Engine compression problems can cause the engine to not start or cause the engine to run with a misfire (rough idle).
In this tutorial, I'll explain the engine compression test and more importantly how to interpret your test results.
So if engine compression problems are causing a no-start problem or a misfire problem, this tutorial will help you find out.
Contents of this tutorial:
APPLIES TO: This tutorial applies to the following vehicles:
- 3.8L V6 Chrysler Town & Country: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
- 3.8L V6 Dodge Grand Caravan: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
- 3.8L V6 Plymouth Voyager: 1994, 1995, 1996.
- 3.8L V6 Plymouth Grand Voyager: 1994, 1995, 1996, 1997, 1999.
Tools You'll Need:
- Compression Gauge Tester.
- A Helper
- Pen and Paper
Symptoms Of Low Or No Engine Compression
Over the years that I've been working I'm cars, I've noticed that engine compression problems usually cause one of two issues:
- The engine is not going to start.
- The engine will start and run but it'll run with a misfire (or a rough idle).
The most common issue is engine compression problems that cause the engine to run with a rough idle or a misfire problem.
On the 1996 and newer 3.8L V6 equipped minivans, the misfire problem will cause the check engine light to illuminate. And a misfire trouble code will be stored in the fuel injection computer's memory:
- P0300 Random Cylinder Misfire.
- P0301 Cylinder #1 Misfire.
- P0302 Cylinder #2 Misfire.
- P0303 Cylinder #3 Misfire.
- P0304 Cylinder #4 Misfire.
- P0305 Cylinder #5 Misfire.
- P0306 Cylinder #6 Misfire.
Generally, when an engine compression problem causes a rough idle, you're only going to feel the problem when you come to a stop light. But as soon as you accelerate the engine, the engine feels fine.
You're also gonna' see:
- Bad gas mileage. This is caused by the simple fact that the engine is now running on less than all 6 cylinders.
- Engine pollutes more. This is also caused by the simple fact that the engine is now running on less than its full complement of cylinders. The live cylinders have to compensate for the ‘dead’ ones.
If your 3.8L V-6 Chrysler equipped mini-van is NOT OBD II equipped, you won't have any codes lighting up the check engine light, but you'll definitely feel a rough idle condition.
OK, having covered the most common scenarios of low compression and no compression, let's get testing to see if this is the case on your 3.8L V-6 Chrysler equipped mini-van.
Spark Plug Access For Cylinders 1, 3, And 5
Testing the engine compression on the 1994-2000 3.8L V6 engine can be a challenge, this is due to the fact that the spark plugs for the 3 cylinders that face the firewall are hard to access (these are cylinders 1, 3, and 5).
Yup, the spark plugs for cylinders 1, 3, and 5 (which face the firewall) are not easy to remove due to the lack of space behind the engine.
I've done this job several times and the one thing that has helped me to remove them (the 3 spark plugs), is to remove the alternator and the alternator bracket first.
Once the alternator and the alternator bracket are removed, you'll able to access them and remove them.
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
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
TEST 1: ‘Dry’ Engine Compression Test
If you don't have an engine compression tester, you can run down to your local auto parts store and you can rent one or buy one there.
If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?
IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!
These are the test steps:
Disconnect the coil pack from its harness connector. This will prevent the ignition coil pack from firing off spark to the spark plug wires.
NOTE: Disconnecting the ignition coil pack's electrical harness connector is important, since it'll prevent damage to the coil pack!
Remove the alternator and the alternator bracket to gain access to the cylinders facing the firewall. This applies to the 1994-2000 3.8L mini-vans.
Remove all 6 spark plugs.
As you're 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.
NOTE: Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When everything is set up, have your helper to crank the engine. Your job is to keep your eye 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 on a piece of paper. Include the number of the cylinder this reading belongs to.
Repeat steps 4-7 on the other 5 cylinders.
Let's take a look at what your test results mean:
CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.
The most common issues would be: Broken timing chain. Or a blown head gasket. Or the engine threw a rod. To test for a blown head gasket, see this tutorial: How To Test A Blown Head Gasket (3.8L V6 Chrysler, Dodge, Plymouth Min-Van).
CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).
But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.
The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.