Engine compression problems can create a misfire condition. Engine compression problems can also cause the engine to not start.
In this tutorial I'm going to explain the engine compression test and more importantly, I'm going to explain how to interpret your test results to find out if there's a problem.
This tutorial covers the engine compression test on the 1998, 1999, 2000, and 2001 2.5L Ford Ranger (Mazda B2500).
Contents of this tutorial at a glance:
Symptoms Of Engine Compression Problems
Engine compression problems fall into two major categories. Either the engine runs but runs with a misfire (or a rough idle). Or the engine doesn't start.
It's been my experience that testing the engine compression is usually the most overlooked test when testing either a hard to solve the misfire condition or a ‘cranks but does not start’ condition.
When an engine compression problem causes a misfire problem, it's usually because you've got one ‘dead’ cylinder with low or zero compression. When this happens you are going to see a misfire trouble code (P0300, P0301, P0302, P0303, P0304).
When the engine doesn't start due to a compression problem, it's usually because the timing belt is busted, or you've got a blown head gasket on your hands, or the engine has thrown a rod or two.
Whether you're testing a misfire condition or a ‘cranks but does not start’ problem on your 2.5L Ford Ranger (Mazda B2500), this tutorial will help you find out if it's due to engine compression problems.
TEST 1: Dry Compression Test
As you're already aware, your Ford Ranger's 2.5L 4 cylinder engine has eight spark plugs.
You've got 4 spark plugs on the exhaust manifold side of the engine. You have 4 on the intake manifold side of the engine.
To test the engine compression, you're gonna' need to remove only 4 of them. These 4 spark plugs will be the ones on the exhaust manifold side of the engine. The 4 spark plugs on the intake manifold side will stay in their place.
OK, these are the test steps:
Disconnect the spark plug wires from the spark plugs.
NOTE: Label each spark plug wire with the number of the cylinder it belongs to. This will prevent losing the firing order of the spark plug wires.
Remove the spark plugs from the exhaust manifold side of your Ranger's engine.
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plug's porcelain insulator crack (then you'll have a misfire on your hands).
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.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining cylinders.
Let's take a look at what your test results mean:
CASE 1: Two or more cylinders have 0 PSI compression. This test result lets you know that the engine has severe internal/mechanical problems.
CASE 2: All cylinders have 0 PSI compression. This test result usually means that the timing belt is busted.
CASE 3: All cylinders have compression, but the values vary between them. This test result needs to be investigated a bit more. Your next step is to do a simple calculation to see if the lowest compression reading is causing a misfire problem. Go to: Interpreting The Compression Test Results.