Burning Oil on Decel
 

About 6 months ago I put on a junkyard 97 head onto my 94 block cause I had cracked my 94 head. Ever since the head "swap" I've noticed an oily smell on decel/high vacuum. I didn't think much of it as I was anally attending to everything and checking oil/topping it up every day so I never realized how much it was burning off. That or its become worse. Couple days ago, after 900ish miles of driving after an oil change, I was down a little over a quart. More recently I've noticed smoke out the back of the exhaust (see video). I never looked for smoke before so I'm not sure if it was smoking a few months ago or not.

PCV is the GTX Mazda unit. I have resealed the valve cover baffles for the PCV system as well. It seems to point to valve seals or the guides right? I just replaced the stem seals about a month ago (FelPro) to see if the oil smell on decel would go away but it didn't help. Are the valve guides to blame? Rings??? The head had about 124k miles on it when I pulled it off the junker and the block had a hole in it :jerkit:. Maybe PO had ran the car low on oil and wrecked the guides? The last compression test (about 2k miles ago) read 150 dry 180 wet across all cylinders. I might do another one soon. Should I do a leakdown too?

I have clips of WOT runs in there too to show it doesn't smoke when on throttle and what not. Flames too :giggle:

EDIT: Stock 1994 bottom end with 171k miles, stock 97 head now with ~127k miles. I ran 5W-30 Supertech for a bit, then Mobil1 5W-30 when I realized I was burning oil. I switched to T6 as of last night and these video clips are me running T6.

Get OEM valve seals and do it again. Fel-Pro seals are crap.

I know oem is preferred but it seems like people have had good luck with the FelPros.
https://www.miataturbo.net/general-m...-seals-100002/
https://www.miataturbo.net/general-m...ls-best-95838/

Maybe they are bunk, I'll probably wait for some more opinions before I go buy some $130 stem seals.

I bought a "rebuilt" head through an ebay cylinder head exchange service. After finding all sorts of issues including a valve stem seal deformed by the 12 point socket used to install it I installed fel pro seals. I still had classic stem seal issues like yourself. I put in OEM seals and saw no noticeable improvement. I'm now confident the guides in my "rebuilt" head are shot and I'd wager you have the same issue.

Yeah, the other threads mentioned that worn valve guides becoming more of an issue now that the heads have been used longer. I wonder if that's it then. Is there any way to check for worn guides with the head on the car? Wiggle the valve around or something?

You'd have to pull the spring and drop the valve a little to wiggle it around a bit. Definitely not a scientific test. To definitely diagnose it you'd have to pull the head and measure it.

All signs point to guides IMO.

Bummer, well I guess I'll get back on the grind soon. Replacing the guides kind of warrants a whole head rebuild right? As in they have to regrind the valve seats and requires full disassembly of the head.