Pele

iTrader: (28)

When would an engine likely have higher numbers? Warm or cold?

I'm guessing cold.

I'm basing this on the fact that hot start problems can be due to low compression. Therefore if the engine starts better in the cold it must have better compression then.

Can anyone confirm?

GoodfellaFD3S

iTrader: (213)

you are correct.

let the engine get up to operating temp, shut it down and let it sit for 5 mins to get nice and warm. Make sure you floor the throttle during testing, and remember spec cranking rpm is at 250.

Pele

iTrader: (28)

So if one wanted to mislead buyers, they'd do the compression check cold.
Hence I should always pack my compression tester when checking out cars.

GoodfellaFD3S

iTrader: (213)

Verdad.

Pele

iTrader: (28)

Is there a graph available that relates temperature to compression numbers?
I know I've seen the graph that'll let you correct for slower or faster cranking RPM.

TwistedRotors

iTrader: (3)

I haven't seen one, though I have seen a graph showing how to compensate for altitude while checking compression. All the Mazda instructions I've read insist that you completely warm up the engine prior to checking compression.
-John

Pele

iTrader: (28)

^

The Mazda instructions should come with burn ointment then.
I warm it up with a drive around the block or a couple minutes of idling... Just into the normal range of the temp gauge... But not fully up to the third of the way up where it normally resides... Upper radiator hose is hot indicating that the thermostat is open.

I just don't want to be misleading people.

BTW, still loving that compression tester.

TwistedRotors

iTrader: (3)

Ain't that the truth...I despise checking the compression on the FD, doubly so when it's hot.

Glad to hear you're liking your tester!

-John

trochoid

iTrader: (6)

Graph relating compression results to cranking rpms is in the FSM. I don't recall if there is a temperature gradient in there or not.