2nd Generation Specific 1986-1992 Discussion

Compression Test

Old 05-15-2004, 07:58 PM
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WHen you do a compression test, does anything have to be put on? Manifolds, injectors, cooling system? Also when trying to start, it makes a thunk noise when turning the key for just a second. This is an 87 TII.
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Old 05-15-2004, 09:15 PM
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Your suppose to do it with a fully dressed engine. If it is not, it can throw the reading off be giving a higher number because it will be able to suck more air in to the housings. You can get a rough estimate. I also think that Mazdas numbers are with the TB wide open. I may be backwards.



Thunking noises are gererally not a good thing. Is this when you are trying to do a compression test. If so, it is normal to get some kinda thunk or a noise.
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Old 05-15-2004, 09:18 PM
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Wel, when just starting to crank it, it kind of makes a small jerk or thunk. ANd is the compression tester for the lower or upper spark plug hole?
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Old 05-15-2004, 09:26 PM
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Leading spark plug, but I dont really think it hurts where you do it.
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Old 05-15-2004, 09:28 PM
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I was getting some really low numbers, but then forgot WOT. Waiting for daylight to try again. Would a loose starter make a clunk noise? I don't think it is loose but it might be.
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Old 05-15-2004, 09:34 PM
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I really cant tell you what it can be without hearing it. Since it only happens when you start the car, most likely it is the starter, or something that is loose that can flap around when the torque of the starter is applied.
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Old 05-15-2004, 09:37 PM
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So what would all the things that affect the compression test? oil,fuel,?
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Old 05-16-2004, 11:34 AM
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Fuel will not affect the compression. You should take your EFI fuse out when doing a compression test so you dont get gas inside your gauge and so you dont waste it. Oil will affect the results +/- a fraction of a psi due to the need for oil pressure. There is always some oil in there. If you use a thicker oil, your compression can be a little higher because it can create a better seal.



The real things that would affect the results would be what altitude you are at,how much air you are allowing the engine to take in, and the temperature of the charge going in to the engine, besides the whole apex seals and side seals being good. All of these, except so seals, will throw the results off by a fraction, just not enough to really worry about it.



If you really want to know, you can search around for the correct equations to figure it all it, but its not really worth it since most compression testers give the results in increments of 5 anyways, unless you go digital.
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Old 05-16-2004, 03:50 PM
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I had just dropped my engine back in on the tranny and mounts, and I wanted to see the compression before I had put all the gasket material(Messy),and hook everything back together. I just did a reading and it gave me some pretty low numbers, but it seemed strong before I took it out.
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Old 05-18-2004, 06:03 PM
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help me?
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