Cylinder not firing, then suddenly started firing again

engines
intermittent
cylinder

#1

I took my car into the shop when the “Check Engine” light came on. I noticed the engine was running a little rough, and the mechanic told me the code said one of the cylinders wasn’t firing. He checked the spark plug, the spark plug wire, and the coil, and told me that all three were working, that I’d have to bring the car back in the morning for a compression test in order to figure out what the problem is, and that it probably won’t be worth the cost to repair it.

I was planning to bring it in Monday morning for the compression test, but when I was driving home this evening, the “Check Engine” light suddenly went off, and I noticed the engine was no longer running rough.

I don’t understand what kind of problem would clear itself up as suddenly as it originally occurred. When I bring it back for testing, what else should I ask the mechanic to check for?

Thanks!

The car is a 2005 Buick LaCrosse, and it has 152,000 miles on the odometer.


#2

Was it damp or rainy? If so, that is a typical plug wire failure. I would replace all the wires and wait and see.


#3

I’d do the same thing. Replace the wires with that mileage. Then the coils. Cheaper than the price of the mechanic. If it were valves, it’d be doing it all the time.


#4

Another wild guess . . .

Perhaps an injector was partially plugged up, and then it passed the crud, and it’s now opening and closing properly again

I agree with the others. Replace the plugs and wires. Consider it preventive maintenance


#5

I wholeheartedly agree with changing the sparkplugs. And “read” their condition when they come out (have the mechanic mark the cylinder number on each and give it to you if you’re having the work done).

As to wires, you have “coil on plug” ignition, where each plug has its own coil connected directly to it. You don’t have the old fashioned “sparkplug wires”. At your mileage it is possible that a coil was misfiring, but they don’t usually heal themselves.

Have the plugs changed and given to you, take photos and post them (it’s a personal request: I’m curious), and enjoy your car. If it begins to misfire again, just get the codes read again and start over… but with new plugs. New spark plugs cure perhaps more misfires than anything else ever has. Your odds of having that fix it are excellent.


#6

Hmm, the same, the 3.6 V6 uses coil on but the 3.8 V6 uses wires and coils the same as on my 86 Riviera. Given that he said the mechanic checked the wires, but didn’t say which engine he had, seems like he’s got the old system.


#7

Offhand, maybe a spark plug going in and out, an injector doing the same, or an iffy connection at the injector plug, etc, etc.

It seems a bit premature on the part of the mechanic to be talking about compression issues and the car not being worth fixing although lowered compression could cause a spark plug to fade in and out.


#8

When was the last time the plugs and wires were changed? If it was in the last 30,000 miles, I would say that they are probably OK. If it was never done, it is a good time to do what should have been done around 100,000 miles.


#9

Properly diagnosing an intermittent problem can be an expensive proposition but as already mentioned replacing the plugs using the original brand and part number along with wires if they are used on that car would be worthwhile and when it is safe and convenient, when the engine is running poorly touching the injectors and comparing the feel as they function and comparing them from cylinder to cylinder, paying special attention to the one on the cylinder that misfired might give an indication that the injector is the problem. More often than not I have isolated and diagnosed faulty injectors by feel and used a scope and leak down test to verify the diagnosis.