BklynRX7

I tried to do a quick compression test on my newly aquired shady 13b-re. Ok so the battery seemed half dead so i did the test with the battery being jumped from my fathers car. Upon trying to start it I hear a loud whining/whirring noise coming from under the hood, it does slowly crank and i hear whooshes but the compression tester read below 25psi.

SO im hoping the combination of the car being cold, the weak battery, and the whining (maybe the starter) caused this incredible low reading.

Any input?

88rxn/a

iTrader: (6)

use any oil or ATF to build compression?? you prob. did but just askin...

Beater86

the low battery will cause low compression readings

BklynRX7

No I actually didnt, ive heard it before but never thought about it. Ill try tomorrow. Just inject some 2stroke though the spark plug holes?? Or should i use the 10w30

311unity13B

ATF, not oil

ilike2eatricers

what?? ATF or oil will raise the compression artificially.... something you dont want to do when doing a compression test to see the true health of an engine

t-too

anything can help when youre just trying see if its even going to run

ilike2eatricers

I dont see anywhere in the post where he says he's trying to get it to run... just that he wanted to do a compression test

MaczPayne

iTrader: (5)

I agree, he wants to see if he has a perfect engine before dropping it in. And if it has problems, a rebuild would be simple instead of pulling it out after installing it because of inaccurate compression readings.

Aaron Cake

iTrader: (1)

Oil, not ATF.


As for the original question, a week battery will make poor compression numbers. And if the engine has been sitting then the sealing surfaces have likely dried out. A few squirts of oil into each leading plyg hole will bring compression right back up.

BklynRX7

I knew the atf thing didnt seem right, ive only heard baaad things about atf. Ill try it tonight with a fresh battery.

iSP33D-for-J3SUS

Let us know how it goes.

iSP33D-for-J3SUS

BklynRX7

I just tested it with some 10w30 dropped into the trailing hole, its read 75psi completely cold, with a slacking throttle cable and a leaky piston tester. MAYbe itll be better hot and with a different tester???

SexInDaRex

i heard good things about the ATF treatment. it breaks down the carbon you might have to change plugs and yes it will smoke for a good 10 minutes before its normal again.

Alex6969

atf also eats the ruber oil seals

vtecgsr95

dont rotrys have to use a special mazda compression tester since there a 3 strokes pere 1080 deg. of revolution. check it out.

WonkoTheSane

iTrader: (2)

Yeup.. and it's **** expensive, if you can even find one/aquire it.. unless ou're working on them all the time, a normal compression tester will work fine..

BklynRX7

So any opinions on my reading?

RotaryResurrection

iTrader: (8)

Man, there is so much bullshit in this thread it is pitiful. I havent been posting much in this section lately, looks like I'm needed back.

You can use either, they do about the same thing. ATF is a bit thinner, and gets in all the crevices a bit more so than a thicker oil. ATF is also better to use in cold weather, it flows easier.

IF you're trying to start an engine, fine. IF you're trying to test it, this will give you an artificial high number, which is just as useless as an artificial low.

Again, either will work. I've never had, or heard of, ATF damaging an engine when it was injected for the purpose of lubrication or to unflood.

Right, and that's cool if you're trying to start it, or if you want to know whether it's completely dead or not. If you want any semblance of an accurate test number, this isn't the ticket. You'll get anywhere from 15-40psi artificially inflated numbers.



With oil in the chambers, and cranking at the specified 250rpm, that is not a really good number. In my experience you see more compression with a cold engine than you do with a warm engine. This is why many borderline weak rotaries have starting problems when warm, but start fine when cold...because the few psi they lose when warm is enough to let them flood at startup, and the few psi they gain when cold is enough to let them start normally.

The throttle should always be held wide open (or at least halfway) during a test, or the results will be artificially low. Compression test is a measure of how efficiently the engine moves air at low rpm, but the throttle plates block air from entering.

The line needs to be tight on both ends. To check each individual face/seal's status, you hold the valve in, or remove it. The bounces should be even. This tells you all 3 faces are working equally, though the actual number is not to be trusted. Then you replace the valve and let the chamber build up it's highest number. This value is usually within 5psi on all faces if the previous test passed okay. So you have a very good idea of the engine's health.

no, not really that. It does get in all the seal slots and keep carbon from making them stick...the carbon is still there, but some lube keeps the carbon from locking the seals in place over time. It's used to lube and coat.

Maybe on a work bench, but not in an assembled engine. I've atf'd many an engine in my time...

You are supposed to use the $1600 mazda tester, yes. BUt a piston tester can give very accurate results if read properly and tested within the proper perameters. I have both, and VERY rarely can the mazda tester tell me something that my ears or my piston gauge cannot.

BklynRX7

Ok so if i get a fake reading when dry, and a fake reading with oil injectied, then when wil I get a real reading without the engine being totally working/running??? I need to know how much legal action I need to take on this guy.

RotaryResurrection

iTrader: (8)

You won't...you can't.

Consider that rotaries smoke on startup when cold, even when in new/great condition. The reason for this is that a little bit of oil drains off from the OMP injection as well as from the oil orings when the engine isn't running. This oil runs down to the bottom of the chamber and pools up.

Now realize why a cold comp test will ALWAYS be inaccurate to some degree. Because you are testing with oil in the chambers, giving an artificial high, even if you did not inject any.

The only way to find out the 100% accurate compression is to start the engine, burn off all contaminants from inside, let it heat up, and then do the test. Just like it says in the FSM.

All other versions are just guesswork...they can get you fairly close to a number.

stevensimon

iTrader: (11)

rotary ressurection... man... i love it when you post. its so refreshing to not wonder about the legitimacy of your posts like practically everyone else on this board. these guys rule!

Aaron Cake

iTrader: (1)

No, it does not. ATF has been shown through tests on this forum (and my own) to have no effect on carbon at all.

BklynRX7

So basically my only options are to install this bitch and hoep for the best, or get it running in the crapshoot it sits in just to get a comp reading, right?

RotaryResurrection

iTrader: (8)

IF you want an accurate reading, the engine has to have been run and fully warmed up.

Keep in mind that sometimes old engines sit and the seals stick a bit. But with some lube a few days prior, and then getting it running, the seals free themselves and everything seals up BETTER. You just never know which way it will go with these...it could get better, could turn out to be shitty. You have to run it to know for sure.