Compression

Post by **chrisd87** » Thu Jun 15, 2006 4:27 pm

I ran a compression test on my car for the first time today - the reading was 165 - 100 - 85 - 160

So I have a few questions...
Could those two low values be caused by burned out valves?
If so, would that cause rough running and a lot of vibration and noise at above 3000rpm (I had thought it was the main bearings but am now not so sure)?
What do burned valves look like? - I have a spare head, but is there any way to find out if it's any good before fitting? The valves in it look like this:

<img src="http://i31.photobucket.com/albums/c390/ ... C01091.jpg" width=400 height=300 >

Thanks.

Post by **Benjy** » Thu Jun 15, 2006 4:41 pm

Yes, burned out valves could be the source of poor compression. I once took a head off to discover that there was a large slice missing from an exhast valve!

I'm not sure about the rough running though. I guess the uneven compressions won't help, but I'm not sure that they're bad enough to cause severe vibrations. I'm sure others on here will have a better idea...

P.S. The valves in the spare head look OK. A good way to test is to pour parrafin into the chambers and leave overnight. If none leaks, the valves are seating well (you'll need to put the plugs in though!).

Post by **Kevin** » Thu Jun 15, 2006 4:54 pm

Do you also get a lumpyness at tickover as its also possible the head gasket is on its way, although the vibration sounds more like a possible propshaft problem, if you do a second test with a bit of oil put down the plug holes and nothing improves that will confirm a head problem however if it gets better that confims wear in the bores, give it a try.

Post by **rayofleamington** » Thu Jun 15, 2006 5:46 pm

when a valve is badly gone it can have a piece missing like a piece of cake had been cut out .
However if the valve is bad enough that you can see the piece missing then compression will probably be closer to ZERO.
Even with a burnt valve, the running at higher revs should be not so much unaffected - apart from a loss in power, but you will often get uneven running and maybe even a missfire at low revs.

As mentioned already - putting oil in the bores should help identify if it is the valve or the rings. Please do note that oil in the bores will increase compression a bit even if the rings are fine as there will be less air leakage down the sides of the piston and a pool of excess oil on top of the piston. If it affects the readings more than 20% then that is a clear answer.
If it's not the rings/bores then either valves or headgasket, but the other guys already beat me to it ;-)

Post by **246gts** » Thu Jun 15, 2006 7:42 pm

whilst we are on the subject what is the correct procedure for taking compression readings?
Dave

Post by **Onne** » Thu Jun 15, 2006 7:47 pm

I always run the car for like 15 minutes, so everything is all nicely lubricated. I then take the sparkplugs out (all of them!)
next thing is to screw in the compression thingie.

I can start the car from under the bonnet (ignition off!!!)
Pull the throttle wide open, and let the starter motor turn ( no longer than 15 seconds at a time) You should get a reading like this

hope this helps

Onne

Post by **chrisd87** » Thu Jun 15, 2006 8:07 pm

OK, Looks like the rings then

Retook the middle two cylinders with a little oil down the bores - readings improved from 100 to 130 and 85 to 115. The smoke was quite impressive afterwards!

Thinking of smoke, I'm quite surprised that the rings are shot as there is no visible smoke from the exhaust and it consumes virtually no oil

Suppose this means I need to get my spare engine ready and put in rather quicker than I'd hoped...

Thanks for your help!

Post by **bmcecosse** » Thu Jun 15, 2006 10:21 pm

No - these readings are still very poor - it's either the head gasket or the valves. So - grind in the valves on the spare head - and swap em over. It doesn't take long - and I'm sure it will sort most of the problems. No harm having a spare engine ready to go in though!

Post by **Kevin** » Fri Jun 16, 2006 12:37 am

Chris Bmc`s right as the improvement isnt that much it really does point to the head as suggested.

Post by **chrisd87** » Fri Jun 16, 2006 3:24 pm

Righto then, I'll get the spare head ready and swap that on when I have a minute. I'll try the 'paraffin test' on it also.
Bear in mind though that the oil I put into the cylinders was that fairly thin 3-in-1 stuff - maybe I should have used engine oil...

Post by **blueangelical** » Fri Jun 16, 2006 4:33 pm

where do you get compression testers from, anyone know...?

is this what it is http://www.screwfix.com/app/sfd/sea/sea ... ion+tester

Post by **Nigel2** » Fri Jun 16, 2006 4:52 pm

That's the one, screwfix is a good price. The problem is always buying 'extra' stuff so you get it sent free (unless there is a depot down the road!!).

Post by **blueangelical** » Fri Jun 16, 2006 4:53 pm

cheers., and yes, there is one just down the road..

mmmmm., I need this and this and this and this.........

Post by **bmcecosse** » Fri Jun 16, 2006 9:39 pm

Yes - you should have used engine oil! But it's still the valves or the gasket!

Post by **Onne** » Fri Jun 16, 2006 10:55 pm

I am willing to take my compression tester to tha national, if anyone wants to have a go!

I'll do mine after the second torqueing of the cylinder head nuts

Post by **chrisd87** » Fri Jun 23, 2006 8:59 pm

Update: I've now replaced the head with the spare and the comression is now 160 - 165 psi all round

It was burned out valves & seats. Strangely enough there was also quite a lot of grey water in no.3 cylinder

I suppose the moral of the story may be to use an additive with unleaded...

Some pics of the damage, anyway:

Post by **bmcecosse** » Fri Jun 23, 2006 11:38 pm

That very white plug tells me there was indeed water getting in that cylinder - either a small crack in the head - or gasket was weeping - maybe at 2 o'clock on the picture ?? See how the water hole is clean on the side adjacent to the combustion chamber. Anyway - your comps are fine now- job well done !