E46 M3 - Compression Test, How To And What It Means
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- Опубликовано: 17 окт 2024
- I run a compression test on my S54 and explain what we are trying to accomplish, what the numbers mean, what to do if they are low.
A compression test basically shows how well the combustion chamber seals, that means valve seats, valve guides, piston rings/bores and head gasket.
Make sure your battery is charged and strong, or is on a trickle charger, you want it to have the same power cranking on first cylinder as on the last one.
Number of cranks - should be the same on all cylinders to keep it consistent, but really, you want to crank for as long as the pressure keeps climbing (normally 5-10 cranks will accomplish this).
Cranking should be done with your foot to the floor on the gas pedal to open the throttle body butterflies, so they don't add any restriction.
Test should be performed on an engine at operating temp (specs are printed based on this assumption) to get most accurate results (after all the metal parts have a chance to expand and seal).
On the S54, compression range is 160-174 psi (11-12 bar), with max variance between cylinders around 7psi (0.5 bar). Healthy engines come in at mid-high 170s.
Don't look at absolute values (some gauges measure differently), but look at variance across cylinders. You want to make sure all cylinders are wearing evenly without one sticking out and having a particular problem.
The easiest way to tell a head gasket issue is if 2 cylinders that are next to each other have lower numbers than the rest. This is because the head gasket between them has a hole and allows air to leak out.
If your numbers come in low, make sure engine is at operating temp, make sure the tester is screwed in tightly so that air does not leak out past the spark plug hole threads, then try to add a teaspoon of engine oil into the spark plug holes on the cylinder. This will allow the oil to flow down into the bore and top piston ring and seal it. So if after this the compression number bumps up, you will know it's a worn piston ring/bore.
On the tester itself, important, make sure you get one that has the checkvalve at the very bottom of the hose that you tighten into the spark plug hole, rather than at the other end of the hose by the gauge dial. This will make sure that you are reading only the compression in the cylinder and that the hose will not be part of the "volume" (which will make your numbers look lower). I believe you can rent a tester from O'Reilly or Autozone if you don't want to buy one.
If you have a problem cylinder and don't know what is causing erratic numbers or a lower than the rest number, you might want to do a leak-down test. It uses a separte tester/setup, but it pressurizes the chamber at TDC for that piston and then you can hear/see/feel where the air is leaking out of and how much air the cylinder loses.
-If you hear air leaking out of the oil filler cap then it's bore/piston ring
-If you hear air leaking out of the intake or exhaust port then it's valve seat/guide
-If you hear air leaking out from adjacent cylinder then it most likely is the head gasket.
Other FUN car and house DIY videos on my channel: bit.ly/37t11Y1
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Авто/Мото
Good informative video! About to do a compression test on my ‘04 M3 w 180k+ miles to determine if i have a failing head-gasket since it’s now running hot on track w no cooling system issues. Favorited for reference
Your engine had perfect compression numbers! Can you post the link for the tool you used? Im shopping for one on Amazon but wanna make sure i don’t get a junk one that doesn’t give consistent numbers. I just know to make sure it includes a 12mm adapter
Thank you for the feedback.
You have any misfires, or any oil in coolant or coolant in oil or other symptoms? Could be that you've killed enough bugs/caught dirt or rocks with your oil cooler or radiator that are blocking the flow and causing higher temps, if veins get clogged cooling efficiency goes down. I've blasted my radiator and oil cooler from inside the engine bay outward with a garden hose and that helped some, until you drive on roads again and pick up more dirt. At some point just have to keep replacing them (use FCP for warranty).
Here is the one I used: www.amazon.com/dp/B000EVU89I/ref=cm_sw_r_cp_apa_fabc_HPQSQV5ABV4A07F06NYZ?_encoding=UTF8&psc=1
@@mrgizmo7975 nice thats one of the testers I was eyeing on Amazon.
Nope no issues at all, car and engine runs super strong w no issues. Even after the track days the engine feels like it has half the miles it does (guess the S54 loves the italian tuneups lol). Yeah i figured could be debris in the cooler fins, I’ve hosed both CSF rad and oil cooler before the last track day, and did coolant flush w distilled and wetter, but water temps still got hot up into the 3/4 mark on the cluster gauge :/ A clog in the system is indeed another possibility, I’m thinking maybe even faulty/sticking thermostat? (tho was replaced not even 10k miles ago)
Apparently on these engines coolant mixing in oil isn’t always a sign like most cars, since the hg is super thin between the bores and thats where they usually fail I’m finding out, thus coolant/oil mixing usually doesn’t happen, only low compression being the clue!
Yep, could be thermostat, sometimes things fail even after 1k miles. You running stock fan/clutch?
@@mrgizmo7975 nope, mishimoto efan! Which while not regarded as the best option, it seems to be working fine. It came w the car and this running hot issue is a recent thing so makes me want to rule it out, but who knows lol
Yeah mishimoto is a popular upgrade from the mech fan. I run no fan (aux still in place), and car doesnt like it at the track in ambient north of 100.