Compression Testing
 

How do you do it and what do you need? simple really :p

compression test..
 

You can get a thing called a compression tester, from halfords and other motor factors. (Mine cost £15).

It i slike a 12" (approx) length of flexible hose, with a guage on one end, and a screw thread on the other. (With adaptor usually.. for different size spark plug holes.).

The screw thread end goes into your cylinder head (in place of the spark plug).

You simply crank the engine a few turns, and the guage will move to the relevant reading. (The better ones have a system whereby the needle stays at this "max" position, so you can read it yourself after doing the cranking, then "cancel" the needle back to zero with a push of a button).

The idea is that all 4 cylinders are meant to be pretty similar in terms of their readings, and one "low" reading is (usually) symptomatic of a blown gasket (or worn piston ?).

I have never actually done this on a scoob, as the sparkies are a bit inaccessible, and when my gasket(s) went (plural!), it was so obvious , I didn't need to do the test. (Towed back by AA).

Incidentally (might not be relevant .. but..) If by any chance you are overheating, and you suspect gaskets, .. be aware that it is very easy to get an airlock in a scoob , so that even if the coolant looks full in both the header tank and the one at the front left (facing car) , it STILL might mean you haven't got enough coolant.

Thats what happened to me, and thats WHY my gaskets went in the first place.

Good luck
Mark

Ok full procedure here we go!

<b>Compression testing</b>

A compression test is basic measurement of an engines health. An engine needs compression in order to run and there are several problems an engine can have that will cause low compression.

http://www.bobstools.net/Store/media/20250clp1x11x1.jpg

<b>Preparing</b>

Run the engine up to normal operating temperature. With the engine warm you get a much truer picture of what?s happening.

Remove all the spark plugs. Remember to note where the ignition leads go so you don't put them back in wrong and mess up the firing order.

IMPORTANT - you need to careful with fuel and sparks here or you can cause a serious fire! On most minis disconnect the negative ignition coil connection. If you have a fuel injected mini its best to also pull out the fuse for the fuel pump and disconnect the coil/coil pack.

http://www.terragon.com/troop77/tt3plugs.jpg

<b>The test</b>

Connect up the compression tester to cylinder 1.

Spin the engine over on the starter with the throttle pushed to the floor so that you get at least 6 "chuga" noises (approx 10 secs).

Note down the reading.

Repeat for all other cylinders.

If all the figures are within spec (see Haynes) and all reading are equal within around half a bar (10psi odd) of each other then you have a healthy engine.

If not say they are all low or one is then continue:

Put a squirt of engine oil down the 1st bore you tested and repeat the test in the same way and again note the figure.

Then repeat for all other cylinders, each time adding a squirt of oil before you do the test.

<b>diagnostic</b>

OK there are many reason for low figures but lets look at some theory of why they are low 1st.

Compression is a factor of two things. How well the rings on the pistons seal to the bore and how well the valves seal. If either are a poor seal you will get low compression.

(1) Head Gasket - If two cylinders next to each other (say 2&3) are both way down on compression it could be the gasket as failed between those bores.

http://www.tinotopia.com/graphics/austin_headgasket.jpg

(2) Readings are all low on both tests - You probably have a valve sealing issue.

http://www.tomstickland.co.uk/astra/...b/doreb17s.jpg

(3) Readings improved on the 2nd test - adding the oil temporarily improved the bore to pistons sealing. You have worn rings and/or bores.

http://www.tomstickland.co.uk/astra/...eb/doreb10.jpg


words copyright AFRacing LTD 2003

Alex