Bad Compression Test, Help!!!

[3VMS3]

Well I am new here and recently purchased a used 2007 Mazdaspeed 3 with approximately 80k miles. To take all the precautions I have already, changed the plugs, oil, transmission flush, radiator flush, cleaned out the EGR, checked the turbo for any shaft play (happily very minimal side to side , checked the transmission for any bad syncro, all the gears seem in good shape and so on and so forth.... Things were going well, we could tell that the car had been previously modded from poor installation of the OEM parts... Anywho, the last step was to check the compression of each cylinder. I am not sure the order of the cylinders right now since we finished this late last night, so going in order from right to left (im assuming cylinder 1 is on the right)

Cyl 1: 190
Cyl 2: 185
Cyl 3: 155!!!!!!!!!!!!!
Cyl 4: 190

As you can see Cyl 3 is very low and has me seriously worried, my first thought is piston rings that are fried...

I am curious to see if this specific Cyl is common for losing compression and how terrible this is, also to see if there is any other possible problems causing this.. I understand without getting in there and taking a look its hard to tell, im just trying to gauge what i am in for.

Is this compression dangerously low or something common with higher mileage engines, and am i worrying too much? Any help, advice or assistance would be greatly appreciated!! Thanks Guys sorry for the long post.. and i apologize if there is a link or sticky explaining all this, i could not find one.

 

[chief_wiggum]

that's not normal compression and you should be concerned. There's no way of knowing exactly what the problem is without further inspection, but based on your report that the car was previously modded, it could be as bad as a bent rod. These cars have a bit of a history of poorly tuned cars (and some stock cars) popping motors under certain high load/low rpm conditions. I'll let others with a little more mechanical expertise chime in. Good luck.

 

[njaremka]

when you see low compression in a test like that, a quick check for cause is to squirt a little oil into the cylinder and re-run the test. if the compression goes up, its rings. if the compression doesn't change, its valve related.

 

[3VMS3]

ok ill have to check it out, thanks for the advice, if it makes a difference, the car still drives very well and does not burn any oil that i can tell.. someone also suggested bent rods !?!?!?!?!?!

 

[MSMS3]

Bent rod usually produces vibes in clutch pedal. Squirt oil in plug hole and retest on warm engine per above test.

 

[3VMS3]

ok great i plan on figuring this out on tues, the car drives fine, no burning oil, no vibration in clutch pedal as MSMS3 said, no loss of power and i just drove it 264 miles on a half tank of gas, im hoping its a valve problem at the worst!!

 

[3VMS3]

oh and to make sure i do this right, when i put a little bit of oil in the plug hole, do i put the oil in first then screw on the compression check valve/gauge? and thats a warm test as well correct?

 

[MSMS3]

Warm test. Let engine run up to normal operating temp first. Then remove IC, etc, and ALL spark plugs. Then screw in compression gauge fitting and crank over engine while watching gauge, like you did before. Write down that number. I assume it will still be 155. Then, remove the gauge fitting, squirt in the oil, screw the fitting back in and retest. Write down that number. If pressure comes up to normal or increases substantially, it's most likely your rings or cylinder wall wear for whatever reason. If it stays at 155 or whatever you first number was, classic thinking is a burnt or leaking valve or something in the cylinder head not sealing on #3 cylinder on the compression stroke.

EDIT: One other thing: It could be a weak or blown seal on the #3 cylinder's fuel injector. This is a direct injection engine. The injector is bolted directly to the cylinder head and shoots fuel straight into the cylinder. An "o" ring type seal keeps cylinder pressure from escaping. The seals are known to harden over time and use. They are relatively easy to replace, but not something for a total novice. I would speculate that if the shot of oil does not raise pressure, the injector seal might be even more likely than a problem with a valve or valve train on that cylinder.

 

[3VMS3]

ok fantastic thanks for the help!! im starting to feel a little better about the situation, just praying its the injector seal or a valve problem rather than rings or the cylinder wall.... but overall i think the worst case scenario of a bent rod is not the problem which relieves some stress lol.... Luckily i have a few buddies that run a small shop here in raleigh that know their stuff, i am just trying to get some good info from you mazda pro's before we go snooping for the problem.. thanks again! ill let you guys know what the problem is this week!! Oh and last question, how much oil should i put in the spark plug hole/cylinder?

 

[njaremka]

a few drops or so. doesn't take much.