I recently learned of a new (to me) way to quickly diagnose engine compression problems. If I understand the method correctly, you simply measure the starter motor’s current during cranking. The idea is the more starter current required during cranking, the higher the compression. You can compare the relative compression of all cylinders in a jiffy. So if no. 5 on a v8 has a significantly different current than the other 7, suspect something wrong with no. 5. For those w/ experience here using this method, what are the benefits and disadvantages?
Or, the starter is worn out.
Not an accurate way to measure engine compression.
Tester
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I’ve done it many times. Very helpful. It tells you if you have a cylinder(s) that has lower compression relative to the others but it doesn’t tell you which one. From there, you can do an actual compression test.
It’s not designed to measure engine compression. It’s designed to compare the relative compression of all cylinders against each other. A tired starter won’t affect this test.
It seems like it could also ID the cylinder number of the weak one with the right instrumentation. But even if it didn’t yield the weak cylinder’s number, that info could be pretty useful in some situations.
What are the five things that could cause a starter motor to have high current draw? Shorted Armature, grounded Armature, bent Armature, shorted field Armature, worn bushings.
Tester
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I have seen and used a technique where you pull the fuel pump relay and crank the engine. The note of a dead cylinder as the starter blows through it. It is a quick and dirty method of identifying a really dead cylinder
There are other things that would show up… a buggered ring gear, all the stuff Tester listed.
Those are irrelevant. The starter will pull less amperage on the compression stroke of a cylinder with lower compression.
IMO, starter issues would not reveal themselves during this test. A worn starter may pull higher amperage but the relative draw of a lower compression cylinder will still be apparent.
Hey, aren’t you supposed to be taking cover soon? Hope all is well done there in your neck of the woods. Stay safe.
Thanks, we are in a safe place.
I am bored waiting for the hurricane about 50 miles south of my home and roughy 125 miles or so south of the landfall point. Storm surge max for the Naples area is about 5 ft lower than the condo I am in.
My cell still works so I can web surf.
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I see @weekend-warrior is still online. Milton looks to make landfall much closer to you than me!
Please stay safe! And dry!
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Stay safe!
As for the thread’s subject…whole different and more informative “method” for a quick engine health check is a vacuum gauge. (Useful article here.) I’ve had one for many years and have used it a lot. (E.g. I used to take it when looking at used cars and get permission to hook it in. I could also tell a long story about how it helped me figure out the existence of the bad valve seat insert issue in Ford Escorts.)
They also can’t definitively pinpoint a cylinder or even if a vacuum issue / weirdness is certain to be a compression issue. But they give great clues.
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A vacuum gauge seems to be a forgotten tool every mechanic should have and know how to use.
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Many benefits and no disadvantages. It’s sometimes surprising how many people have no understanding of basic electrical principles. This process has been in use in the field for decades. Ford and Toyota (probably others) have a cranking compression test as a function of their factory scan tools, and the aftermarket scan tools (like Snap On) use this as well.
Remember those “big-box” engine analyzers that your corner service station had in the 80’s? The one with all the wires that connected to your engine that the mechanic used for tune-ups? How do you think that machine determined you needed a valve job? It measured cranking amperage and a cylinder that’s low in compression causes the starter to draw fewer amps.
Fast forward to the early 2000’s and I was using a handheld 2-channel labscope with an amp clamp and ignition trigger to watch the starter waveform and figure out which cylinder allowed the starter to spin faster. That’s the low compression.
Today I can sit in the driver’s seat and click a few icons on my laptop screen and have the car run a relative compression test. It will disable the fuel injection, engage the starter, look at battery voltage, crank and cam signals, and determine relative cranking compression. So if I have a car with a fault code that says cylinder #3 misfire, and my relative compression shows cylinders 1, 2, and 4 at 90-100% and cylinder 3 at 60%, I know I have an internal engine issue without even opening the hood. Remember, the actual compression readings aren’t really relevant. I just want to know if one cylinder is lower than the rest.
Now I understand that the average DIY’er will not have this level of equipment and are stuck doing things the old-fashioned way. 
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Since it seems to yield a lot of bang for the bucks, maybe that function will be incorporated into a new car’s drivetrain computer at some point, so if you own the car, you have access to the test. I didn’t realize this test is old-hat to those in the know.
They already have it.
It"s called the Check Engine light.
Tester
It’s already there. I just described how it’s done. Or do you want the car to come with a laptop and software, OBD2 interface, and repair manuals?
Your new car also needs regular oil changes. Do you think the car should come with jack stands, oil filter wrench, and a drain pan?
You guys need some hills and mountains there. I live about a mile from the shore but I’m 400 feet above sea level.
Good luck and stay safe.
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Actually, all the Sun testers I used performed a cylinder balance test to determine low compression, not cranking amps. It cut spark to each cylinder and measured the respective RPM drop to find weak cylinders.
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Cylinder balance was just one of the options on the Bear and Allen machines I used. Both the Bear and the Allen had an option to do an engine cranking test with battery and amp probes connected to determine compression. Just cutting spark to a cylinder will tell you if it’s not firing, but the underlying cause could also be a bad injector, spark plug wire, valve spring, etc.
The Allen SEA machine I owned had a nice scope and you could see cylinder bore taper on the vacuum waveform so you could identify if low compression was caused by upper end or lower end wear.
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The highest point in Florida is reputed to be Disney’s Space Mountain ride!
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