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:
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.
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!
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.
When ready, crank the engine as you observe the needle 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.
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% and this is how you can find out.
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: Substract the result (the number that was rounded) from the highest compression value.
- ANSWER: The result of this substraction 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:
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.