Number 4 low on compression!

[briancoates]

I t has taken a while but I finally got the Healey out of the garage, so it was time to diagnose the no compression in number 4 cylinder of the p car. A leak test showed pressure in the crankcase so I knew it was not going to be pretty. Anyway I will let the pictures tell the story.
If anyone has suggestions on what may cause this type of failure I would appreciate feedback.


were getting there be patient....




"pissed'n broke"

[ruf-porsche]

Looks like you overheated the engine at some point in time.

See Link

http://images.google.com/imgres?imgurl=http://www.smellofdeath.com/lloydy/images/piston25.jpg&imgrefurl=http://www.smellofdeath.com/lloydy/piston_diag_guide.htm&h=264&w=200&sz=13&hl=en&start=25&um=1&usg=__aqpQtEU8ZizSEE1DZ8nNcB63yto=&tbnid=DKrPBKmcgnHpQM:&tbnh=112&tbnw=85&prev=/images%3Fq%3Doverheated%2Bengine%2Bpistons%26start %3D20%26ndsp%3D20%26um%3D1%26hl%3Den%26sa%3DN

[Davz912/911]

+1 on the overheat....that hole may have leaned out....

Dave

[bigsid]

looks to me like a case of excessive clearance on rings in ringlands. there doesn't appear to me to be any seizure marks on the rest of the piston. It looks to me as if the ring started breaking apart and started destoying the piston and cyl. I can't tell, but it looks as if your circlip is gone too.

[briancoates]

Good pickup bigsid.... I removed the circlip.

I have considerable experience with engine rebuilding although this is my first Porsche experience.

A couple of points to note:
There are no signs of seizure on the rest of the piston nor on any other piston.
The combustion chamber is unusually clean when compared to the other cylinders.
The pistons are numbered although not all are fitted to the corresponding cylinder (this one is).
No sign of pre-ignition or leaning out on the crown of the piston.

Will check the ring clearance using a ring from another piston, the remainder of the ring glands show no damage or wear so this may reveal something. Although past experience with ring failure normally presents itself as damage to several positions around the cylinder. This appears to be a instant failure at one section.

Anyway as said I do not have experience with these motors, just trying to fuel ideas.

[Steve@Rennsport]

Thats caused by detonation,.....

That engine really hammered on for awhile before it failed. There are many reasons for this including lean mixtures & excessive timing, and/or bad fuel.

It really underscores the fact that one should never, ever rely on their ears as the defacto "knock sensors" to detect such occurances. These problems need to be addressed proactively by ensuring each cylinder receives the correct AFR and the timing values are correct.

[briancoates]

Thanks Steve

By detonation I pressume that is what I refer to as pre-ignition, with a failure of this type I would normally expect to see a hot spot or ignition point somewhere in the combustion area, including the spark plug itself. This combustion area does not show any signs of detonation. Although saying that I also reiterate what I have said earlier I have had no experience diagnosing these engines.... so at this stage i will go with your diagnosis. The CIS injection had been poorly assembled and many fasteners were loose so a lean condition could have been present causes by possible air leaks.
Oh well I bought the car at the right price because this type of problem was expected.

Thanks

[Grady Clay]

Brian,

The detonation that Steve is referring to occurs after ignition has started. It is not pre-ignition.

Somewhere around TDC compression an unburned part of the mixture explodes rather than burns. There can be many reasons for this, discussed elsewhere on Pelican. Common is too low octane.

The shock wave from the detonation causes the piston to deform slightly above the top compression ring. This causes the ring to no longer ‘float’ in the ring land. The ring no longer seals and combustion gasses pass the ring, overheating it. The ring breaks. A broken piece gets past the side of the piston. The damage you see is the result.

The good news is that you found a problem before the piston completely broke up damaging the case and crankshaft.

There are situations where the compression is only slightly low – say 80 psi while the rest are 125 psi. The cylinder leak may show 20% while the others are 4%. The top ring can be in multiple pieces and not (yet) broken past the piston. Everyone shood heed those warning signs.

Best,
Grady

[wowzer911]

+1, the deformity seems almost visually apparent on underside of the top lip.

[briancoates]

Thanks Grady

Good explaination, I will definately be draining the fuel tank before I start the new engine. I have only recently bought the car and have not driven it so history is unknown. There are some very slight marks on the other cylinders so it looks like I will have to replace all 6 cylinders and pistons. At over $6k Aus. it may be a while before I get to drive it, oh well it looks good in the garage.

Can these coated cylinders be bored and honed or not?

Thanks