High oil consumption

I would start with replacing the pcv valve, after that I think oil control rings would be next on my list. Good compression is often mistaken for good rings, but to much oil round the rings seals them and gives a misleading result.

Did you test compression by spinning the engine or by a compression leak down test?

 

Difficult to read the actual reading on that chart, but it looks like average 27bar or 400psi that suggest an engine spinning test on a Prius. The readings are even a good sign for the valves but as I said before a compression test does not really help with oil control rings. I would change the PCV valve (not expensive) and take it from there.

 

Try put a can or Rislone engine treatment in your oil, then drive at least 2000 miles or 3200 km before you change the oil. I tried this method on my 02 Acura at 140k miles & it did help a lot because the Rislone will free-up the oil rings if they get stuck, if that's the case.

 

I had a honda long ago that had great compression but used a lot of oil. I replaced the valve stem seals and problem fixed. I find it a little odd they replaced the valve guides and seals twice, and valves once. Replacing these is not a trivial task on an engine like this with the shimless bucket lifters, and must be done precisely. You may need to get a whole rebuilt engine, unless a mechanic can do all the tests to pinpoint the problem better.

 

My guess is that the root cause was replacement of the valves, valve guides and seals.

Since you say that the valve guides and seals were replaced for the second time, and the oil consumption was normal for the first 1,000 km after that work, that tells me there is nothing wrong with the piston rings or PCV valve. (However it doesn't hurt to replace the PCV valve.)

I agree that the quality of the machine shop work is very important. Obviously if the valve stem clearance to the valve guide is not perfect, the engine will consume oil.

It may be necessary to purchase a remanufactured cylinder head from Toyota, if you are serious about solving the problem. Hopefully the reman head will have been reworked more precisely compared to the work performed by your mechanic.

 

Patrick I feel that at 150,000 miles it would not be practical to fit a new head, surely it would be better and cheaper to fit a complete engine from a low mileage wreck.

If a new head was contemplated before purchasing, the old one should be removed and the pistons taken out for inspection of rings and bores, after all once the head is removed not a lot of work is required to drop the sump and undo the big ends. It would also be prudent to replace the bearing shells upon replacement.

John.

 

Maybe, it depends upon the labor cost to remove and replace the engine and transaxle, vs. just removing and replacing the cylinder head.

The least costly alternative might be to purchase a used cylinder head coming from an low mileage engine and have that installed.