Doing a compression test (and more importantly, interpreting the results of it) is not hard to do.
In this article, I'll walk you thru' the whole test in a step-by-step fashion and then I'll explain how to make sense of your test results.
Why do an engine compression test? Well, it may help you to find out if the engine is shot and if this is the reason why it won't start or the reason you have a hard to diagnose misfire condition.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Ford 1.9L, 2.0L) (at: autotecnico-online.com).
Tools You'll Need:
- Compression Gauge Tester.
- A Helper
- Pen and Paper
Symptoms Of Low Or No Engine Compression
I've noticed, over the years I've been wrenching on cars, that engine compression problems will cause one of two problems:
- The engine does not start.
- The engine starts and runs, but runs with a misfire (rough idle).
On OBD II equipped vehicles, low compression problems will usually set a misfire trouble code:
- P0300: Random Cylinder Misfire.
- P0301: Cylinder #1 Misfire.
- P0302: Cylinder #2 Misfire.
- P0303: Cylinder #3 Misfire.
- P0304: Cylinder #4 Misfire.
You're also gonna' see one or more of the following:
- Bad gas mileage.
- Engine burns oil when accelerating the vehicle on the road.
- Rough idle (or misfire) that goes away once the engine is accelerated.
TEST 1: Engine Compression Test
To make sense of the results of the engine compression test, you need to know the number of the engine cylinder you're testing and illustration above will help you with this.
Although the compression test is not that hard to do (as you'll see shortly), I suggest you read all of the test steps first and familiarize yourself with them.
By the way, you'll get the most accurate test results if you perform the compression test with the engine in your Ford vehicle slightly warmed up (but not hot).
If your Ford 1.9L or 2.0L vehicle does not start, then no need to worry about -you can still do a compression test.
If your Ford car does start, crank up the engine and let it run for about 10 minutes and no more. If the engine has been running for an extended amount of time, it's important that you let it cool down for at least 45 minutes.
NOTE: If you don't have a compression tester, check out my recommendations here: Which Compression Tester Should I Buy?
OK, this is what you'll need to do:
Disable the fuel system. You can do this by removing the fuel pump relay.
The reason you need to disable the fuel system is to keep the PCM from injecting fuel into the engine cylinders as you perform the compression test.
Disable the ignition system. This can be done by simply disconnecting the ignition coil pack from its electrical connector.
It's important to disable the ignition system so that you won't have spark firing off at the spark plug wires (while you do the compression test).
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!
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.
Ask your helper to crank the engine once the compression gauge is set up and hand tight in the spark plug hole.
Once the needle stops climbing on your compression gauge, have him or her stop cranking the engine.
Record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
Repeat steps 4 thru' 7 on the remaining three cylinders.
Let's interpret your compression test results:
CASE 1: You got 0 PSI on all four cylinders. This is not good. The engine won't start if all four cylinders have 0 PSI compression.
0 PSI compression across all four cylinders is usually the result of one the following conditions:
- Broken timing belt.
- Blown head gasket.
- Blown engine.
CASE 2: One or more cylinders had low compression. Up to a certain point this is normal. But if the low compression value or values are lower than 15% of the highest, then those cylinders will misfire.
Thankfully, we can easily figure out if these cylinders (with the low compression values) are causing a problem by doing a little math. Go to: Interpreting The Results Of The Engine Compression Test.