RotaryResurrection

iTrader: (8)

You know, actualy, with a rotary anyway, you get better compression when COLD. IN my experience, anyway. You tend to lose 5-15psi when the engine warms up to op. temp. This is why a lot of worn out rotaries crank right up when cold (more compression) and flood/hardstart when warm (less compression).

RotorJoe

That is the exact opposite from what I have heard and seen.

RotaryResurrection

iTrader: (8)

The mazda compression tester doesnt lie.

RotorJoe

Ok, I believe you. I just have seen people start dead engines with ATF. Before they put ATF in the engine had hardly any compression. They were using an modified normal one, so it was tough to see the exact compression. After ATF the compression numbers increased. Once the engine started and warmed up they were able to let it die or shut it down and restart it. Yes it was hard to restart but it had more compression then when it was cold and was able to fire on its own. Although it didn't last long.

su_maverick

I am going to have to have them do the test again. When I took it in they did the test before they fixed anything. They replaced the o-rings, a crack in the intake tube, and a fuel leak.....would those make a difference?
btw....still trying to find the boost prob.

whitekingsnake

a cold engine has higher compression.....NO

whitekingsnake

your mazda tool must not be working too well.

Nathan Kwok

Yes they do. This has been tested, I still have the post from the "big list" saved from the guy who did it. This is also what Mazdatrix told me and they've done quite a few compression tests in their day. In any case, the proper procedure is to test the engine at operating temperature, meaning you should warm up the engine first, then test it.

*Edit: dug out the old post, in this example we see about a 20psi increase when doing the test cold. Admittedly, there is more mileage during the hot test, but it was done at the same altitude and barely a month later:

quote:
I had them do the compression test twice cause the dealer screwed it
up-go figure! Anyhow the first test (2/14/01, 61,678 miles, engine cold)
was-
Rotor 1; 9.1, 8.2, 9.1
Rotor 2; 9.0, 9.1, 8.4
Second test, (3/21/01, 66,312 miles) after I complained about the test
procedure (showed them their own Mazda manual on how to do it) and made
them do the test hot-
Rotor 1; 7.9, 8.1, 7.2
Rotor 2; 8.3, 8.0, 7.1

RotaryResurrection

iTrader: (8)

Now what.

RotaryResurrection

iTrader: (8)

I have NEVER seen a higher compresison number when hot. To check this again, I tested a car I just rebuilt, an 88 nonturbo. IT has 7 miles, and is still a bit weak on compression.

Test performed using mazda digital tester, trailing plugs out, using trailing hole for test.

When cold (sitting overnight) @ 240rpm
7.5, 7.3, 7.5, 7.4, 7.2, 7.4

Ran 15 minutes out on the road, immediately pulled plugs and rechecked warm @ 275rpm
7.1, 6.9, 7.1, 7.0, 6.9, 7.0

Had the cold test cranked @ 275rpm, it probably would have bumped to 7.7's. So, we can say that it lost about 10psi compression when warm.

This backs up my theory about rotaries and hard warm starting. When warm, they lose compression, which can sometimes cause cranking and starting problems. Anyone who's ever dealt with an old FC or a really tired FD can tell you that they crank right up when cold, but have trouble (or won't at all) when warm. BEcause the lose some compression, enough to cause a problem.