Can Compression Test Diagnose Tiny Leaks?

So, I’ve got rough idles and misfires on cold starts. Before I start replacing parts my diagnostic plans is as follows:

  1. Remove and inspect spark plugs.
  2. Inspect and test starter coil/distributor (still have to research how to do this)
  3. Test resistance on fuel injectors.
  4. Test pressure drop on fuel rails.
  5. Test compression for possible small head leak.

I’m wondering, if I do have a tiny head leak that is letting in a drip or two of coolant after hours of the engine sitting, will I be able to see this on the gauge doing a compression test? I’ve never done one before and still need to buy a gauge, but maybe I’m better off just doing the other tests and if they all check out I can be certain it is a minor head leak.

My assumption is that I have a leaking injector; just trying to do things the proper way.

A compression test will not detect small leaks.

A compression test is too dynamic where the piston moves up and down so fast that it builds the proper pressure even if there’s a small leak. And the compression gauge pegs at the highest pressure and stays there.

If you want to find a small leak, a leak-down test is more accurate.


Thanks @Tester‌

Any recommendations on a good budget friendly leak down/fuel pressure test kit?

Errr wait, I guess I should be doing a compression test on the cooling system to diagnose a small head leak, no?

A leak-down test is where compressed air is introduced into a cylinder with the piston at top-dead-center.

Here’s the leak-down-tester.

At top-dead-center, both the intake and exhaust valves are closed. When the compressed air is introduced, it can be determined if there’s a problem with that cylinder.

If air is heard coming out the exhaust pipe there’s an exhaust valve problem. If air is heard coming out the throttle body/carburator there’s an intake valve problem. If air is heard out of the oil dip stick tube there’s a ring problem. And if bubbles form in the coolant in the radiator there’s a head gasket problem.


Just like a skipping record…that’s right most people wouldn’t know what a record is anymore,


I don’t know what’s going on with the site.

Let’s see how many duplicate posts result from this.


Year, make, model of the car you are working on??

Rough idles and misfires on cold starts could be caused by an assortment of things. I suggest before you start making up a testing plan, start with the actual symptom(s) and what others here think the most likely causes could be. Rough idle problems are often caused by faulty or dirty IAC, dirty throttle bodies, dirty fuel injectors, or air leaks. Diagnosing misfires occurring on specific cylinders often involves a process where you move things around and note whether the problem tracks what you moved. General misfires where it occurs seemingly at random over all he cylinders is often a sensor problem.

If you want to eliminate the head gasket as the source of the problem, my advice is to take the car to a good inde mechanic and ask them to do the necessary tests. I think you’d get better results that way and spend less time and money in the process. I’m all in favor of DIYers, but sometimes it is better to trust the job to someone who has done it many times before. Especially when the job is a narrowly defined task like: Does this engine have a head gasket leak.?

A good answer for you is impossible without info about your car. For example, bad spark plug wires could cause your problem, but we don’t know if your car has plug wires. Bad spark plugs also, but we don’t know how many miles on your car or plugs or your change interval.
I could go on and on with possible causes but without information it is pointless.

1991 Regal, 3800 v6.

So here’s an interesting observation. When i turn the ignition to ON, I hear no sound nor feel any vibration from the fuel pump. Is there any scenario where a fuel pump fails to prime yet kicks into gear after a start?


In some cases, the fuel pump will not get energized without a crank signal

I commend you for doing diagnosis before changing parts. It’s a far, far better way to do repairs, and often saves tons of cash.

Start with a repair manual. Newer cars often don’t have “generic” manuals available, but you can buy manuals for most over the internet… albeit at significantly higher costs, since they’ll generally be the manufacturer manuals. Read through the procedure for every test you want to make, and be sure you feel comfortable with it and have everything necessary before starting.

A lot of good information is available on the internet too, but be very wary of those not written by an official source. Bozo enters internet postings too. Generally a manufacturer’s site is the best resource, for example an NGK site has good info on evaluating spark plugs. Don’t underestimate your dealer’s parts counter as a possible source of free diagnostic, repair, and technical info as well.

As already stated, there are numerous things that cause misfires. More detailed info would help us help you.



And to check this I put a fuel gauge on the rail and see if it pressurizes?

Electrical problem or just a pump replacement?

So why not let us in on the results of this test.

What was the pressure when you turned the key to the run possition, and what was the pressure while the engine was running…and did those pressure readings hole steady or fluxuate.


Usually, you can hear the pump run for a few seconds…On the fuel rail is a test port where rail pressure can be checked and monitored…

@Tester: Skipping record? A lot of the younger folks won’t even understand it if you compare it to a skipping CD…

Check the fuel pump relay first–it may be a bit sticky and the vibration from starting the engine makes it function.

That 3800 engine is somewhat imune to head gasket failures. And if the engine starts quickly and will accelerate from a stand still to 60 mph at wide open throttle without bogging down fuel pressure is not a problem. You might listen closely for the indication of timing chain slap and chatter at a cold start and inspect the spark plugs, plug wires and coil bases. High resistance in the ignition secondary will cause coils to arc into the module and destroy it. Any corona on the coils or on plug wires is an indication of arcing due to high resistance.

A vacuum gauge is less than $20 and can help diagnose driveability problems.

Heavy carbon deposits in the intake and on intake valves can cause rough cold starts.


I haven’t tested the pressure yet.

I’m shopping for a gauge right now. I’ll update after I do some fiddling.