Help!! Need advice.

Level 2 User Joined: 26 Mar 2004 Posts: 52

Hi Guys,

After my MOT, and the guy saying about a cylinder misfire, i got it checked out (after replacing the plugs, leads, dizzy cap, rotar arm, coil, and module!!!)

Got a compression test done.

Cylinder 3 is fucked. It had 50 odd bars on the guage. Matey said that it could be a number of things, but most likely to be burnt valve or piston rings.

That sounds expenseive.

What do i do now then? How do i go about sorting this.

Any comments gratfully received.

Cheers
Ben

By the way. Phase 1 R19 1.4energy GTS

Level 6 User Joined: 20 Jun 2004 Posts: 155

Glad you isolated the problem (kind of anyway)!

Low compression means that there is a weak point in the airtight seal that makes up the combustion chamber (above the piston and below the valves), so that when the cylinder compresses the fuel air mixture before ignition, and when the burning fuel and air expands, the high pressure caused by this escapes through the weak point.

It means that either the valves are not sealing properly when closed, or the piston rings are duff and are leaking pressure, or the head gasket is dead. Since the spark plug would have been out when doing the comp. test, a loose plug won't be the cause unless the plug hole is buggered in some way (pretty unlikely I reckon). If the car has had a cambelt failure in its past, then this would probably have bent valves (at least), and a crap repair job on them might be causing the problem you have.

Are you loosing any water or oil? This is a good indicator that the head gasket may be knackered, or if you are really unlucky it could be a cracked cylinder head, but unless you haven't been using any antifreeze or running with no oil or water or something I would think its a bit unlikely. Head gasket failure also tends to affect pairs of cylinders rather than one, due to the engine's construction.

Did you get a full compression test report from the garage, or just "a cylinder is low". You can test for piston ring wear by testing the compression once, then using a turkey baster or similar to squirt in some oil through the spark plug hole, then testing the compression again. If it goes up noticeably the second time, the rings are shagged. If ithe compression doesn't improve much, then it is going to be down to valve related or head gasket failure (maybe piston rings too, but they're harder to get at so do them as a last resort) - valve related includes bent or excessively worn valves, knackered valve seats in the head, and probably a bunch of other things. If you are super lucky then you may be able to fix it by adjusting the valve clearances and that will solve the problem, but I think that would be very lucky indeed Smile

Basically, you need to see if the piston rings are bust with the oil test described above. If it ain't the rings, then you will have to get the head off probably and have a bit of an explore to find what is leaking in there. Any mechanic worth his salt should be able to find and probably fix any of the possible valve/head gasket issues, and it shouldn't cost the earth. The parts are cheap, but the labour will soon rack up at a garage - if you have a mechanic mate then defo get him to help you out in exchange for a few pints Wink

Cheers
Alex

Forum Moderator Joined: 02 Mar 2004 Posts: 19978

50 odd bars! Wow 700 odd psi, you mean 50 odd psi.

do thew oil test but if he was any sort of decent engine man he would have checked things out with a vacuum gauge as well.

saying a cyl is low on compression is like saying its 2 foot long, whats 2 foot long, useless without the item!