A compression tester is an invaluable tool to have around when it comes to diagnosing problems with a vehicle. It can tell you so much about the overall health of an engine and help pinpoint any particular problems, providing you know how to use them properly.
I won’t get too heavy on technical details. But, volumes, compression ratios, and a little background information is useful to understand how a comp tester works, and what it can tell you.
What Is It?
An engine is essentially one big air pump. It sucks air in one end and pushes it out the other. Of course, there are a few things happen along the way. It gets a bit of gas mixed with it, then explodes before exiting via the exhaust. But, it’s the compression inside the cylinders that helps to dictate the level of explosion.
If the gas/air mixture gets pumped into the cylinders and escapes via the side of the piston or a faulty valve, then power is dramatically reduced.
To combat this, engineers do a couple of things. They fit steel or iron rings around the piston which seals the piston to the bore. Then, they make sure that the valves are perfectly matched to their seats. This is a process called lapping or grinding. It all helps to ensure that the combustive gas can only go where it’s meant to when it’s meant to. Quite simply.
But if any of those processes start to fail, it can lead to a number of running problems. It can cause weak cylinders and low compressions and poor idle. It can also cause it to be down on power and perhaps running on fewer cylinders than it has. This is what we’d call ‘losing a cylinder’, and it’s very noticeable.
A compression tester can tell you exactly how much pressure that each cylinder is making. It can do that just through pumping air as the engine turns over. While an absolute reading of ‘XX’ may not mean much, you’ll be able to compare that reading across all cylinders. Anything that’s more than 10% out would be considered as faulty.
The Best Compression Tester
We’ve compared three of the best compression testers. We’ve looked at what makes them work for everyone; from a home mechanic to a professional technician. It’s worth pointing out that all of the following kits are for gasoline engines only. None of them are suitable for diesel work.
This is a full kit, contained in a blow-molded storage case. That means everything you need will be on hand, without having to scramble around searching for adapters or hoses. Of course, it helps to keep the kit safe and damage-free.
It includes all the regular adapters that you’d expect to find a kit like this, and some that maybe you wouldn’t. Like the specialist Ford Triton engine adapter, and the 8” deep well adapter for deeply recessed spark plugs. It’s a nice touch. Although, personally speaking, I’ve always managed with a regular hose. With that said, I’m sure if my kit included this, I’d use it regularly.
So what are you getting for your money? Plenty. An analog gauge that has dual calibration for 0-300 psi and 0-2,100 kPa and measures up at a useful 2.5” diameter. An 8” deep well solid adapter (14mm), 5” flex Ford Triton adapter (16mm). 12” flex standard reach (14mm). 12” flex long reach (14mm). 12/14/18mm thread adapters and a repair parts kit.
The main hose itself (that connects to the gauge) is 25” in length. The 25” means that you can place the gauge out of harm’s way. Or lay it on the windshield so that you can physically see it while cranking the motor. If you’re intending on doing compression tests as a regular thing, a longer hose will always win out.
On the whole, I’d say the OTC kit is certainly capable of being used in a professional environment. But, the price is low enough for a not too large investment for the serious amateur spanner man. I like the kit, and it’ll give you a trouble-free service with accurate results.
If you’re looking for something with more features, more adapters, more … of everything, then the Mityvac compression tester is the one for you.
It’s a digital gauge rather than analog that gives you the options for more fancy tricks. It will read in psi (0-300) bar (0-20) and kPa (0-2,000). It allows you to store readings across the cylinders (no need for pen & paper!)
It also allows you to store the maximum highest reading. You can work through the cylinders with the ‘next cylinder’ function. Plus, the display is also backlit – particularly useful under the dark hoods.
You have a wide array of adapters and flex hoses:
- 14mm long reach hose x 12”
- 18mm standard reach hose x 12”
- 14mm standard reach hose x 12”
- 14mm standard reach x 6”
- 10mm male thread adapter
- 12mm male thread adapter
- 18mm long reach thread adapter
- 16mm male thread adapter for the Ford Triton engine
Air hold adapter and servicing kit.
All held nicely in a hard plastic storage case that’s blow-molded for convenience. (The standard reach and long reach refer to the length of the threaded portion).
All in, this kit is similar to my Snap-On compression tester. The only difference is it uses a round gauge. Thanks to the functionality, I’d be happy recommending it to anyone that needs something a little more professional.
