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I just lost an (old, 230K miles) engine with no compression in two cylinders. It had been drinking oil liberally for a while, but just prior to this failure, had begun seriously drinking oil (6QT per tank of gas). They said that a likely failure was burnt valves, but since there was no compression, there was no reason to investigate further than the compression test.

I had not done well in changing oil for a while, essentially going downhill after it had been leaking/burning/whatever more than 6 qts between oil changes. Arbitrary, unwise, cringe-worthy, to let an engine go without oil changes for too long but there it is.

SO, my question is this; assuming it was burnt valves (50/50 shot, given that the engine had been seriously drinking oil), at what point of oil burning does it become a lost cause and likely to harm the engine seriously just because of the oil burning? And at what point should I have tried to figure out if my valve seals were OK to prevent burning my valves and prolong the gradual slide of performance and oil usage increases?

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    I don't think I'd ever equate burnt valves with an engine drinking oil. The only thing valve related is if you have bad valve seals and shot valve guides, but with those the valves can still be seating/sealing correctly. Just tends to pump oil into the engine, is all. 6qts per fill up is a LOT of oil, though. Has to be going somewhere. If it's burning that much oil, you'd be seeing plumes of smoke coming out the back end of the vehicle. It would be VERY noticeable.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 19, 2017 at 17:48
  • What would I watch for to prevent burnt valves? It sounds preventable if you're paying attention. And yes, with a plugged Catalytic converter, there was a lot of smoke, but some sign of leaks, just not 6 qts leaking... at 230K miles, it didn't make a lot of sense to get aggressive about the oil loss as I'd somewhat outgrown the car as well...
    – BenPen
    Sep 19, 2017 at 17:55
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    It's hard to say. The way valves become burnt is by exhaust gases leaking past (valve not sealing). It can occur due to a buildup of carbon on the sealing surface, or if the valve is sticking (not closing as fast as it should). There's no real way to prevent a burnt valve other than proper maintenance.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Sep 19, 2017 at 18:00
  • Hah, sounds like an answer given to a serial engine abuser. I'll have to do better with my next car. It sounds like the dead Catalytic converter is a sign of impending doom I didn't act on, though without taking the head off, there probably wasn't anything to be done, and I'd still probably have an oil burner... (That there was a cascade of bad signs reminds me a bit of a multiple system failure at the end of an old person's life. One thing leads to another, to another and it all snowballs out of control, even if the pieces used to be under control...)
    – BenPen
    Sep 19, 2017 at 18:11
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    You asked at what point you "should have tried to figure out" what was going on. That was back at 1 quart between fill-ups.
    – CharlieRB
    Sep 19, 2017 at 19:38

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Knowing that a Ford engine has gone 1.1 million miles with proper care; why subscribe to letting you think something other than performing maintenance might lead to a known point where once damage has occurred: one might get the vehicle back into a fully functional operation short of correcting all the damage to the point of all specifications restored. Obviously I don't concur with negligent auto care. But to your point, that kind of oil consumption means the rings are worn out, the seals then had oil being pushed out them from excessive blow by from the rings. And following that the valves got oily and started caking carbon, losing compression further. If you want to conserve an engine; never overheat it.

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