Practical Classics (UK)
Q My friend has just bought a Cavalier 1300 that was running well when sold, but on the journey back it started to run on three cylinders. By the time he got home, one plug had oiled up. He's no idea what's wrong, but I have a very bad feeling. Any advice? Dave Smith, Worcester
A Start by swapping ignition components between cylinders. If you can't clean the fouled plug, just replace it. See if the misfire follows the plug or lead, or even the distributor cap. If not, remove the plug lead (using insulated pliers) from the offending cylinder with the engine running. If no difference at all, at any revs, then the ignition system is still a suspect. Test for a spark by holding the lead a good 5-10mm away from the cylinderhead. There should be a strong spark. If it appears that the engine slows or becomes rougher without the lead, then that cylinder is firing – but isn't burning very effectively, which is perhaps why the plug oiled up. Investigate the inlet manifold for air leaks around that cylinder. Look for cracked and broken castings, loose nuts or missing washers, a warped manifold or a servo hose that's loose or perished. A leak will unduly affect the cylinder closest to it, which will draw in outside air instead of air/fuel mixture. If all this draws a blank, perform a compression test. Remove all plugs. Disconnect coil LT lead. Depress accelerator fully and crank engine through about 5 compression strokes, or until the gauge reaches its maximum reading.
All cylinders should read about the same. If your suspect cylinder is low, put a few squirts of engine oil down the spark plug hole and quickly retest that cylinder. If the compression improves dramatically, then the piston is the culprit. If not, suspect a valve and its seat. Do check the valve clearance before tearing the cylinderhead off.
If the piston or rings are defective, it's possible the cylinder is scored which may require a rebore. It may be more cost effective to replace the engine.