How To Check Engine Compression (GM 3.5L, 4.2L)

TEST 1: Dry Engine Compression Test

Although it isn't always possible (the engine may not start), you should do the compression test with a slightly warmed up engine.

In case you're wondering why, it's because metal expands with heat and this expansion may have an effect on your compression tester readings.

Now, this isn't a super critical thing, so if your particular pickup or SUV doesn't start, don't worry about this.

Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.

OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results. Let's get started:

1. 1

   Disable the fuel system. You can easily do this by simply: Removing the fuel injector fuse or disconnecting the fuel pump relay.
   
   This step is important because it will prevent fuel from being injected into the cylinder as you do the compression test, so don't skip it.

2. 2

   Remove all of the spark plugs. As your taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

3. 3

   Install the compression tester. Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

4. 4

   When ready, crank the engine as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
   
   Write down the compression value. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.

5. 5

   Interpret the results. After testing all cylinders and having written down all of your compression test readings, now you need to interpret the results.

OK, let's interpret the results of your compression test:

CASE 1: If you got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested then you've got serious engine mechanical problems.

It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually means a broken timing chain.

What is common, if you have a very high mileage engine, is low compression across the board. Low usually means anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days. The other symptoms you'll see is that the idle will be very rough.

CASE 2: One or two cylinders gave a low compression value. This might be normal, since each cylinder will not give the exact same compression value.

What is NOT normal is if the pressures vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15% and this is how you can find out:

1. Multiply the highest compression reading that you registered on the compression tester by 0.15.
   
   Let's say for the sake of this conversation that you got the following compression readings on a 4.2L 6 cylinder:
   • Cylinder #1 175 PSI.
   • Cylinder #2 160 PSI.
   • Cylinder #3 165 PSI.
   • Cylinder #4 95 PSI.
   • Cylinder #5 160 PSI.
   • Cylinder #6 165 PSI.
2. This is the math you would do: 175 x 0.15= 26, then 175-26= 149.
3. So then, 149 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
4. This means that cylinder #4 is the one causing the misfire.

To further pinpoint the problem, the next step is to do a ‘Wet’ compression test on the dead or low compression cylinder.

The Wet compression test will let us (you mainly) if the problem is due to worn out cylinder head valves or worn out piston rings. Let's turn the page and get testing.