN. 5 PSI difference is mint, any more than that and the motor isn't running very well.

-If your numbers are consistently low, try another gauge (one from a professional mechanic, theirs should be accurate).

-If your numbers are STILL low, look at your modlist. Pistons can change the compression, as can stroking/boring/decking. A cam can change the compression as well (why the K8 and KLDE make different compression even though they are the same ratio).

-If they're STILL low, just face the fact that you need to do some rebuilding.

[mitmaks]

I thought you only take one spark plug at a time and test that cylinder, reinstall plug and move onto next cylinder?

[MrMazda92]

I did too, but after Ryan posting this elsewhere, my research shows it's really a preference thing.

[Pritchett]

Informative and to the point...nice write up, great job as always!!

[Ryan]

Once you determine there is an issue at all, narrowing it down the HG, rings, or valves is done with new procedures.

I've yet to write that part, I will this evening I think.

[Daninski]

Easier on your starter and battery with plugs out besides you have to remove them any way to do the test and this way it's faster and more accurate (engine temp stays close to consistent for all cylinders).
Ryan thanks for this write up I forgot about disconnecting the injector plugs. I plan to test two engines this week a DE and a ZE. One I can start and warm up the other, it's on an engine stand so cold test only.

[MrMazda92]

hmm, makes sense enough for me.

[Daninski]

hmm, makes sense enough for me.

HOLY SHEET, HE FINALLY GETS SOME THING. THIS IS AMMAZING. Ok I have to pour myself a drink. Lights are finally coming on and he's finally home.

[Ryan]

There, I think I'm done. Goodnight. G4 and ZE max numbers are up for debate. I can check G4 numbers at my old work, but I have never seen or heard of a Eunos 600/800 manual, or MS8, or whatever else came with a ZE.

[MrMazda92]

Thanks Dan, way to make me feel big inside

[mikeinaus]

I just did a compression test with a brand new gauge and my numbers were kind of low. My ze is putting out 195+/- psi on all cylinders except #3 which makes 175psi. Are the numbers alright or is my motor looking at a rebuild in the new future?

[Ryan]

That is not ideal, but still acceptable. Its within the 30 difference between cylinders. The low RPM compression is just a surface test... it doesn't necessarily tell you what goes on at high RPM.

A lot of stock class cars can hardly run at idle, but make 10% more power than a real stock motor because some of the modifications only work at high rpm. (backwards pistons, for example)

If it still feels good and runs good, don't lose too much sleep over it. My KL only tests 170 PSI, but thats because of my cams. 170 PSI should make it hardly start and run (but it does, just fine), also it runs great up high.

Did you find out why #3 is low? Did you do the rest of the tests outlined here?

[mikeinaus]

no ill give the oil trick a try sometime this week.

other then the lower compression the motor runs fine. idle is more or less steady 90% of the time (i think i have a small vac leak somewhere), no smoking, doesnt burn much oil and there is plenty of power. the only "problems" ive noticed is the friction gear spring tick and some lifter ticking in cold mornings.

[Polonius]

Thanks for the write-up, Ryan!

I've got a rough idle going on, and have confirmed that cylinder 3 (it's a B6 SOHC) has 50 PSI, while the other 3 have 160-170. I added the oil, and there was no change to the compression, so it's obviously valves at fault.

Now what? I'm not really sure where to go from here, short of opening up the cover. Which I'd have done already, but I'm not entirely sure what kind of "sealant" the manual is referring to when it says "if re-using the gasket add sealant to the groove first". If I can get my hands on whatever's appropriate and open it up, I think the manual's drawing might start to make more sense.

I believe I'd be checking the HLA and the valve seat (and anything else that might look more catastrophic). Anything else to look for once I'm in there?

[Ryan]

You won't see valve damage from the valve cover, you need to remove the head to look at the combustion chamber. In the very least, you'd have to remove the intake or exhaust manifold, shine a bright light down the plugwell and look for light in the ports. But I think if you did the test properly, you've come to a correct diagnosis already.

Sounds like its BP time.

You can use any sort of engine type sealant for the valvecover, but you get what you pay for.