evilfocus

would a near-dead battery cause low compression? my battery died and i recharged it. it seems to crank fine, but drives with no power at all. i done a compression check with a regular comp. gauge, but held in the valve. my readings only came up at only about 35 psi per face on both rotors. each rotor seems to be exactly the same. any possibility of internal engine damage?

btw- its an 88 gxl

classicauto

iTrader: (3)

well if you got numbers that low and you're confident the readings were accurate then you need a rebuild...


But a low battery WOULD give you low readings - if its not cranking over with the proper speed....however if it ACTUALLY has low compression - the battery is not the cause

evilfocus

this is a rebuilt motor with about 1800 miles on it. if it were blown, would it read evenly across both rotors?

evilfocus

i just done some more searching and found a few people saying the throttle has to be all the way open when testing compression. is this true? would this explain the low numbers?

Archangels

yes, you need to have it at WOT for a proper reading if i remember correctly....

evilfocus

would it drop to 35 psi though?

HAILERS

True to some extent.

What is the compression if you leave the relief valve in the tester? As in don't crack it open?

evilfocus

a little over 100 psi

HAILERS

I'm betting you have a compression gauge that has a valve on the side of it, and you've been depressing that valve while doing the check.

If so, that won't work. You need to remove the valve that is in the end of the compression gauge (if it has one there), then do the check over again and not press the relief valve on the side of the gauge.

Make sense? That way each time a rotor face comes up, the gauge pressure will peak and then immediatly drop off til the next rotor face comes up ....and so on.

IF you depress the relief valve on the side you'll never see much more than the thirty you saw before.

If you gauge does not have a valve to remove at the bottom ...........I'm not sure what to do. Although the 100 sounds alright but you don't know if it's one face that has 100 and the others maybe 75 or whatever. Although if you saw the bounces were all about the same when you did this before, then it's probably ok. Anyway, look for a schrader valve in the bottom of the gauge where it enters the sparkplug hole.

EDIT: Since its not a series four turbo, that compression isn't all that good. A series four turbo has a lower compression ratio so they'll read a bit lower. On your engine I'd expect closer to 110 for a so,so engine.

hanman

Hailers, how good would 95 lbs be on a S4 turbo engine? Good enough to run for a few years before a rebuild?

HAILERS

Yes, but nobody really KNOWS because the apex seals wear down and just turnover when they get too worn, with no real warning. If it's an original engine you might just watch out. If it's a rebuild it could live a long time. Rebuilds do not ever get to stock/new compression, in my humble opinion. New rotor housings are the way to go. Just a humble opinion.

hanman

Yea, I agree with you on the new housings, but christ Mazda is awfully proud of them. $$$
Have you ever had any experience with the cermet coatings? There is a guy here in OKC building engines that likes them.

LAN

iTrader: (1)

That's good !
It can only run for a day, a week or so if you go crazy on it...
Other than that, it should last you good depending on your maintenance...
You're wondering how I know...from experience. 85PSI.S4TURBO.

Boostmaniac

A little over 100 psi is a fair reading with the relief valve in. If it was bouncing even 35's across all faces of both rotors with the valve out, that is also a good reading.