Has my piston ring gone?

[JohnD]

Paul,
Do your compression test, and then pour a teaspoonful of engine oil into the chamber, via the spark plug hole.   Turn the engine over two or three times and then repeat the test.
If it is a failed piston ring that causes the low pressure in No.3, then this will raise it, as oil soaking down into the ring lands will help to seal it.  If it doesn't raise the pressure, then you have a damaged valve.

That's what Brendan means by a burnt valve - one that has ceased to form a seal between the edge of the valve and the valve seat.   Wear or damage due to overheating are some of the causes.

NB  The compression test should be done with:
1/ a well charged battery!
2/ All the spark plugs out.
3/ the ignition turned off or disconnected
4/ the fuel supply disconnected
Else you are pumping an air/fuel mixture out of the spark plug holes, with fire risk.

Then connect the gauge, and turn the engine over on the starter with the throttle wide open.
Allow the engine to turn only until the gauge reading stabilises.

Good luck!

John

[GT6 M]

Kev, i think he meant ..bar.. !!!!  or some other weired readings,



have you checked the rockers yet,!!!

[PaulB]

Ok, so it took me some time but I eventually got some spare time to work on the car today. So I took the head off.

And yes it was the valve that had gone. A small bit around the edge of the valve and broken off.

The pistons look good except that 3 of them have black hard deposit on the face of them. What should I use to clean this stuff off?

[cook1e]

The black deposit is carbon and it's normal when a head is off to do a decoke, ie remove the carbon. I usually use an old very very blunt screw driver to gently scrape the carbon away and then finish off by polishing with solvol autosolv or similar polish. Decoke the both the piston tops and the cylinder head chambers of carbon.

[sparky_spit]

This may cause some comment, but I would leave the piston tops alone, and do nothing to them. Bearing in mind the mileage your engine  has done, my opinion is that removing the carbon and polishing the pistons may do more harm than good. Think about it; until your valve went the carbon on the piston tops was doing no harm, and probably some good in that it is helping to keep up the compression. I think it would be unwise to clean it off.

I think the bit missing from your valve is likely to have burnt away rather than broken off. Unless of course you are talking about a great big chunk? I take it there are no witness marks in the head or on the the piston top showing where a chunk may have rattled about in there before exiting the exhaust? Is it an exhaust or inlet valve? Once you have the old valve out, what does the seat look like? Part of it burnt away?

[lancepar]

Don't bother polishing, just remove the coke as in De-coke.

[GT6DavidMK1]

I agree with sparky_spit, leave the tops of the piston as they are!!! or you will cause more problems :(

David.

[PaulB]

Ok, I migh leave it then.
This is a picture of the piston. One is fairly clean (the one which had the broken valve) the others have a layer of carbon.

[PaulB]

This is a picture of the faulty valve

[lancepar]

De-coke or not to De-coke, here is one opinion from the interweb on the effects of carbon build up.
http://www.autospeednet.com/sites/3dauto.com/viewterm/3681/0/

[PaulB]

sparky_spit wrote:

I think the bit missing from your valve is likely to have burnt away rather than broken off. Unless of course you are talking about a great big chunk? I take it there are no witness marks in the head or on the the piston top showing where a chunk may have rattled about in there before exiting the exhaust? Is it an exhaust or inlet valve? Once you have the old valve out, what does the seat look like? Part of it burnt away?

I think you're right Mike that a part of it burnt away as there is no mark. I've posted a picture of the valve. See above.

[Alex]

Whats the plan Paul?
New valve and refit or unleaded conversion?...... assuming its not already?

[PaulB]

Alex wrote:

Whats the plan Paul?
New valve and refit or unleaded conversion?...... assuming its not already?

I think I'm going to go for replacing the head with an unleaded head. Although more money now, it should save me money in the long run.

[Alex]

PaulB wrote:

I think I'm going to go for replacing the head with an unleaded head. Although more money now, it should save me money in the long run.

Sounds like a perfect plan!

Good luck with it, its great that you've had a go on your own and will feel so rewarding afterwards.I always asked a friend to help but that doesnt happen now as its easier and more satisfying to do it by myself.........unless its full engine and box changes  ;D

Good luck,
Alex

[Tim Bancroft]

When ordering your gasket set, I'd attempt to buy a Payen made set.

Also have a go at cleaning out the waterways as they seem to have a fair amount of scale present.

Totally agree with Mike and Dave, leave the carbon alone!

[PaulB]

timbancroft61 wrote:

When ordering your gasket set, I'd attempt to buy a Payen made set.

Also have a go at cleaning out the waterways as they seem to have a fair amount of scale present.

Totally agree with Mike and Dave, leave the carbon alone!

I've got a Payen head gasket from Canleys.
Yes, I was surprised by the gunk in the waterways, the rad was always clear, until I took the head off when there was a load of gunk at the top.
How best is it to clear the waterways? Should I wait until I put it back together and run a rad flush product through it or try and clean it while its apart?
  

[PaulB]

Alex wrote:

Good luck with it, its great that you've had a go on your own and will feel so rewarding afterwards.I always asked a friend to help but that doesnt happen now as its easier and more satisfying to do it by myself.........unless its full engine and box changes  ;D

Good luck,
Alex

I quite agree. I was so chuffed with myself that I got the head off all by myself when I'm a real novice to mechanics.

Admittedly I be even more amazed that I can put it back together and get it working!  :o