Checking Cranking Speed Compressions

A quick and easy check of compressions can be done with the starting handle.  With the gearbox in neutral and the ignition off, insert the starting handle and turn the engine over. You should feel 4 strong compressions which are all the same.  If this is the case then it is unlikely that there is any major problem as it is most unlikely that all four compressions will be low by an equal amount. If one or two compressions feel weak there could be a problem and a compression tester will be needed to proceed further.

Compression testers are quite cheap to buy but get one which screws into the plug hole not one that has to be held in place while the test is done. The tester I like to use is an Aerite Dubltest Compression tester and tyre pressure gauge. I bought one of these many years ago and it is still giving excellent service. Although these are no longer made they appear regularly on auction sites such as Ebay and, while I was writing this, I bought one off Ebay for just £5 plus postage. PCL made a very similar tester too. It can be screwed into either a 14mm or 18 mm spark plug hole and when the engine is spun over it is pumped up just like a tyre pressure gauge. It is graduated in 5psi intervals which is sufficient for our needs.

To check the compressions remove all four spark plugs and screw the tester into the front plug hole. Sitting in the car, hold the accelerator flat to the floor (throttle wide open), turn on the ignition and spin the engine a few times. This will cause the piston inside the tester to rise a bit with each compression. After a few spins of the engine it will not rise any further but will stay at that position for the reading to be taken. The tester can then be remove from the plug hole and the pressure within it released by pushing up the one way valve inside the bottom of the tester. The test is then repeated on the remaining cylinders and the results compared. Bearing in mind that a compression tester is not a precision instrument the reading it gives may not be exact but will be close to the truth and most importantly any errors in the tester should be the same for all cylinders.

So what should the readings be? The figures quoted by Riley are 150psi for a 2½ litre engine and 123psi for a 1½ litre engine. If the readings obtained are close to these values and very close to being the same on all cylinders then all is well. If the readings are low and vary a lot between the cylinders there could be a  problem.  In general, if all the readings are within 5psi of each other there is no need to worry and if they are within 10psi of each other there is no immediate need to panic but the readings should be rechecked soon to see if they are getting worse. If the difference is over 15psi it is time to investigate further.

Before hitting the panic button there are a couple of things to check. Was the engine cold when the test was done and did it loosen up during the test? An engine which cranks over slowly is likely to give a lower reading than one which spins freely so the battery should be well charged before the test starts. Was anything blocking the free flow of air into the engine? The throttles need to be wide open. A dirty air cleaner could impede the air flow so remove it and recheck. The piston in the carb. can also obstruct the flow particularly if fitted with a damper. It might be worth removing the carb. top cover and removing the piston completely and then rechecking. This can be a problem for cars with twin carbs. If during the test one carb. piston does not rise as much as the other a false low reading will be obtained on one or more cylinders. If with all obstructions out of the way the readings are still low and uneven there is problem within the cylinders.

Before lifting the cylinder head pour a small amount of engine oil into each cylinder before retesting that cylinder. The aim of this is to give a temporary seal around the pistons. If when the test is repeated the readings are significantly higher then it is probable that the pistons and their rings are not sealing well. If the results are little different then the pistons are sealing well and it is the valves that are not seating properly. Reseating the valves is a much easier job than correcting pistons problems.

Since all remedial work will start with the head being lifted the valves and their seatings should be checked first.  If these are in poor condition reseating them or fitting new valves if the edges are getting thin or they are damaged  may cure the problem. After doing this it is worth refitting the head and repeating the compression check. With a bit of luck that may be all that is needed. Of course, if the engine emits blue smoke under acceleration it is burning oil and it is likely that there is a problem with the pistons/bores. If the engine smokes on the overrun then oil is being sucked down between the valves and their guides so fitting new guides while the valves are out would be a wise precaution.