Well I went and broke my Miata

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Yeah, Vincent I do

Quote:

Charlie, you do know what 'non-interference' means right? :wink:

What intrigued me was the possibility of reving the engine so fast that the connecting rods actually stretched a few to many thousandths of an inch. I would think that the rpm required to do that would be in excess of 10K, but what the heck....inquiring minds want to know. When forces exceed the design parameters, things move in unexpected ways.

And yes, air still escapes from holes :lol:

Charlie G

Andrew Jonell · **Joined:** Mon Mar 23, 2009 6:43 pm **Posts:** 1350

Carbon build up would seem to be pretty premature for the engine, though... however, who knows how the PO took care of it?

IIRC, it was pulled out of a 2001 with just around 60k +/- 3k miles on it.

JamesMilko · **Posted:** Thu Mar 03, 2011 9:27 pm

Finally got around to a compression test.

1 - 180
2 - 120
3 - 75
4 - 180

I don't have the equipment to do a leakdown test so next update should be pictures of something horrifically broken.

JamesShort · **Posted:** Thu Mar 03, 2011 10:26 pm

Unfortunately with numbers like that, you don't need a leak down test

.

Did you put oil in the 75 psi cylinder to see if it improved any to confirm rings?

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

JamesMilko wrote:

Finally got around to a compression test.

1 - 180
2 - 120
3 - 75
4 - 180

I don't have the equipment to do a leakdown test so next update should be pictures of something horrifically broken.

I'll put $5 on head gasket... 2 adjacent cylinders with a problem...

Now to hijack this thread.... how do you explain a motor with compression readings of

1 - 50
2 - 25
3 - 50
4 - 60

Valvetrain appears intact, so I doubt it's valve seats.

DickRasmussen · **Posted:** Fri Mar 04, 2011 8:33 am

MikeWhitney wrote:

JamesMilko wrote:

Finally got around to a compression test.

1 - 180
2 - 120
3 - 75
4 - 180

I don't have the equipment to do a leakdown test so next update should be pictures of something horrifically broken.

I'll put $5 on head gasket... 2 adjacent cylinders with a problem...

Now to hijack this thread.... how do you explain a motor with compression readings of

1 - 50
2 - 25
3 - 50
4 - 60

Valvetrain appears intact, so I doubt it's valve seats.

Details? You must be "hiding" something since you know at least as much about the possible causes as any of the rest of us. :lol:

Regarding the head gasket theory for the Miata, can it blow part of the gasket without getting a water leak into the cylinders? It doesn't look like he had that.

JamesMilko · **Posted:** Fri Mar 04, 2011 9:18 am

It's possible but I'm not holding out much hope. I'm thinking that the oil fouling in 1&4 is sealing the rings and bringing the numbers up.

It seems possible to have a cyl-cyl blow in a Miata HG. They look like http://i.imgur.com/lUO6F.jpg

Charlie Guthrie · **Joined:** Thu Aug 28, 2003 8:14 pm **Posts:** 832

Replying to Mike's hijacking threat. The Dirtmaster 1600 had an almost identical problem with compression on all cylinders. The problem was so bad that we decided to pressurize a cylinder to see if air was just blowing out the exhaust or inlet. With all plugs removed we applied air to the #2 cylinder and we noticed a breeze coming from another plug hole.
The problem is that the car ingested a load of sand when the air filter loosened. The grit lapped the valves into the head so far that the valve lash went to zero, and the valves refused to seal. The mystifying part of the diagnosis is that we drove the car onto the trailer and shut it down. It just didn't have enough compression to start at cranking speed. It ran well enough to get onto the trailer, but that was the last time it started.
I believe that the new owners of the Dirtmaster 1600, Jasper and his dad Jerome, are rejuvenating the car.
Charlie G

JamesMilko · **Posted:** Fri Mar 04, 2011 8:07 pm

Ran a wet compression test and #3 came up to 90. #2 held at 120. Finally got Andrew's car back together and out of my garage so I can start tearing into this motor soon.

DickRasmussen · **Posted:** Fri Mar 04, 2011 8:34 pm

I stopped by Dennis Shaw's shop today (parts for my FF engine) and asked about your problem with the closed plug gaps. He showed me a piston out of a Spec Miata where the engine went lean due to a leak in the air inlet plastic tube at the bellows. The outer portion of the piston was melted but the piston looked pretty good in the middle. Dennis said the molten metal would/could have been in the combustion chamber and would/could have closed the plug.

The tube had been bent out of the way during a thermostat change and apparently cracked in the bellows area.

JamesMilko · **Posted:** Sat Mar 05, 2011 1:55 pm

This x4.

I guess I'm in contention for the burnt piston award now being as my current count is 4. Need to spend some time looking at the exhaust valves, at least 2 are burnt.

DickRasmussen · **Posted:** Sat Mar 05, 2011 4:09 pm

Yuk. Much worse than the piston from the Spec Miata. :cry:

Is this a boosted engine?

JamesMilko · **Posted:** Sat Mar 05, 2011 4:18 pm

Nope. Managed that NA.

JohnByers · **Posted:** Sat Mar 05, 2011 9:33 pm

This is absolutely a new piston failure on me. Never seen one fail on the edges like that.

<--- going to be checking failure causes CLOSELY on the brandy new engine in Frank... Don't want any of that happening this season.

Sorry to hear about the bad luck James

JamesMilko · **Posted:** Mon Mar 07, 2011 10:17 am

Does anyone have the tool or a tool that works to remove the Miata valve retainers? They are recessed into the head casting so most normal tools won't work.

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Well I went and broke my Miata

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