If your engine has an engine compression problem, you'll usually see one of two things: The engine might not start at all or it'll start but run rough and misfire.
Checking engine compression isn't difficult. In this tutorial, I'll show you how to run the test and make sense of the results. You'll easily figure out if a compression problem causing your engine's no-start or performance issue.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 3.9L V6 Dodge Ram B150 Van: 1989, 1990, 1991, 1994.
- 3.9L V6 Dodge Ram B250 Van: 1989, 1990, 1991, 1994.
- 3.9L V6 Dodge Ram B1500 Van: 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
- 3.9L V6 Dodge Ram B2500 Van: 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
Symptoms Of Engine Compression Problems
Engine compression problems usually lead to one of two things.
- Engine won't start: The engine cranks, but refuses to start.
- Performance problems: The engine starts but runs with a misfire.
If the engine does start, you might notice a few of these:
- Rough idle: The engine runs rough but doesn't throw a misfire code.
- Misfire codes: The PCM (on OBD II cars) logs these:
- P0300: Random misfires.
- P0301: Misfire In Cylinder 1.
- P0302: Misfire In Cylinder 2.
- P0303: Misfire In Cylinder 3.
- P0304: Misfire In Cylinder 4.
- P0305: Misfire In Cylinder 5.
- P0306: Misfire In Cylinder 6.
- Misfires during acceleration: You'll feel it when you hit the gas.
- Idle misfires: Engine only misfires while idling, but smooths out when you rev it.
- Bad gas mileage: You're filling up way more often than usual.
- Strong exhaust smell: You'll smell a heavier exhaust coming out from the tailpipe while the engine is running.
Important Tips And Suggestions
TIP 1: If the engine's been running for any length of time, wait until it cools off all the way before pulling the spark plugs.
Removing the spark plugs from a hot engine can strip the threads in the spark plug holes —and believe you me, that's a headache you really don't want.
TIP 2: When unplugging the spark plug wires from the spark plugs, use a spark plug wire puller.
This tool makes it way easier to unplug the wires —and more importantly, it helps prevent the metal terminal (the part that connects to the spark plug) from detaching and getting stuck on the plug when you pull the wire off.
If you don't have one, you can check it out and buy it here: OEMTOOLS 25542 Spark Plug Pliers. (at: amazon.com).
Which Compression Tester Should I Buy?
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.
TEST 1: Dry Compression Test
To get the most accurate picture of your van engine's health (whether it won't start or it has a rough idle or misfire) you'll want to test the compression of all six cylinders.
The instructions below assume you're testing all six cylinders, but you don't have to. That said, even if you know there's just one dead cylinder, try to test at least two others as well.
This way, you can compare the compression readings from the "dead" one to the two "good" ones and see if the compression is within an acceptable range.
No compression tester? No problem. You can swing by your local auto parts store —most of them let you rent or buy one. Wanna save some cash? You can check out my recommendations here: Which Compression Tester Should I Buy?
Alright, let's get started:
- 1
Disconnect the spark plug wires from the spark plugs.
IMPORTANT: Label each spark plug wire with the cylinder number they belong to. You want to avoid losing the spark plug wires' firing order. - 2
Remove the spark plugs.
Be extra careful not to drop any. If the porcelain cracks, you'll be dealing with a misfire later —and that's a pain in the neck you want to avoid. - 3
Install the compression gauge into cylinder #1's spark plug hole.
NOTE: Just hand-tighten it. Don't use any tools to tighten it down. - 4
Have someone crank the engine until the needle on the gauge stops moving up.
- 5
Write down the pressure reading and note which cylinder it came from.
- 6
Repeat steps 3 trough 6 on the rest of the cylinders.
Now let's break down what your results mean:
CASE 1: All six cylinders show 0 PSI. That tells you something's seriously wrong inside your 3.9L V6 engine. Usually it's one of these:
- The timing chain snapped.
- The engine threw a rod.
Even if you get a reading under 100 PSI (but not zero), that still points to internal engine trouble.
CASE 2: All cylinders have compression, but the numbers are all over the place. It's normal for them to be a little different. But if the difference is too big, you're gonna get rough idling or misfires.
To figure out what those numbers are telling you, head over to: Interpreting Your Compression Test Results.