Oil burps

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Oil burps

Post by PeteH » Mon Dec 03, 2018 12:27 am

Mod note: I suggest Dave has a look at this, to decide if it's worth putting in FAQs, and if anything needs amending or adding.

An oil "burp" is a very dramatic event, and you will be in no doubt if you have one. The symptoms are that a huge plume of white smoke will be emitted from your exhaust, you engine will faulter and loose power, and it may continue to run roughly afterwards.

It happens when sump oil passes up the oil fill tube, through the breather system, and into the intake system. From there the oil will coat much of your intake, including the air filter, concertina tube, MAF & IAT sensors, and throttle body. The smoke is caused when the oil enters the combustion chambers, and is partially burned (and further burned in the catalyst). The initial hesitation and loss of power is caused by the excessive oil in the combustion chambers. The continued poor running is caused by the contamination of the MAF & IAT sensors, and possibly by a clogged air filter.

If you do get a burp it may have just been a random occurrence, and it may not be anything to worry about. The remedy is simply to clean out and oil in the concertina tube, replace the air filter (if contaminated), and carefully clean the MAF & IAT sensors with MAF cleaner or isopropyl alcohol (do not use anything else, because they are very delicate sensor).

If you get regular burps then you may want to fit a catch can, to try to prevent the oil from reaching the intake. This is just a "sticking plaster" though, and the root cause should be identified.

So what does cause burps? The oil has to travel quite a long way "uphill" to reach the intake, and that requires a pressure difference. That means, either the sump is pressurised, or the intake is in vacuum. In reality it may well be a combination of both. You also need the sump oil to be covering the oil fill tube. So the first thing to observe is that excessively filling the oil will be a contributor. Most owners don't fill their oil above the 3/4 mark on the dipstick. Note that the oil level tends to rise a bit in a hot engine. Also note that the oil is more likely to cover the dipstick tube when accelerating, and when going around right hand corners. But there is also a second way to get oil into the breather system; filling the oil too quickly can cause the oil to back-up into the breather tube, and flow down into the little catch can on the side of the intake. It then sits there, waiting for a bit of pressure or vacuum to force it into the intake. Early cars (pre-2006) were prone to this, but Mazda revised the breather system in 2006 to make this problem much less likely. The next point to note is that burps are only possible if the sump is pressurised. If the oil is covering the dipstick tube then the sump can't breath, and if pressure builds it can only be relieved by blowing that oil up the tube. How can the sump get pressurised? There are many theories on this. Possibilities include:
- side seal failure, causing combustion gasses to pass into the oil galleries, and into the sump.
- water seal failure, again causing combustion gases, and coolant, to get into places it shouldn't.
- normal thermal expansion of the oil and air in the sump.
- moisture in the sump (my favourite). If moisture (condensation or coolant) gets into the sump then it will sink to the bottom of the oil. As the oil gets very hot the water boils, rapidly expands, and blows the oil up and out of the sump.

Regular oil burps almost always indicates an engine coming to the end of its life. If your burps are getting more regular then it is time to start saving for a rebuild.
These users thanked the author PeteH for the post (total 3):
warpc0il (Mon Dec 03, 2018 9:52 am) • Shagrington (Mon Dec 03, 2018 4:23 pm) • mrspiller (Sat Aug 24, 2019 2:41 pm)

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