I had a fairly frequent P0300 code popping up 2 months ago so I replaced all the plugs and wires with oem reccomended plugs then and the code went away for a bit. I also had taken the car to a shop to have it diagnosed for the misfire and that is what they recommended was to change the plugs and wires.

I've recently been getting the code again.

My car also consistently burns oil, however I don't really notice the oil level dropping ever, but I do see it in the exhaust and smell the oil for a bit after the car is shut off and hot. This may have something to do with the misfire, I'm not sure.

The first time I ever got the P0300 code was probably in january of 2021 and I have had the car since february of 2020 so I believe it is something new of an issue. I replaced the plugs and wires as I mentioned in March 2021.

It also may possibly correlate with my alternator. I have the cheapo oreilly's alternator which I swapped in in december 2020, and that on it's own is a mess because it doesn't have a proper voltage regulator or something.

You can smell the gas slightly out the back. my friend suggestted my fuel injectors may be pumping in too much gas and that is causing the misfire, but it could also be lack of spark I suppose.

If you listen to the exhaust carefully you can hear a slight stutter every few seconds when the misfire occurs. The engine doesn't throw or catch every misfire so right now it usually will throw one P0300 H C then not detect it anymore and switch over to P0300 H.

Could using 91 vs 87 octane cause the misfire?

Any suggestions and tips are appreciated

~ Tyler

 

If the injectors were “pumping” in fuel, your fuel trims would give a rich condition. That would show up as a cooler cylinder for the offending injector, so an infrared thermometer might help isolate the cylinder giving a misfire. A P0300 is called a random misfire, but often there is only one offending cylinder. The real meaning of P0300 is “ I’m detecting misfires, but the count rate per cylinder isn’t high enough for me to declare a given cylinder misfire, so I will warn the owner, misfires are starting to happen”. It does not necessarily mean one cylinder is misfiring this time and another a short time later.

To rule out plugs and wires (I’ve seen spark plugs new in box with cracks in the porcelain). Fill a spray bottle with water, add a half teaspoon of salt, and spray the coil packs and wires one at a time with the engine running. If this causes the engine to stumble, then you know the cause. If not, then , fuel air or mechanical are next. Vacuum leaks are a common occurrence.

 

A diagnostic shop “recommended” that you change your plugs and wires ??...sounds to me like they didn’t have a clue why the car was misfiring..your issue isn’t alternator related but I’d say that’s sounds like a comeback for that shop...don’t know how long away you took it there but I’d tell them to diagnose it properly for what you paid them !!... without a scan tool and a scope you’ll only be guessing and that can get expensive...a quick check of a mechanical issue is to do a clear flood crank (gas pedal held to the floor while trying to start the engine)...you should hear a consistent even cranking of the motor...you can remove a few plugs and see if they looks oil fouled... that can certainly cause a misfire...you really need a scan tool to diagnose it properly...without it you’ll only be guessing unfortunately.

 

The shop you took it to just threw parts at it hoping maybe their quick and profitable plugs and wires job would get it done. Clearly it didn't. You could take it back to them if you want, but there's a good chance they have no idea what they're doing. That's why I'm VERY suspicious of people calling themselves "mechanics" and I really REALLY caution against just "taking it to a mechanic". Any idiot and his ******* brother can call themselves a "mechanic", and it's not hard to load the parts cannon and fire it at customer's cars and hope they take care of the problem, which is exactly what they ripped you off for. If I were you I'd take the L and never go back to them.

Now, about your miss. I don't think it has anything to do with your alternator. Like a previous response said, P0300 comes back as "random/multiple misfire". Usually you'll only be missing on one cylinder, though in this case the computer isn't detecting the miss regularly enough to set the P030X code, where "X" is the number for the cylinder that is missing. P0307 means a miss in cylinder #7. Get an IR thermometer and take the temperature of the exhaust manifold at each cylinder port. If one is cooler than others, that indicates the cylinder that's missing. If you don't find your miss that way, start unplugging plug wires. With the engine running unplug each cylinder one at a time at the coil pack. Listen for the change in engine sound. If SHOULD sound like it has a miss when you unplug the wire. If there's no change in sound on a cylinder that can indicate your miss.

