Interpreting Your Compression Test Results
If your Escape's engine compression test indicates you've got one or more cylinders producing a low compression value, then the next step is to figure out if they are causing a problem (or not).
As I mentioned before, up to certain point, it's not unusual to see the compression values vary between a bit (especially in high mileage engines).
But when the difference is too big, you'll have a rough idle or a misfire condition on your hands.
To find out if the lower compression value is causing a problem, you need to find out if it's lower than 15% of the highest compression value you got.
You can do this (figuring out the 15%) in 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 I got 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 Compression Test.
TEST 2: Wet Compression Test
If your test results (from TEST 1) indicate one or several cylinders with low compression, the next step is finding out what is causing it.
What's usually causes a cylinder to have low compression is one of two conditions: either its piston compression rings are severely worn or the its intake or exhaust valve are severely worn.
Thankfully, you and I can find out by adding a little oil to the affected cylinder and testing its compression again. This is known as a ‘wet’ compression test.
If the low compression is due to worn rings, the compression value of the affected cylinder will go up. If the problem is caused by worn cylinder head valves, the compression value will stay the same.
These are the test steps:
Add a tablespoon (or two) of engine oil to 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.
Install the compression gauge, and as before just hand tighten it.
Have your helper crank the engine till the needle stops climbing on the compression gauge.
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.
Whatever value your compression tester reads, write it down again.
Repeat steps 1 thru' 4 on any other cylinder with a low compression value.
Let's take a look at what test results mean:
CASE 1: The compression value went up after adding motor oil and retesting. This tells you that the low compression problem is due to worn piston compression rings.
Here's why: The motor oil you just added helped the piston rings to create a tighter seal. With the piston rings now sealing the compression inside the cylinder, the compression value on your compression tester went up. This type of test result only happens when the problem is due to worn piston rings.
CASE 2: The compression value DID NOT go up after adding oil and retesting (in other words, it stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
The Intake Manifold Plenum Has To Be Removed
The thing that complicates doing a compression test on your 3.0L Ford Escape (3.0L Mazda Tribute) is the fact that the upper intake manifold plenum must be removed to access cylinders 1, 2, and 3 of bank 1.
I don't include any instructions on this procedure, so you may want to buy a repair manual for this procedure.
For the DIY'er that doesn't have a lot of wrenching experience, this might be a job that's best left to an experienced automotive mechanic/technician. Since paying for his expertise may end up saving you from a tremendous headache.
If you do tackle the compression test job, here are some pointers:
- As you're removing bolts, nuts, etc., place them in a container and away from the engine. This is important since you'll want to avoid having anything fall into the open intake manifold runners (that will be exposed when the plenum is removed).
- You WILL NOT be able to place rags in the open intake runners while doing the compression test. Why? Because if you do place rags in the open intake runners (to avoid stuff falling into them), then these rags will get sucked into the cylinder as the engine cranks.
- Use new gaskets. Do not re-use the old intake manifold plenum gasket. Also, when installing the new one, do not coat it in any type of sealer (like RTV Silicon). This is not necessary and could back fire on you since the sealer can cause a vacuum leak.
Yes, testing engine compression on the 3.0L Ford Escape (3.0L Mazda Tribute) isn't for the ‘faint of heart’. So take all necessary safety precautions and avoid having anything fall into the open intake manifold runners. Or consider letting a professional mechanic do it.
If this info saved the day, buy me a beer!