Low compression on FSI one cylinder.

S

Simon_Blackburn

Member
Happy Sunday… well having managed to make the fuel and air system work properly, it turns out that the underlying fault is low compression in cylinder #4.

Does anyone have any experience or dealing with this please? I assume as injecting oil into the spark plug hole that it’s replace rings. Can this be done with the engine in situ?

Am I overly optimistic in hoping that a long overdue oil change may bring it back?

Any thoughts much appreciated.


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Simon_Blackburn said:
Happy Sunday… well having managed to make the fuel and air system work properly, it turns out that the underlying fault is low compression in cylinder #4.

Does anyone have any experience or dealing with this please? I assume as injecting oil into the spark plug hole that it’s replace rings. Can this be done with the engine in situ?

Am I overly optimistic in hoping that a long overdue oil change may bring it back?

Any thoughts much appreciated.


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It could be valve not closing properly when it should
 
Injecting oil into the plug hole and retesting brought the compression back to normal limits ?
If yes then it’s rings that need changed possible a re bore depending on bore condition
An oil change will not do anything to improve the low compression
How low is it compared to the other cylinders ?

Looking for a replacement engine is likely the cheapest solution. The gain is a complex engine so you may get a good block and pistons from a poorly running engine as the problem is likely elsewhere where on the donor engine

Paul


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How low is the compression? Is it a blown head gasket?
I’ve replaced a cracked piston in my FSI with it in situ. 20,000 miles on and it’s still going well. It is a hell of a lot of stripping down but just spannering really. You need a trolley jack to pull the engine forward a little.
When taking off the sump be sure to clean up what you can first, apply penetrating oil to any exposed screw threads which you can get to where they poke through on the top side of the block flange . Some of the hex key screw heads will probably round off, so be prepared to cut slots with a grinder or Dremel. There are 2 sump screws hidden by the flywheel you’ll need a long ball driver to get to.


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depronman said:
Injecting oil into the plug hole and retesting brought the compression back to normal limits ?
If yes then it’s rings that need changed possible a re bore depending on bore condition
An oil change will not do anything to improve the low compression
How low is it compared to the other cylinders ?

Looking for a replacement engine is likely the cheapest solution. The gain is a complex engine so you may get a good block and pistons from a poorly running engine as the problem is likely elsewhere where on the donor engine

Paul


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Easier to locate a breeding pair of Unicorns than a, (worth having), FSI engine.
Ask @Evripidis
Better refurb the known one you have, in my opinion. Easy to say, but not so easy to do. Best of luck, and if we can help, don't hesitate.
Think carefully about continuing to drive it. If it is a ring problem, scoring the bore is a real risk.
Any chance of a look with a borescope? If the bore looks good, then a piston swap, as described by @BiggDogg might be favourite 🤞
Mac.
 
Last edited:
I would suggest to do both. Start spannering and looking for a block. That is what I should have done.

The sump bolts need a ball allen driver and a bit of glue between the driver and the bolt to put back on.
 
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