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Tuesday, May 6, 2014

Blow-by: not just how you pass someone...

Hey all, Machine Whisperer back again, this story is a bit sad, but its a short one too. Today, I had to do one of those things that I loathe doing: I had to tell a car owner that their engine was only going to go another 20,000 miles at most. Since the car already has 262,000 miles, I don't feel too bad about having to tell them- plus they have a year or more to decide what to do about it.  So how did I come to the conclusion that the engine will only last another 20k miles? I had a few clues to work with...
It all begins with a bit of blue smoke in the exhaust recently that tells me that somewhere oil is leaking into the intake system or getting past valves or piston rings and into the combustion chamber. Its this blue exhaust that brought the car in for an appointment in the first place. To rule out the basics, I checked that the oil was not over-filled, then changed out the PCV (positive crankcase ventilation) valve and cleaned out the majority of the crud from the breather system hoses.

 This system is designed to move alot of air through the crankcase to mix with and flush out all of the acidic, partly burned vapors and fumes and burn them in the cylinders. This PCV valve was clogged with waxy, oily sludge, and was not moving fumes through it properly. After cleaning, I tested the flow in the system and realized it is BLOWING the fumes, under pressure, out of the engine breather system instead of being pulled out by engine vacuum through the PCV- no bueno. 

 With the oil filler cap off and open, the amount of gasses blowing out was considerably more than normal or acceptable. In this case, the PCV system can no longer remove all the gasses and thus leaks are forming, blowing oily fumes out of the hose connections and onto other parts under the hood. This added engine case pressure will cause increased oil leaks soon, and will increase oil consumption overall because the higher flow of gasses carries oil into the intake. 

It is the cause of those vapors under pressure in the crankcase that is my reason for the dire prognosis. The term used by us mechanics to describe this situation is 'blow-by'. It refers to the amount of unburned or partly burnt gasoline and air fumes that get past the piston and rings during combustion. Having no where else to go, the fumes are supposed to get mixed with fresh filtered air and flushed out of the crankcase into the air intake to be re-burnt. In the PCV system, the vacuum created in the intake manifold before the throttle is used to keep a gentle 'suction' on the crankcase. In a younger engine, newer piston rings form a much tighter seal, therefore less of those gasses sneak past them. In this older engine though, the rings have worn and softened so the leakage past them is increasing. 

Extra blow-by is a sign that the engine will not be doing its job efficiently for much longer. It may continue to run for years to come, but the added cost of constantly dropping fuel economy and increasing oil costs will make it less and less economically viable.  

We will likely put a new engine in this car since it had a newish used transmission put in it last year, and the body/chassis is in excellent condition. The car is still in extraordinarily good condition for its age and is a great ride in all respects- the chassis and frame are rust-free , the interior is clean and nice, and everything works still. The engine runs smooth and quiet, starts great, still makes reasonable power output for now. 

So for the time being, nothing changes, except for the knowledge that all good things must end, and that this way we can find a good deal on an engine when its convenient instead of waiting for an emergency and paying double or more. Just another day with the MachineWhisperer...

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