low compression

[esprit_1985]

my 1985 has low compression in one of the cylinders. Is it a big cost item to fix? thanks

[GSimpson]

You need to determine whether it is a valve problem or piston ring.

If you can temporarily restore compression with a squirt of oil down the plughole then it is rings otherwise likely to be damaged valve. Check also your valve clearances are OK.

If bore wear is low then one can get away with new rings only. However only economical if you can strip motor yourself

Graeme

[Splashout]

could be any of the things already mentioned or a light weeping in the head gasket, do you have any cooling issues? is the system being pressurised? this could point to a small weep in the gasket.

[WayneB]

If the compression reading is within 10 percent of the other cylinders that is acceptable. If it is much lower than you have a prob. As you dont mention cooling system pressurisation then it prob aint a head gasket. If its a turbo 910 could be dettonation damage to top of piston or rings. Or as the others suggest a valve prob. Do a leakdown check to see if its the valves.If you cant get your hands on a leakdown checker another method is to remove the sparkplug on the suspect cyl., and squirt some 50 weight oil in. Then take a compression reading. If the reading is higher, your piston seal is bad, if not its your valves or the headgasket or a bloody hole in the piston(very bad)