Experts in the house, please enlighten us on how spark plugs affect the performance of a car engine.

Is it possible to start and run a car with only some of the spark plugs working?

The car will fire nup, and run with some of the spark plugs working, yes.

But the engine will missfire, or in extreme cases, only run on the cylinders with good plugs. We'll use the more common 4 cylinder as an example - the firing order of most 4 cylinder engines is 1-3-4-2. So say the plugs on cylinders 1 and 4 are dead, the engine will be very reluctant to start, as it'll have to fire from cylinder 3, depending on where it stopped in its last cycle.

It'll be lumpy, and idle very badly. As the injectors on both cylinders 1 and 4 are still spraying fuel into the cylinders, the fuel will not be burnt, and will instead end up in the sump, mixed with your oil. It will also continiously wash the lubrication from the cylinder walls of the defective cylinders, causing "bore wash." This of course will lead to premature wear of the piston rings in the affected cylinders, and later on, the cylinder bores. If this happens, even when the plugs are replaced, these cylinders will be down on compression, so will not seal. The oil control rings will also allow oil to blow past, visible as blue smoke from the exhaust.

So yes, spark plugs affect the performance of an engine, as well as fuel consumption. Every engine has a recommended spark gap, and temperature range. Too big a gap, and the spark generated will be weak, at high rpm, the compression will actually blow the spark out on the compression stroke, causing missfires. Too small a gap will cause missfires at lower rpm.

It's important to run the correct plugs for your engine - although others will fit, and appear to be fine, not a good idea. If you run "cold" plugs in a hot engine, the electrodes are liable to overheat, melt, and drop into the cylinder, with catastrophic results. It will chew up the bores, and destroy the piston crowns. Most cars will have an alternative plug choice, always run plugs specified as alternatives, never run any old plugs, just because "they fit, and look the same".

Thanks a mil, Sienna. I recently replaced the spark plugs on my '99 Camry V6 which I bought 8 months ago.

I had them changed because I discovered the car was starting at very low rpm, making me have to rev the engine so it didn't go off. After the replacement of the plugs the car is now faster, making me think that probably not all the cylinders were working previously.

Yes, perhaps the old plugs were worn, or could well have been the wrong grade for your engine.