Expert: Sean Halliday - 2/8/2009

Question
QUESTION: Hi, This truck has 150, on it. I just had one of the heads replaced, as it was cracked. It has never used any oil. Since the head replacement, the truck is losing a lot of oil, 2 quarts per 100 miles. It runs fine,no loss of power and it has no smoke . There are no drips or drops in the driveway, and no oil coming from the exhaust. my mechanic is completely baffled. Any suggestions would be appreciated.

ANSWER: Its going somewhere if its using that much.Its either being burned off mixed with the coolant(check coolant for oil) or burning by friction/heat and evaporating thru the breather.It may be there is too much crank pressure and it will blow oil vapor when hot out the breather and the cause of this is the valves/timing may be out of adjustment or when a engine such as yours has a new head fitted,the pressure increases on the crank and big end bearings increasing crank pressure.Other possible causes are breather blocked or pcv valve blocked or fitted wrongly.Ask your mechanic to test the crank pressure when the engine is hot.If it is high,test the filler cap also.I am assuming all the breather hoses and vacuum were replaced correctly.

---------- FOLLOW-UP ----------

QUESTION: Okay, so the same mechanic has now replaced the head twice more. He says the first remanufactured head had the wrong valve guides or seals (or maybe was the wrong head!).  The next reman head was cracked also and he has now replaced it again from a different reman outfit.  It is still losing oil , about a quart every 200 miles. I think this guy is trying to do right as I have not paid for anything but the first repair, but it seems obvious he doesn't know where else to look.  My thinking is that the first "wrong" head put too much pressure on the lower engine, and that's where the oil is going now. I don't know if he's run a compression test yet.  I also know that he implied at one point that  when he put the new head on, because the engine had a lot of miles, it may  have caused the lower engine to leak. This truck never used any oil before the head was replaced.

Thanks!

Answer
I would agree that the upper rebuild can put pressure on the lower end and its nearly best to rebuild the complete engine rather than the upper.It may well be that the upper repair has increased the pressure in the crankcase to where it is too much for old worn rings and hence it will burn the oil at a faster rate.My advice is to stop what he intends to do and replace the engine.With a used one.This engine will never stop losing oil and may in time run the crank dry and seize the lower half as i have seen this happen.The Mazda is not a good engine to rebuild and does not do well with a half built engine.