Engine disassembly diagnosis

[adampecush]

Last year, this engine (approx. 40k miles post-rebuild) began blowing an excessive amount of smoke out of the PCV system when the routing was slightly modified (manifold vacuum source removed). A compression test revealed 165 psi from #1-5, and 120 psi in #6. Obviously there was a problem. Car idled poorly at 14:1 afr, but aside from blowing oil out the air filter, ran OK under moderate boost levels. Also, each time the plugs were changed, the #3 plug was always blacker than the rest, without good reason. I should note that the engine did not consume coolant over a span of several years and multiple autocross events. There were, however, some "coolant pouring out the overflow" incidents way back, attributed at the time to poor burping.

Anyhow, disassembly of the engine immediately revealed the cause of the low compression on 6, a cracked ringland, which was suspected. FYI, pistons are OEM toyota, 0.020 overbore:

Sadly, close examination of the head revealed the cause of the odd spark plug color in #3. It also explained the odd cooling system behaviour observed ~12k miles ago. Strange that this cylinder gave good compression results given the crack. Looks like new head time.

For reference, here are pictures of the head and block. Lots of carbon from all the oil entering the engine. In #1, a champion spark plug lost its insulator - there was a surprising amount of damage to the head and piston from this. Not so strangely enough, every little oddity over the years showed itself when I tore the engine down.

 

[suprarx7nut]

Very neat examples of what can cause various symptoms. Also good example that passing certain tests (ie compression) doesn't always mean all is well.

Thanks for sharing!

Sent from my ADR6400L using Tapatalk 2

 

[Dan_Gyoba]

That crack in #3 is scary. I guess that definitely proves that a compression test won't necessarily show you that things have gone awry. I imagine that there are BHGs out there that aren't as big as that crack.

 

[Poodles]

The crack in the head is pretty interesting as it looks like it might be from the valve seat. The damage around the headbolt holes in the head is odd as well...

Broken ringland looks scary, did it damage the cylinder wall?

Kinda sad to see since I know you did an amazing job on that entire car.

 

[adampecush]

I always thought something might have been up with this engine...this just confirms it. Still impressive that it made good power given the difficulties. This just gives a good excuse to upgrade to a forged bottom end and built head. Poodles, the walls of #6 are still in pretty good shape, no scoring and cross-hatching still visible for the most part. Hopefully I can get away with a hone and go back to 0.020" over pistons. I also have no idea what the hell was happening around those head bolt holes - all I can think of is galvanic corrosion from trapped water or coolant.

 

[djshoester]

Wow that crack looks very scary.
Is that crack only affecting the head? and not the head gasket?

I apologize if this is a amateur question since I do not know very much yet. Just trying to learn as much as I can.

 

[adampecush]

Crack was in the head. Looking back, the crack was likely the reason that coolant was blown out the overflow a couple of times. It seems as though combustion products (carbon) served to plug the crack and prevent further leaking. Head gasket was intact by the crack and the warm compression of that cylinder was good when last measured.

 

[91Supra313]

Let me know if you need a head. I still have my built 7MGTE sitting together in the driveway. I want to get rid of it to fund this 2J swap I'm undergoing. I can ship it bare or assembled. I would just split it from the block, box it up and send it off. PM me if you need one. Mine was rebuilt around 3k ago, and had no issues at all holding 18psi from a T70. Has an HKS stopper under it.

 

[radiod]

Exactly what happened to my first head . I was lucky enough to find a bare head from a Cressida off eBay for $120 in AMAZING condition. Got the new head hardness tested and came back good, so I swapped everything over and all was good. I wish I got pictures of my dead head, didn't have a camera when it was at the machine shop unfortunately