Testing the compression of the 4 cylinders of the 2.5L engine is not hard, especially since the spark plug are very accesible.
A compression test will not only help you find out the overall health of the engine, but it'll also help you diagnose a misfire or rough idle condition.
In this tutorial I'll explain how to do and interpret the engine compression test.
Contents of this tutorial at a glance:
APPLIES TO: This tutorial applies to the following vehicles:
- 2.5L Dodge Caravan: 1991, 1992, 1993, 1994, 1995.
- 2.5L Plymouth Voyager: 1991, 1992, 1993, 1994, 1995.
Symptoms Of Low Or No Engine Cylinder Compression
Engine compression issues usually cause one of two problems. Either the engine is not going to start due to a lack of compression across all four cylinders.
Or the engine will start and run, but it'll run with a misfire or a rough idle.
You'll also see one or more of the following symptoms:
- Bad gas mileage.
- A heavier exhaust smell coming out of its tailpipe.
- The engine is not as peppy as it was once.
Whether the engine in your mini-van doesn't start or it starts but runs with a misfire, this tutorial will help you find out if an engine compression problem is causing it.
Important Tips And Suggestions
TIP 1: If the engine has been running for any length of time, let it cool down completely before the spark plugs are removed. Removing the spark plugs from a hot engine can result in damage to the spark plug hole threads.
One method that I have used to cool the engine down in a matter of 15-20 minutes is placing a box fan on top of it to cool it down.
TIP 2: The engine has to be cranked to test the compression, for this reason take all necessary safety precautions while working around the engine when it's being cranked.
TIP 3: Have your helper wait outside of the vehicle till you're done setting up the test. This way you'll avoid having your helper accidentally crank the engine while you're installing the compression tester.
TIP 4: Disable the ignition system before starting the engine compression tests. You can easily do this by disconnecting the ignition coil from its electrical connector.
Which Compression Tester Should I Buy?
There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
TEST 1: Dry Compression Test
We're gonna' start off by testing the compression of all 4 cylinders.
Once you've tested all four cylinders, we'll interpret those compression values, to see if they're normal or not, in the next page.
If you don't have an engine compression tester, you can buy or rent one from your local auto parts store. If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?
Let's get started with the test:
Disconnect the ignition coil from its electrical connector. This will prevent the ignition coil from sparking during the test.
Disconnect the fuel pump relay from its electrical connector. This will prevent the fuel injection during the test.
Remove the spark plugs. Remove them from a cold 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 and 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.
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 steps 3 through 5 on the remaining cylinders.
Let's examine your test results:
CASE 1: All four cylinders had 0 PSI. This test result tells you that the engine has an internal mechanical problem.
The most common cause of this condition is a broken timing belt or a blown head gasket.
Your next step should be to check the condition of the timing belt and perform a blown head gasket test.
CASE 2: One or more cylinders had a low compression value compared to the others. This could be normal or it could be causing a problem.
To find out if the compression values are normal or not, go to: Interpreting The Compression Test Results.
CASE 3: All four compression values were similar and above 120 PSI. This lets you know that a compression problem is not behind the no start or misfire problem you're trying to troubleshoot.