How many of you still use a compression tester to diagnose a rough-running problem?
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 are a list of reasons.
You start by removing all of the spark plugs or removing one, then testing and reinstalling 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 relay, or have a helper do the cranking. On many of today's engines that may include removing the intake plenum to access.
Problem #1
The first cylinder you test will naturally have 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 recorded the readings, where can you find the specs for that particular engine to compare it to? You won't find one. If you do find published specifications, they will be general guidelines, not factory specifications. 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. So 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.
Some shops may still have a stand-alone engine analyzer capable of performing many tests. Check and follow the manual or on-screen menus for the procedure for doing cranking tests. But, if your shop is like many, this old 'clunker' has porobably been replaced with more portable hand-held diagnostic tools. If so, you may be out of luck. (See below for a possible solution)
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 will align compression amps results with #1 cylinder in the firing order
- It takes 15 seconds to get your results
This procedure eliminates the problems and variabilities that are inherent with the manual 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?
Whenever a cylinder goes through a compression stroke, the starter motor draws more amperage. The scope matches each of the peak amps to the cylinder in the firing order. You see 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 are 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 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 the #1 cylinder was 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
Example:
#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 compressions 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 handheld 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 of the compression stroke
- 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.
No old engine analyser or scope?
You may still be able to create this procedure with a two to four channel oscilliscope, if you have the accessories to view ignition secondary patterns and cranking amps with inductive pickups.
All you are trying to do is view the cranking amps pattern for maximum amps and align these patterns with cylinder number one.
Since you know the firing order, you can view the maximum amps per cylinder in the firing order.
The trick here is to disable the engine from starting. That means disabling injectors and doing a test crank to ensure the residual fuel has been purged. You need the ignition to function for the test.
Once you are set up and ready to go set a timer for 15 seconds.
Why?
That way you you can also acquire accurate data for a battery load test and a starter draw test under real conditions.
I hope this helps.
