When buying used cars, we usually take a mechanic’s help in determining the condition of the engine. Most mechanics who are not much experienced just check a few things and then comment on the car’s engine. A major check up is the smoke coming out of the tailpipe. This can be manipulated by the owner, if the car has a weak engine, by using sealants or other kinds of oil that reduces smoke. However, a more accurate way of checking engine condition is the compression test.
A compression test, in easy words, tell you the condition of each cylinder, good or bad. It is basically a pressure gauge that tells you the pressure each cylinder is generating in its power stroke. The pressure gauge is mounted in place of a spark plug, after removing the spark plugs of course. Remove the fuse of the injectors in case of EFI cars as you don’t want the injectors to flood the cylinder with fuel. Disable the ignition system (just remove the spark cables or disconnect ignition coils). Crank up the engine for about 6 to 8 compression strokes with fully depressed accelerator pedal. The gauge, by the end, will show you the maximum pressure the piston has generated. Do the same procedure for rest of your cylinders. For a diesel engine, you need to follow the same procedure except remove the injectors as there are no spark plugs.
After you have your readings for all the cylinders, you can now tell the condition of your car engine. Compare the readings to the readings in car owner’s manual.If you don’t have an owner’s manual, this is how you can check. If the readings are about 180 psi to 200 psi for each cylinder, the engine condition is very good. For older cars, if the readings per cylinder are above 135 psi, the engine is fairly good in condition. Less than 130 psi means there is a leak in the engine possibly worn out piston rings, faulty gasket, etc. It’s not entirely the pressure values that matter, what you should be looking for is consistency of pressure in all four cylinders. The pressure readings must be within 10 to 15 percent of each other. Diesel engine compression readings average approximately 275 psi to 400 psi and the differences between readings should not be more than 30 psi t0 50 psi.
If a cylinder shows less compression readings, add about 15 ml of oil to the cylinder and take the reading again. The end results can be interpreted as follows:
- Normal: Compression builds up quickly and evenly to the specified compression for each cylinder.
- Piston Rings Leaking: Compression is low on the first stroke. Compression builds up with the following strokes but does not reach normal. Compression improves considerably when you add oil.
- Valves Leaking: Compression is low on the first stroke. Compression usually does not build up on the following strokes. Compression does not improve much when you add oil.
- If two adjacent cylinders have lower than normal compression and injecting oil into the cylinders does not increase the compression, the cause may be the head gasket leaking between the cylinders.
Now the point is, if this is possible in Pakistan. What are the chances that the seller is so cooperative that s/he is going to let to pull out and disconnect bunch of things off his car? It is a good practice though, and you can’t doubt the science behind it. Some buyers don’t even let you test drive your car. Instead of being a Wolverine and using your super hearing to listen to all the clicks and clanks of the engine, and revving is unnecessarily, why not just compression test it. If you are confident about your product, and you know you are selling a good car, would you be willing to let a potential buyer, a real buyer and not one of those tyre kickers, let do a compression test, under your supervision of course.
Let us know if you think the practice of compression testing can be implemented in Pakistani market or not, in the comments section below.