Testing the engine compression is becoming a standard test when diagnosing a hard to find misfire (rough idle) or a no-start condition, especially if your 3.2L Isuzu Amigo (Rodeo, Trooper or Honda Passport) has a lot of miles on it.
In this tutorial, I'll walk you through the entire process in a step-by-step fashion and help you interpret your compression test results to see if one or several cylinders have low (or no) compression.
Contents of this tutorial:
Tools You'll Need:
- Compression gauge tester.
- A helper
- Pen and paper
Symptoms Of Low Or No Engine Compression
If your 3.2L equipped Isuzu has an engine compression problem, it'll usually fall within two categories:
- Low or no compression in one or more of the engine's 6 cylinders.
- No compression in all 6 cylinders.
Low (or no) compression in one or more cylinders (but not all) is probably the most common problem I've seen.
This type of engine compression problem can usually be seen in vehicles with a lot of miles or that have had a life of severe wear and tear. The root cause of this condition is usually worn cylinder head valves or worn piston rings (in the affected cylinder or cylinders).
Here are some more specific symptoms you'll see when 1 or more, but not all, cylinders have low compression:
- Misfire codes (if your Isuzu is OBD II equipped):
- 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.
- Engine misses at idle (rough idle).
- Lack of power.
- Blue smoke coming out of the tailpipe at idle and/or when accelerating (this is engine oil burning in the cylinders).
If your 3.2L equipped Isuzu has 0 compression on all cylinder, it won't start. The engine may crank, but it's not going to start. This usually indicates that your Isuzu's timing belt is broken, or blown head gaskets, or that the engine is blown.
The most common symptoms, when there's no compression on ALL 6 cylinders, is:
- The engine cranks very fast and this fast cranking speed is very noticeable.
- The ignition system is not creating spark (only occurs if timing belt is broken).
- Fuel pump is working and providing pressure.
Let's get testing to see what's happening to your 3.2L Isuzu Amigo (Rodeo, Trooper, Honda Passport).
The ‘Dry’ Engine Compression Test
To do an engine compression test, you'll need an engine compression tester.
You don't have to buy one since you can borrow one from your local auto parts store (like AutoZone or O'Reilly Auto Parts) for a cash deposit, which you'll get back once you return the tester.
If you need help deciding where to buy one or which one to buy, take a look at my recommendations: 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!
This is what you'll need to do:
Disable the fuel system by removing the fuel pump fuse. This will prevent fuel from being injected into the cylinders as you crank the engine.
Disable the ignition system on the ignition coil pack SOHC 3.2L Engines (1997 and older). This is just to make sure that the ignition coils do not fire off spark during the compression testing of the cylinders.
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!
IMPORTANT: You should label each spark plug wire with its location before you remove the spark plugs so that you can connect them (the spark plug wires) back to the correct spark plug once you're done with the compression test.
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.
Have a helper crank the engine, when the test is set up, as you observe the needle on the compression tester's gauge. Once the needle on the gauge stops climbing, have him or her stop cranking the engine.
Write down the compression test result and the cylinder the test result belongs to.
Repeat the test in the next 5 cylinders.
OK, let's interpret your compression test results.
CASE 1: Low or no compression in 2 side by side cylinders (of the same bank). This indicates a problem that's usually caused by:
- Blown head gasket.
CASE 2: No compression in ALL cylinders. This isn't good and indicates that your 3.2L Isuzu has serious engine mechanical problems.
The most common issues would be:
- Broken timing belt.
- Engine threw a rod.
CASE 3: Low or no compression in one or two cylinders (that are NOT side by side on the same bank). With the compression values you have recorded, you now need to do a simple math calculation to find out if the low compression test result is within normal specs or not.
How do you figure this out? You can find out by using my online compression calculator here: Online Low Engine Compression Calculator or manually this way:
- Grab a calculator and multiply the highest compression reading that you recorded by 0.15.
- So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X 0.15 gives you 26 (25.5 rounded off).
- Next, subtract 26 from 170, which gives us 144 PSI.
- 144 PSI is the lowest possible compression reading that any one of the other 5 engine cylinders can have. Any compression reading below this and that engine cylinder cause a misfire (rough idle).
Here's a more specific example to make sense of the math. Let's say that my 3.2L equipped Isuzu gave me the following compression readings:
- Cylinder #1 174 PSI.
- Cylinder #2 100 PSI.
- Cylinder #3 175 PSI.
- Cylinder #4 160 PSI.
- Cylinder #5 168 PSI.
- Cylinder #6 175 PSI.
The next step is to do the math: 175 x 0.15= 26, 175-26= 149. So, now I know that cylinder #2 is the one causing the misfire!!
Now that you've found the cylinder with the low compression, the next step is to see if this low compression is caused by worn cylinder head valves or worn piston rings. For this test, go to: ‘Wet’ Engine Compression Test.