Iain Ayre tells us how to use a compression tester to check out an engine.
Whenever you buy a car, a quick compression check on the cylinders will reveal whether you’re buying a lemon, or a presentation fruit basket from Harrods. A cheap cylinder compression tester could save you the cost of an engine rebuild or even a court case.
The compression test will tell you the basic condition of all the cylinder bores, and will also check the valve gear and valves. If that’s all in sound condition, and there are no bearing noises, leaks, water/oil contamination or overheating, you can reasonably expect to get some useful service out of the engine in question without a major problem.
The tester itself is simple. It’s a gauge on the end of a stick or hose that temporarily replaces each spark plug to give you a reading of the compression of each cylinder in turn. A reasonably good compression gauge comes with a flexible hose and attachments to suit several sizes of spark plug thread.
The cheapest ones have a rubber cone at the end of a stiff tube: you just push them down on to the spark plug hole and spin the engine. You won’t get a precisely accurate reading, but you’ll find out if the compression’s really poor, and you’ll find out if one cylinder is down.
My own compression tester is a fairly good one that cost about £30. It has a flexible steel-braided hose, and it screws into the spark plug hole to seal properly. I don’t expect ever to buy another one.
The Jeep engine I was testing had an annoying fault: it ran on choke and then died after a few moments and wouldn’t restart. I changed the coil and the fuel filter, cleaned some carbon crud out of the EGR valve and cleaned some jelly kak out of the carb. With some of the mass of emissions spaghetti taken off, blue oil smoke was coming out of the valve cover – usually a sign of worn-out piston rings. Oddly, there was no blue smoke coming out of the exhaust. The compression check was performed in order to find out how bad the bores and rings were.
First step is to disconnect the ignition. There’s still fuel about, and there’s also the chance of a fried ECU if sparks go the wrong way. (Yes, this engine has a computer as well as a carb, the ultimate nightmare scenario: I solved this and future problems quickly and decisively by getting rid of the Jeep.)
Removing the fat central HT (High Tension) lead to the distributor and the thin LT (Low Tension) wire to the coil kills the current to the distributor. For more complex ignition systems, take the plug leads off, having marked their positions first.
Next, take out the spark plugs, after waiting ten minutes for aluminium heads to cool. The engine will spin faster and more easily if all the plugs are out. Mark the plug wires from one to eight or whatever in order to get them back in the right place.
Wedge open the throttle and the choke plate on carbed engines to make sure that there’s no restriction to air getting into the engine. (Note – remembering to release the throttle later before starting the engine will help keep the pistons inside the crankcase when it fires up.)
Push the compression tester in if it’s a cheap one, or screw it in (just hand tight) if it’s more expensive. Use the correct adapter to match the plug length, as the wrong adapter may be hit by the piston.
Then get somebody to crank the engine over for ten seconds or so, using the same amount of time for each cylinder. You don’t need anybody to help when using the more expensive gauge because the reading stays on the gauge, so you can turn the engine over and then walk around to the front of the car to check it. However, what you’ll miss seeing is the speed at which the compression pulses get up to maximum on the gauge. Puffing up to 150psi (pounds per square inch) or more in just a few revolutions is good news: taking longer suggests compression ring wear.
The Jeep engine puffed straight up to the following readings – 140psi, 138, 135, 138, 136, 130. A healthy engine, tested when hot, should have between 125-175psi, and should have no more than 15-20psi difference between the strongest and the weakest of the cylinders. This engine wouldn’t run for long enough to get hot, and a cold engine gives a lower reading, so my average cylinder pressure is okay. The difference between my cylinders was also a max of 10psi, so I didn’t have a dead cylinder.
That was quite clear, then. If you don’t get a clear result, or you get very low pressures, you can investigate whether the problem is in the block or the cylinder head by doing a “wet test”. This means tipping a tablespoonful of 20/50 engine oil down all the spark plug holes and testing again. The oil seals the rings and ensures full compression from the rings and bores, so if the pressure reading rises (improves) that means the rings are suspect. If the reading remains the same, the problem is more likely to be a damaged or sticking valve, or maybe a broken or maladjusted rocker or a cam problem.
Two bad cylinders next to each other are usually on either side of a hole in the head gasket. Unusually high compression can be caused by carbon buildup on the head.
A high compression engine with say a 10:1 compression ratio should be giving 170-200psi on the gauge. The internet is unable to provide reliable information on Jeep compression ratios, but this engine would either have medium or low compression, so cold readings above 130psi are okay.
A compression tester is a very useful piece of kit. Don’t waste money on a good one unless you’re going to get enough use out of it, though – a cheap one will work accurately enough to either keep you out of trouble… or to tell you just how much trouble you’re in.