Interpreting The Compression Test Results
It's not uncommon to find that some cylinders have a lower compression value than the rest (especially on high mileage engines). Within a certain range, this doesn't cost any engine performance issues.
It's when these low compression values vary more than 15% than the highest, that the engine is going to idle rough or suffer a misfire problem.
You can find out if these low compression values are causing an issue by doing some easy math and in this test section, I'll explain how to do that.
The rule of thumb is that the lowest compression value can not vary more than 15% of the highest value (that you wrote down in TEST 1). If any value is lower by more than 15%, then that engine cylinder is going to misfire. This misfire will cause your Ford van's 4.2L V6 engine to idle rough.
How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com) or by calculating this 15% difference manually.
To understand how to figure out this 15% thing manually, I'll use the following compression test results:
- Cylinder #1 175 PSI.
- Cylinder #2 165 PSI.
- Cylinder #3 170 PSI.
- Cylinder #4 120 PSI.
- Cylinder #5 170 PSI.
- Cylinder #6 170 PSI.
The next step is to do the following math:
- Multiply .15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
- Next, we subtract 26 from 175: 175 - 26 = 144.
- So now we know that the lowest possible compression value is: 144 PSI.
This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire because it's below the 144 PSI minimum.
Once we'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 reached a section, that's one has confirmed you've got one or more cylinders with low compression. This low compression is the result of one of two things:
- The affected cylinder's piston rings are severely worn or damaged.
- The affected cylinder's intake/exhaust valves are severely worn or damaged.
Thankfully, a wet compression test can help us identify the source of low compression -be it the piston rings or the intake/exhaust valves- without the need to disassemble the engine for a visual inspection.
The wet compression test simply involves adding about two tablespoons of oil to the low compression cylinder and then checking its compression again.
If the cylinder's compression increases after adding engine oil and retesting, the low compression issue is caused by worn piston rings. Now, if the compression doesn't improve, the problem is due to the affected cylinder's intake/exhaust valves.
These are the test steps:
- 1
Add 1 or 2 tablespoon of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.
- 2
Install the compression gauge on the cylinder you just added oil to.
- 3
Have your helper crank the engine till the needle stops climbing on the compression gauge.
- 4
You'll see one of two results:
1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder.
2.) The needle will not move at all or stay at the same number you recorded earlier.
What ever value your compression tester reads, write it down again. - 5
Repeat steps 1 thru' 4 on any other cylinder you need to check.
Let's examine your test results:
CASE 1: The compression value of the cylinder you added oil to increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.
CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of that cylinder are worn-out or damaged.
More 4.2L Ford E150 And E250 Test Tutorials
If this tutorial was helpful, be sure and take a look at all of the 4.2L V6 E150 and E250 diagnostic tutorials in this index:
Here's a sample of the tutorials you'll find there:
- How To Test The Throttle Position Sensor (1997-2000 4.2L V6 Ford E150, E250).
- How To Test The Fuel Pump (1997-2000 4.2L V6 Ford E150, E250).
- How To Test The CKP Sensor With A Multimeter (1997-2003 4.2L V6 Ford E150, E250).
- How To Test For A Blown Head Gasket (1997-2003 4.2L V6 Ford E150, E250).
If this info saved the day, buy me a beer!
