Testing to see if low or no engine compression is the reason for a misfire condition, or a rough idle condition, or a no start condition... just doesn't really come to mind in a lot of situations. Well, in this article, I want to not only show you how to do one, but also interpret its results... so that you can find out if low or no engine compression is at the root of your 2.4L Honda's problem.
So, in case you're not aware... I want to tell you that internal engine components (mainly cylinder head valves and engine piston rings) that are worn can cause problems in idle quality or simply result in a no-start condition.
In your 2.4L equipped Honda, you can do the engine compression test yourself... and avoid a trip to the shop. In this tutorial I'll show you how to do both a Dry and Wet engine compression test and then interpret the results.
Symptoms of Low Engine Compression
Every cylinder, in your Honda's 2.4L engine, needs 3 very specific things to output power... these are: air, fuel, and spark.
If any one of these bad boys are missing from the mix (or not in the correct quantities), your 2.4L Honda is gonna' misfire (or, in other words, the engine is going to run rough, idle rough, miss, etc.).
The air part is provided by the suction and compression created by the cylinder head valves and engine block pistons... and if they are too worn... they will cause low engine compression.
This low engine compression results in less air entering and being compressed during the combustion cycle of the particular cylinder affected.
Low engine 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.
Although not an exhaustive list of thing your 2.4L Honda engine may experience... you'll definitely see/feel one or several of the following symptoms:
- Rough idle. Simply put: the engines shakes too much at idle.
- Check engine light (CEL) on with one or several of the following diagnostic trouble codes (DTCs):
- P0300, P0301, P0302, P0303, P0304.
- 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?
It goes without saying... but I'm gonna' say it anyway, that the most important tool that you're gonna' need is a compression tester. If you don't have one, don't worry... if you're in driving distance from an Auto Zone or O'reilly Auto Parts, you can rent one from them (for a returnable deposit of $40+ US dollars).
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 also 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).
TEST 1: Dry Engine Compression Test
Although it isn't always possible (the engine may not start)... you should do the compression test with a slightly warmed up engine.
In case you're wondering why... it's because metal expands with heat and this expansion may have an effect on your compression tester readings.
Now, this isn't a super critical thing, so if your particular pickup or SUV doesn't start... don't worry about this.
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.
OK, to get this show on the road, I'll first explain the test steps. At the end of the test steps, you'll find two possible test results that will help you to interpret your specific test results. Let's get started:
Disable the fuel system. You can easily do this by simply by disconnecting the fuel injectors from their electrical connector.
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!
NOTE: Make sure that you leave all 4 ignition coils disconnected from their electrical connector to avoid having spark discharged during the test.
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. Record this compression reading on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other cylinders.
Now that you're done with the test, let's take a look at what your 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... you've got serious engine mechanical problems.
It's absolutely rare for the engine to get to the point of 0 PSI compression on all cylinders, but if it were to, this would usually means a broken timing chain.
What is common, if you have a very high mileage engine... is low compression across the board. Low usually means 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. The other symptoms you'll see is that the idle will be very rough.
CASE 2: One or two cylinders gave a low compression value. This might be normal, since each cylinder will not give the exact same compression value.
What is NOT normal is if the pressures 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:
- Multiply the highest compression reading that you registered on the compression tester by .15.
Let's say for the sake of this conversation that you got the following compression readings:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 PSI.
- Cylinder #4 95 PSI.
- Cylinder #5 160 PSI.
- Cylinder #6 165 PSI.
- This is the math you would do: 175 x .15= 26, then 175-26= 149.
- So then, 149 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.
- This means that cylinder #4 is the one causing the misfire
To further pinpoint the problem, the next step is to do a ‘Wet’ compression test on the dead or low compression cylinder.
The Wet compression test will let us (you mainly) if the problem is due to worn out cylinder head valves or worn out piston rings. Let's turn the page and get testing...