Compression test results. Any problem?

[shiwei]

Hi Guys, I followed ETCG’s video to do my compression test yesterday. My car is 2005 Mazda 3s, 4 cylinder, with 56K miles.

The data required by manual are: standard: 199.1psi, minimum: 139.2 psi; maximum difference: 28.5 psi.
My test results from passenger to driver side are: 178psi, 162psi, 171psi, 168 psi.

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Updated 2nd test results (with wet test):
I rented the compression tester from Autozone and did the dry and wet tests again today(#4 cylinder didn’t take the wet test as I feel the battery is quite weak after previous tests)

Dry results from passenger to driver side are: 170psi, 180psi, 173psi, 178 psi.
Wet results from passenger to driver side are: 185psi, 185psi, 185psi, #4 not tested.

I paid attention to the pressure and the 1st crank reading is only 20~30psi, and it needs 20 cranks to get a steady reading. 🙁

The consistency is good, but all readings are lower than spec 199.1psi. The car has low miles but the previous owner didn’t change engine oil for 1 year (although put only 2000 miles), otherwise changed the engine oil for every 4000miles/6months.

My guesses are:
1. The reading may not be accurate due to the compression tester.
2. The cylinder is worn out so the pressures are lower than specs. Is it possible with the fact that wet pressures are higher than dry pressures?

I am still confused why it takes so many cranks to get the reading? It’s said to be about 5 cranks with the 1st crank reading of 100psi.

Thank you, pros.

Topic

[Gary]

I assume those are the dry test results. Based on data, I see no reason for concern. Your car is within Mazda specs according to the numbers given and the variation is minimal. You have less than a 10% difference between your highest and lowest cylinders which is excellent in my book. Did you preform a wet test? If you did, did it produce different results?

[Lorrin Barth]

What Mazda is telling you is that they feel the motor is shot when compression is less than 140 psi. Will it still run if it gets below that? It will but not with a lot of power.

[shiwei]

yes, it’s dry test. I will did the wet test and then update the data.
Thank you for your reply.

[Gary]

[quote=”shiwei” post=134959]yes, it’s dry test. I will did the wet test and then update the data.
Thank you for your reply.[/quote] Ah I’m interested to see the wet test results. Reason being is that in your other thread, you stated that you saw oil on spark plugs. The wet test will determine your ring condition. If the wet test significantly raises compression, the rings may be worn. And your welcome.

[shiwei]

Hi Pro. I just updated my new dry/wet test results. Please do some analysis.
Big thank you.

[shiwei]

BTW, I didn’t find any engine oil on spark plug this time.

[Gary]

Ya, that’s way too many cranks, the change seems to be most drastic in cylinder 1 going from 170 to 185. It could be the tester, but I’m leaning towards wear. Because the reading went up with oil, it is not the valves. The rings are not that worn based on what I’m seeing and since there is no oil on the plugs, that is also an excellent sign.

Your two lowest readings are a cylinder apart, so it’s not a leaky head gasket. #1 and #3. When you were testing those cylinders did the pressure build more slowly than #2?

It could also simply be the technique your using to run the test, are you blocking the throttle wide open? All spark plugs removed? Fuel/Ignition disabled? Make sure the tester is not leaking. Was the engine at temp when you ran the test?

Meeting the max spec the manufacturers recommend is relevant, but not to the same extent that the variation between cylinders does. At most you want no more than 10% difference between highest and lowest cylinders(which in your case is fine, but borderline)

At this point if all things considered, I’d run a leak down test. This test is easy but time consuming provided you have the right equipment. Eric has a video on it. A leakdown test is the way to be 100% sure theres an issue.

EDIT: I reread your post, it would seem your readings are inconsistent. The first round compression test you ran is vastly different than the second more recent test. Might be the tester, might be the technique. Are you following Eric’s video to a T?

[shiwei]

Thank you for your detailed analysis.

I followed ETCG’s video to do the test, with spark plugs removed, fuel pump/injector disabled, engine at temp and throttle wide open.

But I didn’t find the ignition fuse or relay on this car, so just disconnect ignition coils from spark plugs.

I will try another compression tester before I do the leak down test following ETCG’s video.

The main problem are that 1) the engine cranks at a very low stroke speed, probably 1second/stroke. from ETCG’s video, it takes only about 1 second to finish 5 cranks; 2)20~30psi for the first crank. What would be the reasons for these two problems, please?

[Bluesnut]

The compression is going down for a car with ony 56k miles on it. It will run fine like this but I would advise another compression test at about 100k miles.

As to cranking speed, you can’t compare one car to another as engine compression, battery and starter motor condition, etc all play a part in this.

When running a compression test it should not take more than 4 or 5 revolution to give a proper reading on the compression gauge.
If it’s taking a lot of revolutions you might consider another gauge or try removing the Schrader valve from the tester and cleaning it.
Sometimes tiny debris from the cylinder or the spark plug well hole will get blown into the compression tester gauge and cause the Schrader valve not to seat fully.
This means that when the piston huffs air into the gauge on the upstroke the non-seating Schrader valve will allow some or all of it to huff right back out on the downstroke.

[Author]

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