Bought a used 94 xj with a 4.0, had a few minor problems that i fixed. Started blowing blue/white smoke after warming up at idle so i decided to have the head rebuilt because its old and has some miles on it and figured it was the valve seals anyway. Finished last weekend and the problem still persists. Runs great, compression is great, no exhaust in the coolant etc. pretty stumped on what it could be. I was holding out hope it was just oil inside the exhaust pipe but ive put some miles on it with no improvement. No oil on the spark plugs, i dont know how long it sat maybe it has a stuck oil ring on a piston?
any help would be appreciated.

 

Well, as you would guess blue smoke is usually burnt oil so...It may not be bad enough yet to show any real symptoms...Throw some fuel /oil treatment in it, monitor and change the oil and hope for the best.. Seafoam, Berrymans, Marvel mystery oil, etc... will help clean it out if it's just an issue of it sitting for an extended time...Also, things to check :PCV valve,crankcase vent line and related system...Anything that would have the ability to recycle the engine oil into the combustion chamber...

 

I forgot to mention i also checked the pcv route as well. Unfortunately you may be right that its just not bad enough yet to do anything but smoke. Its just strange that its when it gets warm, and not when its cold. I would expect if it was the rings it would be the opposite. I might experiment with some heavier oil for now im certainly not looking forward to doing any piston work after just replacing the head. Ive seen quite a few older threads with this same problem but no solutions have ever shown up.

 

If it was carboned up and sat for a while..the rings could be sticky or stuck.

If it were mine I'd get it fully up to runing temperature then immediately dump a couple tablespoon of Seafoam down each of the spark plugs holes and let it work overnight.

That and I'd double check and double verify it has the right dipstick...Don't ask.

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I had a slant six once that sat for decades and smoked like a chimney. It stopped after a couple hundred miles.

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Heres to hoping lol

 

Also depending on how long the seals were bad the exhaust could be full of caked on oil that will take a while to clear

 

Yeah if the head had new valve seals that usually would take care of it. I like the sea foam and junk in the exhaust that needs to burn out theory