How To Check Engine Compression (2002-2009 4.7L V8 Dodge Ram Pickup)

Testing the compression of the engine, on your 4.7L Dodge or Jeep SUV or pickup is not hard to do and in this tutorial, I'll show you how to do it and more importantly, how to interpret the results of your compression test.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (4.7L Dodge) (at: autotecnico-online.com).

CYLINDER MISFIRE DIAGNOSTICS:

• How To Troubleshoot A Cylinder Misfire -P0300↔P0308 (2002-2009 4.7L V8 Dodge Ram Pickup).

ENGINE NO-START DIAGNOSTICS:

• How To Test An Engine No-Start (2002-2009 4.7L V8 Dodge Ram Pickup).

Symptoms Of Low Engine Compression

There are 3 things, the 4.7L engine in your Dodge or Jeep vehicle needs to start, idle smooth and accelerate: Fuel, Spark and Air.

The ‘Air’ part is handled by several components, among them are the piston rings and cylinder head valves.

When these wear out (piston rings and/or cylinder head valves), the affected cylinder will have low or no compression and this will adversely affect the amount of air the cylinder can breathe in and then compress.

Low compression in one or several engine cylinders will have a direct impact on the way the engine idles and the way the engine accelerates. No compression on all cylinders and the engine won't start, it'll crank but not start.

Here are other symptoms of low or no compression that you'll see:

1. Rough Idle.
2. Check engine light (CEL) on with one or several of the following diagnostic trouble codes (DTCs):
   • P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308.
3. The spark plug (of the cylinder with low or no compression) will always get fuel fouled.

No compression in all of the cylinders will result in a cranks but does not start condition. The symptoms you'll see will be:

1. No Start.
2. Everything else works, for example:
   • The fuel pump will activate, so you'll see fuel pressure at specification (if testing with a fuel pressure gauge).
   • All of the COP coils will spark.
   • If the COP coils are sparking, then this indirectly proves that the crank sensor is OK too.
   • The PCM will still activate all of the fuel injectors.
   • The spark plugs (of the cylinders with no compression) will always get fuel fouled.

What Tools Do I Need To Test The Engine Compression?

The most important tool that you're gonna' need is a compression tester. If you don't have one, you can rent one from your local Auto Zone or O'Reilly Auto Parts Store.

If you're gonna' buy one and want/need to save some bucks, buy it online and below is where I recommend you buy it.

Since the COP ignition coils and the spark plugs need to be removed, you'll need some of the following basic tools:

1. Ratchet wrench.
2. 5/8" spark plug Socket.
3. Extensions for the ratchet wrench.
4. Motor oil (for the "wet" compression test part).

In case you're needing to buy a compression tester and want to save money by buying it online, you can shop here:

What Is Engine Compression?

In a nutshell, engine compression refers to the fact that the air that the engine breathes in is compressed (after it's been mixed with fuel).

Once the air and fuel mixture is compressed, it's then ignited by a spark from the spark plug (the actual process is a bit more complicated but for our purposes, and since we're not trying to reverse engineer the engine, this brief overview is more than enough).

As I mentioned before, each engine cylinder needs 3 things to run optimally:

1. Air.
2. Fuel.
3. Spark.

The "air" part includes healthy compression in all of the 8 cylinders, among several things.

If any one of these three is missing from the mix, the engine cylinder that's being affected will not produce power (or not enough). This will cause the engine to have a miss (also known as a misfire condition) and you're gonna' feel it when the engine idles and/or when you accelerate the engine.

Alright, let's get testing.

TEST 1: Dry Engine Compression Test

If the engine in your Dodge or Jeep SUV or pickup starts and runs, then the optimal way to do the compression test is with the engine slightly warmed up.

I want to stress slightly warmed up, because you do have to remove the spark plugs and you should never remove spark plugs from an engine that's hot (the main reason being that you could damage the spark plug threads in the cylinder heads as you remove them).

Now, this isn't a super critical thing, so if your particular Dodge/Jeep SUV (or pickup) doesn't start, don't worry about the engine's temperature during the compression test (you'll still get results you can use).

NOTE: Before you start, take a look at the whole article and familiarize yourself with all of the steps. Please remember to always think safety first, since you'll be working around a cranking engine.

Let's get started:

1. 1

   Disable the fuel system. You can easily do this by simply:

   1. Unplugging all of the fuel injectors from their connectors.
   2. Removing the ASD (Auto Shutdown) Relay.
   3. Or, removing the fuel pump relay.

   This step is important because it will prevent fuel from being injected into the cylinder as you do the compression test, so don't skip it.

2. 2

   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!

3. 3

   Install the compression tester. 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.

4. 4

   When ready, crank the engine as you observe the needle on the compression tester's gauge.

5. 5

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

6. 6

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

7. 7

   Now repeat steps 3 thru' 6 on the other cylinders.

Let's take a look at what your test results mean:

CASE 1: If you got a reading of 100 PSI or less (less being 0 PSI) on all of the cylinders you tested, then you've got serious engine mechanical problems.

If the engine, on your Jeep/Dodge SUV (or pickup) has 0 PSI compression on all cylinders, this usually means that the timing chain has broken or something severe has happened to it internally (which will either require a re-build or a replacement).

On high mileage engines, low compression on all 8 cylinders will be common. By ‘low’ compression, I mean anything under 120 PSI (although the service manual says 100 PSI is the minimum). If your engine has reached this point, it's also smoking from the quart of oil it's burning every few days (one of a few symptoms you'll see, will be that the idle will be very rough).

CASE 2: One or two cylinders gave a low compression value. It's normal to see a slight variation, in compression values, between all 8 cylinders and so your compression test results may be normal.

What is NOT normal is if these compression values vary by 15% or more. That's right, the individual cylinder compression readings of each engine cylinder can not vary more than 15%. Let's go to: Interpreting The Results Of The Engine Compression Test.