Blue smoke on overrun

Hi, I've recently been getting some blue smoke on overrun after coasting a hill, when I get on the accelerator there is a large puff of smoke briefly! My garage has just changed the PCV as a possible fix, I was thinking it could be the valve stem seals? Could a bad PCV cause this?? it also blows some smoke when revved a bit at idle!!


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Compression and Leak down test?

Don[emoji768 said:

;2522419]Compression and Leak down test?

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I haven't spoken to the mechanic yet as I'm at work and my father dropped the car off but I would hope they indeed did do a compression test although this would not rule out valve stem seals and he wanted to try the cheapest option first, can you tell me what a leak down test is please? I'm not familiar! Thanks.


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Compression Test - It is a good way to check the engine’s ability to create pressure.

Leak Down Test - Two gauges are used, one measures the pressure of the air entering the cylinder and the other measures the percentage of the air escaping (or leaking) from the cylinder. The loss percentage will indicate the condition of the cylinder and overall condition of the engine.

Don[emoji768 said:

;2522451]Compression Test - It is a good way to check the engine’s ability to create pressure.

Leak Down Test - Two gauges are used, one measures the pressure of the air entering the cylinder and the other measures the percentage of the air escaping (or leaking) from the cylinder. The loss percentage will indicate the condition of the cylinder and overall condition of the engine.

Click to expand...

Ok thanks, I'm pretty sure he would have done a compression test! Is the leak down test known by any other name?? Never heard of it!


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Could a bad PCV cause the car to burn oil like it is?


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In overrun and under normal driving conditions a PCV would normally be open (post turbo, intake side); however, when the engine is overrun, it also pulls higher vacuum - meaning that there's a greater chance to suck in oil from the rings or valve guides/seals into the combustion chamber.

As the piston passes TDC with intake valves opened, there's a pressure drop in the combustion chamber. The vacuum created by the piston on its way down towards BDC creates a suction that will allow oil to be sucked in from the rings and valve guides/seals. As a result, excess oil can enter the chamber and made part of combustion cycle, this is indicated by blue smoke.

Don[emoji768 said:

;2522469]In overrun and under normal driving conditions a PCV would normally be open (post turbo, intake side); however, when the engine is overrun, it also pulls higher vacuum - meaning that there's a greater chance to suck in oil from the rings or valve guides/seals into the combustion chamber.

As the piston passes TDC with intake valves opened, there's a pressure drop in the combustion chamber. The vacuum created by the piston on its way down towards BDC creates a suction that will allow oil to be sucked in from the rings and valve guides/seals. As a result, excess oil can enter the chamber and made part of combustion cycle, this is indicated by blue smoke.

Click to expand...

Thanks for the info, looks like changing the PCV didn't fix it (as I thought), going by the info I've seen here and elsewhere on the web my hunch is it's the valve stem seals and possibly the valve guides, as the seals can be changed without removing the head is it acceptable to change only the stem seals and hope the guides are ok?? Thanks for sharing you knowledge of this it's already proved to be a great help.

EDIT: Changing the PCV has pretty much stopped the oil being burnt but having now checked the spark plug condition I have found that the plug in cylinder no4 was fouled with oil (the other 3 are in perfect condition), having now cleaned it I'm hoping it will remain that way but if not I will be fitting new valve stem seals.

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