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How many of you are still using a compression tester to diagnose a rough running problem?compression_tester 

How many of you are still doing a compression test as part of a tune up?

Not many, I hope.


I still see to many shops using a compression test gauge as part of their diagnostic procedures and for the last 15 years, I’ve wondered why.

The results of these tests are very inaccurate and untrustworthy. Why? Here, I’ll give you a list of reasons.

You start by removing all of the spark plugs or, remove one, test and reinstall the plug. If it’s a V6 or V8 you will do one bank, and then the other. To do this, the engine will probably be cooled down so you can work on it and you will either have a remote starter switch connected to the starter or have a helper do the cranking.

Problem #1
The first cylinder you test will naturally have a lower compression than the last cylinder because of oiling. Since there will be many cranks before you get to the last cylinder, it will have the proper lubrication on the rings for cylinder seal compared to the first. Prove it to yourself by retesting #1 again. You will find a difference in your test results.

Problem #2
How many cranks per cylinder were there? Three compression strokes compared to four will really make a difference.

Problem #3
During this lengthy test procedure the battery gets drawn down giving you a slower crank speed as you progress through the test which will give you inaccurate results.

Problem #4
Now that you have the readings, where can you find the specs for that particular engine to compare it to? You won’t find any. If you do find published specifications, they will be a general guideline, not a factory specification. If you get 160 PSI on one motor, is 140 PSI on another considered bad? Different compression ratios, camshafts, valve timing and oil types on the same types of engines will give you different results.

Problem #5
The point of this exercise is to ensure that all cylinder compressions are within 10% of each other but, with all the possible testing variants listed above, can you trust the results. Plus, if you do find a cylinder lower than the rest, can you tell why it’s low? No, you will probably inject oil into the cylinders to try and seal the rings and redo the test. This, you would say, could tell you if the problem was worn rings or valves. But, if it was a broken ring or damaged piston, this procedure would lead you to believe that the valves are not sealing because the oil injection method would not make a change in the readings.
Remember the Chevrolet engines that were wearing cam lobes? Because of the shorter duration and lower lift from the flat lobes, the compression readings were higher than normal. Does that mean the other cylinders were bad?

Problem #6
This procedure takes a fair amount of time to do. So, who’s going to pay for it? On today’s vehicles with coil-over plugs and tight engine compartments, two hours to complete this test procedure is not unheard of. Is this procedure cost effective?
So, with all these problems, you can see why I have questioned this procedure for quite some time.

There is a better way. 

Most shops have an engine analyzer that’s capable of doing many tests. Check the manual or on screen menus for the procedure to do cranking tests.

This test is designed to test the battery and starter. These tests may include a cylinder peak amps test or a cylinder contribution test. This is what you’re after.

The benefits of this procedure are:

 - You get a battery draw test
 - You get a starter draw test
 - You get an accurate compression test
 - It takes 15 seconds to get your results

This procedure eliminates the problems and variabilities that are inherent with the “old way” and transforms the potential labor loss into profit.

In most cases, you will have the scope hooked up anyway, if you’re doing a tune up or addressing a driveability issue, so this procedure will simplify acquiring the information.

How does it do it?

Every time a cylinder goes through a compression stroke, the starter motor will draw more amperage. The scope is matching each of the peak amps to the cylinder in the firing order. What you see is the maximum amperage drawn per cylinder during the fifteen second period.
You will also get a voltage reading at the start and finish of the test which will let you know how good the battery is. (Also, these amperage readings are the same for a starter draw test, if you’re concerned about a bad starter.)

You now have an accurate representation of the compression of each cylinder without the variations a manual compression test would give you. The 10% variability between cylinders still applies even though you are reading amperage instead of P.S.I.

What if my customer wants an actual compression results in P.S.I?

Not a problem.

The peak amps per cylinder is a much higher scale than pounds per square inch. So, to start with, comparing cylinders to see if they are within 10% of each other will be very accurate. But if you want to transform the numbers into PSI, all you have to do is do a compression test with the gauge to an easily accessed cylinder and then get your calculator.

Let’s say that #1 cylinder tested at 240 amps on the scope and the gauge showed 155 PSI.
What you do is divide the small number by the big number.

Example: 155 divided by 240 equals .6458

Then multiply the other cylinder amperage readings by  .6458

 #2 cylinder’s result was 232 amps.

232 x  .6458 = 149.8256 or rounded to 150 PSI

#3 cylinder’s result was 249 amps

249 x .6458 = 160.8042 or rounded to 161 PSI

And so on.

Your gauge couldn’t give you that accuracy.
There is another benefit that will result in testing this way. While the engine is cranking, you will hear the steady rhythm of an engine with even compression. When you get one that has a weak or worn cylinder, it will not have that familiar rhythm. There will be a “hiccup” in the pattern.

Seeing the test results will verify that “hiccup” and soon you won’t need the scope or even a gauge to determine if all the cylinders have even compression or not.

Now, that is what you would call “cool”.  

Now, when you find one cylinder that is down more than the 10% allowable, use a cylinder leakage tester.

I recommend a small hand held one. I don’t find a need for the large ones with many options. All you want to know is how much is it leaking by percentage and where is it leaking.

(I use one that I purchased from NAPA. Part #ULT95009 for $64.99 cdn)
Here’s what you do 

 - Bring the suspected cylinder to TDC
 - Remove the spark plug and screw in the hose for the tool
 - Hook up the tool and zero the gauge. (you do this a couple of times to insure the gauge is calibrated to zero)
 - Hook the gauge to the hose which will pressurize the cylinder.

Note the percentage of leakage on the gauge. (the scale will also show if it’s low, medium or high leakage 

Now, start looking for where the air is escaping.

 - Open the throttle and if air is escaping there, it will be an intake valve.
 - The tail pipe would be the exhaust valve.
 - Oil filler would be rings or at the extreme, a broken piston, and
 - radiator would be head gasket or cracked head.

As you can see, and you will realize, that by using these procedures to test a motor’s integrity is significantly faster and very precise.

One final note:

Don’t forget to charge for this procedure.

Just because it was quicker doesn’t mean you charge less. You are using the scope which is probably the most expensive piece of equipment you have, plus the best technician you have for drivability issues.

So, don’t feel uncomfortable about charging accordingly.

Good luck.



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