Testing the compression of the engine on your 4.7L V8 Dodge Ram pickup is not hard to do. In this tutorial, I'll show you how to do it and, more importantly, how to interpret the results of your compression test.
So whether you're troubleshooting an engine no-start problem or a cylinder misfire issue, your compression test results will tell you if an engine compression problem is the culprit.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (4.7L Dodge) (at: autotecnico-online.com).
CYLINDER MISFIRE DIAGNOSTICS:
ENGINE NO-START DIAGNOSTICS:
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:
- Rough Idle.
- 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.
- 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:
- No Start.
- 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:
- Ratchet wrench.
- 5/8" spark plug Socket.
- Extensions for the ratchet wrench.
- 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:
Disclosure: As an Amazon Associate, I earn from qualifying purchases. If my tutorials help you, using these links is an easy way to support the site at no extra cost to you. Thank you!
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:
- Air.
- Fuel.
- 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
Disable the fuel system. You can easily do this by simply:
- Unplugging all of the fuel injectors from their connectors.
- Removing the ASD (Auto Shutdown) Relay.
- 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
Remove all of the 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!
- 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
When ready, crank the engine as you observe the needle on the compression tester's gauge.
- 5
Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
- 6
Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.
- 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.
Interpreting The Results Of The Engine Compression Test
If you got a low compression reading (in TEST 1), the next step is to find out if this compression value is causing a problem.
What you have to do is find out if this low compression value varies by more than 15% of the highest compression value you obtained from your tests.
Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered 'dead'.
You can do this one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).
If you want to manually calculate the 15% difference, here's what you'll need to do:
- STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
- STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
- STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
- ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.
Now, let me give you a more specific example: Let's say that my engine compression test produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 160 PSI |
| #6 | 165 PSI |
| #7 | 160 PSI |
| #8 | 170 PSI |
My next step is to do the following calculation:
- STEP 1: 175 x 0.15 = 26.25.
- STEP 2: 26.25 = 26 (rounded to nearest one).
- STEP 3: 175 - 26 = 149.
- ANSWER: 149 PSI. Any cylinder with this compression (or lower) value will misfire.
Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.
To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.
Once you've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: "Wet" Engine Compression Test.
TEST 2: "Wet" Engine Compression Test
If you've gotten a very low compression reading from any one cylinder, the next step is to add a little bit of engine oil (to that cylinder) and re-check its compression to see if the compression pressure will go up or not.
Adding engine oil and checking the cylinder's compression is known as a 'Wet' compression test. This 'Wet' compression test will help you determine if the piston rings or cylinder head valves are the ones causing the low or no compression reading you obtained in TEST 1.
The amount of oil you're gonna' add is about 2 tablespoons. If the piston rings are worn (and the cause of the low compression reading), this oil will cause your compression tester to record a higher compression value.
Now, since oil could never help seal the cylinder head valves, if the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.
OK, this is what you need to do:
- 1
Add oil to the cylinder.
- You should add about 2 tablespoons of oil.
- Remember, you're only adding oil to the cylinder or cylinders that reported low or no compression in the dry compression test.
- 2
Install the compression tester.
- Once again, hand-tighten the compression tester.
- 3
Crank the engine when all is ready.
- 4
Once the needle on your compression tester stops climbing, have your helper stop cranking the engine.
- 5
You'll get one of two results:
1.) The compression value will go up (from the one you recorded before).
2.) The compression value will stay the same.
Let's take a look at what your test results mean:
CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
More 4.7L V8 Dodge Ram Pickup Tutorials
You can find a complete list of tutorials in this index:
Here's a small sample of the 4.7L V8 Dodge Ram pickup tutorials you'll find in the index:
- How To Test The COP Ignition Coils (2002-2007 4.7L V8 Dodge Ram Pickup).
- How To Test The CKP Sensor -P0320, P0335, P0339 (2002-2010 4.7L V8 Dodge Ram Pickup).
- Common Causes Of Spark Plug Failure (2002-2009 4.7L V8 Dodge Ram Pickup).
- How To Check For Vacuum Leaks With Carb Cleaner Spray (2002-2009 4.7L V8 Dodge Ram Pickup).
If this info saved the day, buy me a beer!
