Low engine compression (in one or several cylinders) is one of the most overlooked tests when troubleshooting a hard to diagnose misfire or no start condition.
Using a compression tester, you'll be able to measure the pressure of the air that gets compressed inside the cylinders by the piston rings against the cylinder head valves.
If the piston rings or cylinder head valves are worn out, the pressure will leak past them and show up as a low compression value on your compression tester.
In this article, you'll learn how to do both a ‘Dry’ engine compression test and a ‘Wet’ compression test and more importantly, how to interpret their results.
Contents of this tutorial at a glance:
Symptoms Of Bad Engine Compression
The most direct impact that low compression (or none) in one or two engine cylinders is bad engine idle. Other symptoms include:
- Rough Idle.
- Misfire Condition
- Check engine light on and one or several of the following misfire codes stored in the PCM memory:
- P0300: Random Cylinder Misfire.
- P0301: Cylinder #1 Misfire.
- P0302: Cylinder #2 Misfire.
- P0303: Cylinder #3 Misfire.
- P0304: Cylinder #4 Misfire.
No compression in all of the cylinders will result in a cranks but does not start condition. The symptoms you'll see will be:
- No Start.
- Everything you test works, for example:
- The Fuel Pump activates and pumps fuel to the fuel injectors.
- The Fuel Injector(s) is (are) being pulsed and injecting fuel into the cylinders.
- The Distributor is sending spark to all spark plugs, which indirectly tells you the ignition coil is good.
- In the COP Coil Ignition Systems, all of the COP coils are sparking.
- If the COP coils are sparking, then this indirectly proves that the crank sensor is OK too.
What Tools Do I Need?
The most obvious tool you'll need, of course, is a compression tester. This bad boy you can rent or buy from your local auto parts store (although the ones they have for rent are so badly beat up they're usually not accurate anymore).
Buying them online is another option that'll save you some money and for the price of what a cheap one will cost at the auto parts store, you can buy one with a case that'll keep your compression tester safe and sound for years to come.
Since the spark plugs have to be removed, you'll need these basic hand tools too:
- Ratchet Wrench.
- 5/8'' Spark Plug Socket.
- Extensions for the Ratchet Wrench.
- Motor Oil (for the ‘Wet’ compression test part).
- Spark Plug Wire puller.
In case you're needing to buy a compression tester and want to save money by buying it online.. you can shop here: