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P0302 Random Misfire on 2006 Mazda6 V6 3.0L

So I have been experiencing a problem that can’t seem to find a fix for it. I have a check engine light for a random misfire on cylinder 2. I have changed the spark plugs, the injector, and even the cylinder 2 coil, but still have that random misfire. The funny thing is that it would run great within short (about 30 miles) one way trips. Lately, I have gotten a new symptom: It has started to hesitate and can feel a loss of power but only once the car has been driven for about ten minutes non stop. Can’t figure this out and need some help. Please any ideas concerning this will be great!

It’s possible you have a burned or misadjusted valve that is causing low compression and a resulting misfire in #2. Have you done a compression test?

Another possibility is coolant leaking into #2 cylinder through the head gasket and causing a misfire. Have you noticed any loss of coolant?

I have not been losing any coolant. I was thinking about doing a compression test but, somebody told me that if you had a loss of compression, you would have a misfire from the moment you start the engine. I thought that was a great relief because it is a pain in the butt to do a compression test in that cylinder since it is under the intake manifold and closest to the firewall. Do you think I should still do a compression test anyways?

i had a car that got a plugged cat causing random misfire. but if its narrowing it down to one cylinder i think a compression test would be a good place to start. does it seem to have the problem at higher rpms or all rpms and is it consistent once it starts or does it come and go.

Assuming all suggested routine engine maintenance is up to date per the owner’s manual. If not, that’s the first thing to do. And assuming you’ve already asked (or at least Googled) Mazda repair experts, as this might be a well known problem on this car.

Misfire means the computer senses the crankshaft isn’t getting the expected bump in rotational speed at the time the cylinder is supposed to fire. Think about it this way in order to develop a plan of attack to find out what is wrong: A successful firing means the proper amount of fuel and air is delivered, then compressed, then ignited, then the exhaust gas removed. And that sequence all has to happen with the correct timing.

Since it always occurs on number 2 cylinder and no other, that simplifies the problem. It’s unlikely to be a problem with the air intake system, or one of the exhaust manifolds or plugged cat(s). Or a bad O2 sensor. Or a fuel quality problem, like water in the fuel. It means either a fuel delivery problem, a spark problem, an exhaust gas removal problem, a compression problem, or a timing problem, but only on that specific cylinder.

Given what you’ve already done – assuming you did those correctly – these are possibilities I would consider next

  • Wire(s)/connector(s) going to that spark plug/coil.
  • Wire/connector going to that fuel injector.
  • ECM problem for that injector (most likely would be a connector problem on the ECM circuit board corresponding to that connector).
  • Crank or cam timing sensor(s) (consistent with the fact it works on short trips, these tend to fail only when hot)
  • valve, rings, or other compression problem.

Which you test first is up to you. I’d be inclined to test the compression first.

If a compression test shows a fault in No. 2 then you need to revisit the compression test and determine if it’s related to the valves.
There’s a possibility that it could be related to tight valve lash and the reason it occurs after running for a while is due to heat related metal expansion which could then be holding the valve open a little.

hes already replaced alot of those parts and if it was a wiring issue it probably wouldnt be fine for the first 30 miles then act up the same every time. im not saying its not that i would just assume its something mechanical that heats up consistently. however i did have a car that had a connector on the firewall that seemed to act up every time it got warm so then again i wouldnt rule out anything

Like ok4550 says, a misadjusted valve can work OK for a few minutes until the engine heats up, then metal expansion causes the valve to stop seating correctly, losing compression in the cylinder and causing misfire/loss of power.

A compression test may be a pain to do on this cylinder, but if you don’t do one you won’t be able to rule out one possible cause of your problem. And you’ll need to do the compression test when the engine is hot/misfiring.

Check the vacuum lines. They can cause it too.

Thank you very much!! I will do a compression test as soon as I get a chance and keep this discussion updated. If anymore ideas pop up, just keep posting. I am willing to try anything at this point.

Something you could do before going into a compression test is to run a vacuum test. Vacuum gauges are cheap, easy to use, and can tell you a lot about what’s going on with an engine once you learn to read the needle. New gauges will come with a sheet showing what symptoms may be causing certain readings.

A vacuum gauge is simply plugged into a vacuum source from the intake manifold. Readings will vary based on engine condition, altitude, barometric pressure, and so on but generally a gauge will show about 17-21 inches of vacuum at idle. The needle should always remain rock steady at an idle and only move when the throttle is blipped.

It’s something to consider before wrestling difficult to access spark plugs. Personally, I consider a vacuum gauge an invaluable and “must have” tool. It’s not the answer to every problem (no tool is) but it’s a great starting point.

So Before I could do the compression test, My wife drove my car and ran it out of fuel. After that, it hesitates and misfires once you start the car. I’m starting to think it might be a fuel delivery problem. Any thoughts or should I do the vacuum and compression test still?

If she ran it out of fuel the fuel filter and fuel pump be bad. Fuel pumps use gas for lube and without it they can eat themselves alive.