BENMAN

i'm selling my car and tomorrow someone is coming over to do compression test on the motor. just wanted to know how to do it or a general ideal how to do it so that tomorrow i'm not totally lost when strangers are touching my car. so if anyone can briefly explain how to do it'll be great and what's the gauge is suppose to read.

p4nc7

iTrader: (1)

Compression tester gets threaded into spark plug "hole".

Look for 3 even pulses per rotor, want something above 90.

Assuming this is for an rx.

BENMAN

Does it matter if my battery is low?? Yups it's for my fd.

No_love_for_pistons

iTrader: (4)

You will want to start with a strong battery. You're not actually going to start the car but rather use the battery power (via the starter) to spin the engine in order to build the compression.

ScrappyDoo

iTrader: (7)

Make sure you disable fuel delivery or else your car will try to start. Pull the fuse, I forget which one it is on an FD.

You should put the battery on trickle charge overnight if you think it is low. Your battery is essentially the most important part of the process as it is creating the rpm.

I think a sellor will want to see consistency in pulses as well as the compression rate. I think 90 is low and you'd want to see at least 100 per pulse but hey do a quick search as I don't know the conversion rate from the Mazda machine.

BENMAN

i relocated my battery to the back with 4g wire. will that be a issue? i know the 4g wire is alittle too thin. but i never bother to change the wire because the car still always started.

BENMAN

can i just pick one of these compression tester at canadian tire?? mayb i'll try it my self tonight.

ScrappyDoo

iTrader: (7)

Personally I wouldn't let anyone else mess with my car. I would split the cost of having Mazda or someone with a Mazda tester do it. You'd get a print out that would be useful for this buyer or another.

I've also heard of horror stories where someone has gone too far into the spark plug hole causing apex seal damage when they turned the key.

I don't mean to scare you but it isn't child's play either.

BENMAN

i totally agree. i feel really uncomfortable letting some stranger touching my car. that's the reason why i want to know how to do it myself. does anyone here that has a mazda tester that can help me do this. i'm located in markham. my car isn't insured right now and i can't drive it to mazda. well let me know if anyone is free tomorrow that can help. thanks for all the replys and help guys.

thewird

iTrader: (8)

Just pull the EGI relay and remove both trailing spark plugs. Insert pressure tester in one, crank with your throttle fully down and watch or read results, then do the same in the other. It's actually really simple once you know the process.

The expected compression depends on the tester used. A basic compression tester, anything over 80 is good. A proper rotary tester and you want to see over 90 even pulses. Over 100 is a super healthy motor . The expected compression is also affected by the type of apex seals in the motor and what type of clearances were used when the motor was put together. A stock 2mm 2 piece seal motor with mazda spec clearances will generate really high compression while say a 3mm with "race clearances" will seem like the motor isn't that strong. Also, the most accurate way to do a compression test is when the motor is warm.

I have a rotary compression tester. You can use it if you buy a battery for it

thewird

BENMAN

thanks, lol i know you msg was nice and simple but it was really helpful. i was surfing the net for quite some time and you pretty much comfirmed some things for me. thanks