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Misfire and engine knock

My mom’s 2003 Alero has had a bad catalytic converter for the last few years… at least that’s what she was told when she took it to the shop. The engine light (P0420 threshold below efficiency) has been on over the last few years and I occasionally check it with a code reader to make sure no other codes have been added. Up until today - as far as I know - none have been. Now today when she was driving it the engine light started flashing and running rough and making a knocking sound.
She brought it straight home and I hooked my scan tool up and it gave the same cat code as I just mentioned, plus a P0306 misfire code. So I pulled the plug wire on number six cylinder and hooked up a spark tester to it - and it showed a decent spark. And to verify the spark being normal, I tested at another plug wire and the spark looked about the same…so I don’t think anything is wrong with number 6 plug wire or coil or anything like that but am not sure. Also, the spark plugs were replaced a couple of weeks ago because the ones in the car were worn down about .010’’. I can’t imagine this having anything to do with the misfire because I did them myself and made sure they were gaped properly and that all the wires were put back on the correct plug/cylinder and plus it ran fine after the plugs were installed - but thought I would mention it anyways. And I was very careful not to damage the platinum on the plug when gaping them.

Did a little bit of research and found this: A code P0306 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve 
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

So where should I start? Any ideas?

Very often a bad cat code is actually caused by a bad o2 sensor. However I doubt that would cause your issue. What motor does she have in this car? Is it the 3.8 v6? Any way that gen if GM motor had a problem with intake manifolds, melting by the egr valve and causing an antifreeze leak. When mine went it caused a missfire in number 6 as well, but mine happened on start up sometimes. That would be my first guess, second would be a compression check.

That list comes from the fact that misfire code refers to a lack of combustion taking place rather than from a spark problem. Spark problems are one thing that can result in no combustion.

The next thing to do is pull the plug back out and check compression since if there is some mechanical engine issue there’s not much point in chasing anything else. You’ll really need to check all 6 cylinders since what you need to know is how they compare to each other as well as how high/low the compression is.

Sorry, but I have to ask, what made you think you could ignore the bad catalytic converter for several years?

Before you start replacing things I would recommend you take it to a good shop and let them do the diagnosis. If that isn’t an option check all of the plugs again and do a compression test. At least you won’t have replaced the cat an O2 sensors only to find the engine needs a rebuild.

I’ll do a compression check and report back.

‘‘Sorry, but I have to ask, what made you think you could ignore the bad catalytic converter for several years?’’

My mom decided not to replace it because we don’t have e-check here and I think they wanted something like 6 hundred dollars to replace it.

gsragtop, it’s got the 3.4L V6

A cheap vacuum gauge will come with instructions and should enable you to quickly determine if the catalyst is obstructing the exhaust and possibly causing the knock and misfire. And is it a hard knock or a pinging? Also, is there a misfire at idle or only when accelerating?

It’s a knock.

And yes, it misfires at idle.

Since you just replaced the plugs recently, it could be simply a defective spark plug. I’d get another new one install it to see the results. A bad coil is also something to rule out once you replace the #6 plug. After those things are tryed you could be looking at something more complicated and expensive. I’d think about a compression check on the #6 cylinder next.

I just tested but forgot to warm the engine up first. By the time I remembered that it was supposed to be warmed up first, I already had the plugs out, the fuel relay unplugged and the ignition system disabled so I just went ahead an tested anyways because the three plugs in the back are kinda hard to put in and take out. I only tested cylinders 2,4, and 6… will do the other 3 if it is necessary for a proper diagnosis.



6:120,90,15 (three seperate tests) then added three squirts of oil and got 130. Then let the pressure out of the gauge and got 25.

Piston rings?

#6 piston has a hole in it or a valve is sticking. How did the #6 plug look?

The plug looked fine. It’s only been in there a few weeks though.

Whatever happened in that cylinder seems to have happened suddenly…

How does a piston get a hole in it?

Broken pistons result from foreign pieces floating around, valves hitting pistons, spark plugs breaking apart, and detonation.

You posted that on #6, after running three tests you “let the pressure out of the gauge.” Does that mean that you did not open the relief valve between the other tests?

When I add up the symptoms; running rough-knocking-20psi compression it seems very likely that something is broken in the #6 hole. If you have access to an air compressor you might pressurize the cylinder to see if the pressure is leaking out a valve or into the crankcase.

No, I always opened the relief valve between tests.

I don’t have an air compressor other than a cheap little Craftsman model that I use to air up tires with.

How powerful of an air compressor do I need?

She ended up taking the car to the shop and the mechanic pulled the valve cover and found that one of the rocker arms had come completely loose and was just laying there… Somehow the screw that fastens it down got striped out. Anyone ever heard of such a thing? I’m having a hard time figuring out how this could have happened. And also curious if it’ll happen to the other rocker arms.

And I’d like to know why the absence of the rocker arm caused such low compression because I had it in my mind that the arm pushed the valve open and therefore without the rocker in place the valve would stay shut allowing the cylinder to HOLD compression.

I’m guessing that it must have been the valve clearence adjuster screw that worked its way out. If I’m correct, than it would be because it was not secured properly.

In order to have compression, a cylinder has to be able to pull air in. If the rocker was on an intake valve, the cylinder would not have been able to do so. I’m guessing that although the adjustment screw had backed out, it was probably still opening the intake valve a bit and allowing some air to get drawn in, resulting in some compression.

Understand that I’m left making some guesses without actually seeing the engine.

‘‘In order to have compression, a cylinder has to be able to pull air in.’’

Ahh…now I see. Thanks

I’m having I’m pretty sure the same exact problem. Just curious about how much it cost for the fix, and were all the issues fixed?

you could start a new thread that outlines your issues or just say look at post 1-6 and that is my problem.
the 3.4 motors did have a history of the rocker bolt pulling out of aluminum head. you can helicoil the bolt hole as a fix. a misfire can be many things. burned valve. bad coil. bad plug. bad wire. low compression. stuck lifter. and so on.

Sean, Mystic has not been here for almost a year . The repair cost is 6 years ago than that will not mean much . Also if you are not doing the repair yourself telling the repair shop what to fix is never a good idea. Just give a clear description of the problem and let them do their job.