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Old 01-27-2012, 04:02 PM   #1
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Default 91 Miata low compression help

Hey guys, I am new to the miata game . Just picked up a turbo'd 91 miata with a 1.6 and 230k on the chassis. The seller claims it has 10k miles on a rebuild with all oem parts but has no receipts to prove it. I got it for cheap due to the bad compression test. I am pretty mechanically inclined but have only been working on cars for about a year now so I am hoping I can turn to this forum for good advice.

Here are the numbers:

Cylinder: 1, 2, 3, 4
DRY: 70, 110, 75, 110
WET: 80, 115, 80, 115


If it was a bad head gasket then two adjacent cylinder should be lower than the rest correct?
Also if the piston rings were bad there would be a much greater gap between wet and dry?

So now I am thinking the valves need to be re-adjusted or the cylinder head is warped and the head gasket isn't sealed properly. Also there is some clicking in the head at idle which leads me to think the problem is mainly the valves.


Any input from you guys would be great and thanks in advance!
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Old 01-27-2012, 04:11 PM   #2
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All '90-'97 Miatas use hydraulic cam followers, so there is no adjusting to be done.

To varying extents, all of the early Miatas also exhibit a lot of valvetrain noise. (my '92 used to sound like a diesel at idle.) Over the years, there has been a lot speculated and written about the causes and remedies of this, but it essentially boils down to a crappy design. However, we usually see cam followers which fail to pump up fully or bleed down too quickly (causing too little valve lift) rather than followers which over-extend and prevent valves from fully closing.

A leakdown tester will be helpful here. If you've never used one, it's a device that screws into a plug hole like a compression gauge, but it uses an external source of compressed air and is applied with the engine fully stopped.

With this device, you can run the suspected cylinder up to TDC, apply the air, and observe various points on the engine to determine where the failure is. If you have air coming out of the oil filler hole, it's probably rings. If you have bubbles and/or coolant coming up out of the radiator cap (with the cap removed) then it's probably the head gasket. If you can hear / feel air moving out from the throttle body (inlet tube removed and throttle open) or the tailpipe, then you've got valve problems.
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Old 01-27-2012, 04:17 PM   #3
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Quote:
Originally Posted by Joe Perez View Post
All '90-'97 Miatas use hydraulic cam followers, so there is no adjusting to be done.

To varying extents, all of the early Miatas also exhibit a lot of valvetrain noise. (my '92 used to sound like a diesel at idle.) Over the years, there has been a lot speculated and written about the causes and remedies of this, but it essentially boils down to a crappy design. However, we usually see cam followers which fail to pump up fully or bleed down too quickly (causing too little valve lift) rather than followers which over-extend and prevent valves from fully closing.

A leakdown tester will be helpful here. If you've never used one, it's a device that screws into a plug hole like a compression gauge, but it uses an external source of compressed air and is applied with the engine fully stopped.

With this device, you can run the suspected cylinder up to TDC, apply the air, and observe various points on the engine to determine where the failure is. If you have air coming out of the oil filler hole, it's probably rings. If you have bubbles and/or coolant coming up out of the radiator cap (with the cap removed) then it's probably the head gasket. If you can hear / feel air moving out from the throttle body (inlet tube removed and throttle open) or the tailpipe, then you've got valve problems.
Thanks for the quick response. I have been trying to find someone with an air compressor so I can run the leak down test. I will post the results when I find one or just give up and take it to a shop.
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Old 01-27-2012, 05:20 PM   #4
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What do the plugs look like?They'll give you a good indication of some problems.
You got a vaccuum gauge?That'll help also.
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Old 01-27-2012, 09:21 PM   #5
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What do the plugs look like?They'll give you a good indication of some problems.
You got a vaccuum gauge?That'll help also.
Pugs look fine. No burnt oil or anything. However they also look fairly new so they might not be a good indicator. I do have a vacuum gauge. What should I be reading at idle? I haven't drove it around much because of this problem so I don't know off the top of my head what psi vacuum its at.
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