Engine going out

[johnl]

Though that is a real possibility, I would just treat it like Sea-Foam and only run it for 30 miles and then do another oil change with regular oil.

I have heard too many anecdotal stories related to highly detergent oils (etc.) used in very dirty engines to consider using them unless the engine is at least already reasonably clean(ish). Having said that, the cleaning affect is likely to happen reasonably slowly, so you may well have no problems if you change the oil and filter at much shorter intervals for the first few changes, maybe. This may not be the case for more 'aggressive' cleaning agents...

Something that keeps coming to mind is when the gunk is free from the rings, what's to say you won't lose compression because the carbon was possibly keeping it sealed at this point.

Carbon (etc.) won't itself seal the rings, or assist sealing in any way. For the rings to seal they need to be at least reasonably clean (or rather, the ring grooves need to be clean). Carbon and varnishy deposits can prevent the rings from moving, so they can't push outward against the bore, and / or obstruct gas pressure from getting behind the rings, which can also cause a poor ring seal. The rings seal at the ring edge (at the outer circumference), and at the lower face of the ring, where it bears against the upper face of the ring land (the seal on the ring land at the piston is at least as important as the seal at the ring edge at the bore).

The 'spring tension' of the rings isn't what creates the 'working' seal, it only creates the initial conditions for the ring to seal properly. The compression and combustion pressures inside the cylinder push the ring downward so that it bears against the upper land face, and gets behind the ring and pushes it outward against the bore wall, with more force than the ring tension alone can do. If there is no initial outward pressure from ring 'tension' (or at least too little), then the initial conditions for gas pressure to get behind the ring and push it outward (harder) don't exist, because too much compression pressure escapes past the ring as compression starts to rise (or tries to...).

If the ring is 'stuck' in the groove then the spring tension can't push it outward, and the gas pressures also can't push it outward. The ring then leaks pressure around it's complete circumference. Even if the ring isn't stuck 'hard' in the groove, if there is enough carbon build up in the ring groove then it can fill the space behind the ring (or at least prevent gas pressure from getting into the space behind the ring), and thus gas pressure can't 'get in' and force the ring outward.

It's possible for an obstructed pathway (for gas pressure to get behind the ring) to cause the pressure behind the ring to be less than the pressure in the combustion chamber (even if less than it should be), in which case the higher pressure within the cylinder will force it's way between the ring and bore, causing the ring to compress into the groove. If this occurs then it will open a pathway for compression and combustion pressure to by pass the ring entirely...

The rings rely upon good contact with the bore walls in order to get rid of heat. If the rings (which pick up substantial heat from direct exposure to combustion and from the piston) have poor contact with the (cool) bore then they can overheat, which can cause them to become annealed. If this occurs then the rings lose their spring tension and so even if the initial cause of the poor bore contact is removed (i.e. gunk cleaned by whatever means), the rings will still seal poorly and lose compression. This is because compression pressure will escape before it has a chance to force the rings outward, even if the rings were 'free' enough for combustion pressure to do this (which it may not be, depending on how stuck the rings may be).

Note that the rings are also an important pathway for the pistons themselves to lose heat into the cooler bore metal (and from there into the water jacket). Poor ring to bore contact will cause raised piston temperature, which can potentially cause damaged pistons and other problems such as detonation and (much worse) pre-ignition from carbon deposits on the piston crown becoming hot enough to glow red hot and prematurely ignite the fuel, which can cause very serious engine damage.

Note also that rings leak around the entire circumference, and not because of increased ring gaps (as many people assume). Even substantially larger than spec ring gaps leak very little pressure in the scheme of things, the gap is just too small for a significant amount of gas to pass through in the very short time available at engine running speeds (and even less so as rpm rise).