This kit is aimed purely at the back yard mechanic that really just needs to test the very occasional vehicle. The kind that really may only use it the once, then tuck it away in a rolling cab.
It’s low cost. It doesn’t come with a storage case. The adapters are loose and the hose is a little too short for convenience. But, it does the job and will deliver accurate results and tell you all you need to know.
Think of it like this: an old beater truck versus a brand new F150. They’ll both haul loads and get you where you need to go. But, one does it a bit easier than the other, with a little more convenience.
The gauge itself measures up at 2.5”, making it easy to read. The same as the OTC, it has dual calibration – 0-300 psi and 0-2,100 kPa. The kit includes the gauge, the main hose, at 15” in length (which is threaded for 18mm) and a 12mm & 14mm adapter. It covers most of the popular sizes. However, it doesn’t have anything fancy like long reach adapters or repair kits.
If you’re only after a simple tester for the very occasional job, then this is priced well enough to do just that. Use it to diagnose a problem, tuck it away, and be happy that you’ve saved yourself a few hundred bucks.
A compression test is a relatively easy thing to carry out, and while some people may do things slightly differently, generally speaking, it will all be the same.
Ideally, the engine should be warm, but don’t do it when it’s literally just finished running – there’s plenty of red hot stuff under the hood, and burns are readily available to those that aren’t careful. With the engine cool enough to work on, remove all spark plugs – don’t test one cylinder at a time, with the other plugs in place, because the readings won’t be that useful.
If you can stop the fuel pump from being activated, then all the better, but in some vehicles, that can’t be done without actually physically removing the wires from the pump, which could also mean the car registers a fault.
Screw-in the required adapter – most kits come with a range of adapters, and you’ll find the most common spark plug thread size is 14mm. This only needs to be hand tight, it doesn’t need a wrench, and if you’re using a separate thread adapter on the hose, make sure it’s secured well because if you unscrew the hose and the thread adapter stays in place, that will be a pain.
Now, the next stage is where you may find differing methods. If possible, I like to test the car with WOT (Wide Open Throttle) to get maximum air pumping through the cylinders, but if that isn’t possible, don’t worry too much – you’re really only after a comparison between the cylinders.
With the gauge attached, crank the motor over (usually around 5 times) until the reading settles – this is the max pressure that the cylinder will produce under those circumstances. Repeat the test for all cylinders, in the same manner. As I’ve already stated, ideally you’re looking for a consistent reading across all cylinders, but expect to see some discrepancies … a few psi here and there isn’t what you’re looking for.
Anything that’s over ten percent different from the other cylinders is a problem cylinder, and it will be affecting how the engine runs. But that’s not all …
Assuming that you’ve found a cylinder that’s low on compression, the next trick that you have at your disposal is figuring out where that extra compression is going – through the valves or down the side of the piston (known as ‘blow-by’), and it’s easily done.
Squirting a little oil down the cylinder (on top of the piston) will help to seal the piston to the bore, which in theory will mean the compression reading will raise if it’s the piston rings (or bore) that have worn. If the reading doesn’t increase, you know that the problem lays within the top end of the motor.
Being able to identify problems like these can mean the difference between knowing whether you’re needing some minor top end rebuild, or full-on engine strip down for reconditioning.
A simple compression test, which takes around thirty minutes can identify all manner of problems. When a customer comes into my shop complaining of running problems, I first check the heat of the exhaust headers.
Anything cooler than the rest is suspect, and then I work backward… does the cylinder ‘fire’? If not, is that a fuel or electrical problem – a simple test with a quality multimeter tells me whether the injector is seeing a signal and firing.
Strip the spark plug out and look for damage, discoloration and any signs of fuel or oil, if at all suspect, I run a compression test. These simple tests don’t take long, and usually give you enough information as to where to take the next step to. Within an hour you can be reasonably confident of an accurate diagnosis. That saves you time and the customer money. It really is that simple.
It’s all very well telling you how many uses a pro might find for a compression tester. But, what of the people that want to do some yard spannering, or maybe even buying and selling of cars?
I know a number of professional buyers that take a compression tester and diagnostic reader on every potential purchase they go to. It may be a bind to check every single car, but I can tell you that they’ve never been burned with a lemon. That hour’s worth of work has saved them thousands over the years.
It’s similar for yard mechanics also. Why take your car to a shop for diagnosis at potentially hundreds of dollars, when for the main part, you can do it yourself? Invest in the right tools and a little knowledge, and the tools will pay for themselves many times over.