Usually for a miss I'd recommend the small to big, outside to inside approach. What that means is to start swapping the plug, wire, injector and coil pack one at a time from the bad cylinder to a known-good cylinder. If the miss follows the part you moved that means it is likely the part you moved was bad and should be replaced. The fact that you're burning oil is concerning, and makes me think it's something internal. This can be a bad valve spring, bent valve, bent pushrod, or broken rocker. All of those valve train issues can cause oil to get into the combustion chamber. It won't be a major leak, but will cause the blue smoke like you have seen without a conspicuous loss of oil.

If you've read this far, congratulations, you've gotten to the part where I say what I'd do if it was my car (read on for what I did when it really WAS my car with this exact problem). If this was my car I'd take the valve cover off the side with the miss. Look for any sign of breakage or damage. Pull the fuel pump fuse, and have a helper crank the car. Look for anything unusual with how the rockers and valves move. Hopefully it can show you what's wrong.

So here's my story. I had nearly this exact same thing happen to me. I started with a P0300 code and a miss, with a flashing check engine light. I went through the above steps to diagnose my problem, none of it worked. I boroscoped my engine, and found that my #7 piston had broken at the ring land. The broken ring land was allowing oil to get up from the crank case, and causing a no-compression condition on that cylinder and thus the miss. Long story short, my C5Z is now powered by an LS3.

 

Wow that's a lot of info!

I would like to mention that the shop was correct (christian brothers mechanics) I needed new spark plugs (old ones were crusty af) but I didn't pay them to do it. Only $115 for the 'diagnosis'. They wanted $700 to do the job and that was bs so I did it myself in 3 hours no problem.
Napoleon, your method of removing the plugs to try to listen to the difference. That wouldn't work if the cylinder is not consistently misfiring right? It would only work if the cylinder misfired 100% of the time? Also I think I heard that would cause damage to the engine would it not? Thanks for the detailed instructions though I want to give it a shot. Also with the IR thermometer that would be nice, but I don't know how to get ahold of one. I'll have to look into that. I really would prefer to not buy another engine, I am not that rich yet I'm just a college kid ballin' on a budget haha

C5 Diag when cranking the engine, how do I see if a plug is oil fouled? Does that just mean covered in oil? Also what scan tool would you suggest?

 

It's probably too late now, but when you removed the spark plugs did any of them look different? One with more (or less) soot on it? You can pull the plugs again and look. That might be another way to determine what cylinder is missing.

And no, briefly inducing a miss would not hurt anything. You already have a miss now, so briefly causing another one isn't a big deal.

You can get the thermometer at any place that sells tools, they're common and cheap.

 

You need a OBD2 scanner that can report the mode $06 data. It will give you the cylinder misfire couñts for each cylinder. That way you can tell which cylinders are misfiring and if it's truly a random misfire or 1 cylinder.

 

NO, just remove the plug to do a visual inspection...that has nothing to do with cranking the engine over...cranking the engine over and doing what I had said will possibly reveal a mechanical issue...I personally do relative compression testing and in cylinder pressure transducer testing...no need to disassemble an engine to look for a mechanical issue...to be honest with you I’d take the car to maybe another shop and see if they use a scope to diagnose engine misfires...if they don’t find a shop who does...the ones who don’t use one will only change parts until they find the problem !!..this Autel below looks like maybe it’s a good scan tool !!

 

Alright I just purchased this scan tool. I wonder if it can disable my tpms sensors too because that is starting to get annoying lol.
I'll try to find one of those temp sensors and I'm gonna take a look at my old spark plugs, I still have them just not labeled.

 

That scan tool will only read generic codes, which are the ones generally considered emissions related. The Mode 6 summary will be handy, but it will only report the emissions readiness monitors that most states use to pass/fail cars. Most states requirements, you can have one generic code showing as long as all readiness monitors have passed.

To turn off TPMS, you will need a scanner that has enhanced mode, bi-directional controls, and special functions. Such scan tools start at $800. A clone tech 2 is on the order of $300 and is fairly good, but most the aftermarket scan tools have a better user interface.

 

Actually the mode $06 data shows misfire history and counts for each cylinder. You just need to have a scanner that can decode and present the data for you. There's a lot more to the mode $06 data than emissions readiness.
People are not aware of it's usefulness in diagnosing all kinds of issues the engine control and monitoring.

 

Okay so I'm in mode 6 which is On-Board Monitor Test, and sadly I dont see the option for cylinder misfire test. I watched a video of another person with an autel and his had all cylinders show up here for his ford but I am not getting the same options from this tool. I am definitely lost lol. Maybe I can figure it out eventually.

 

Not sure what scanner you have but my scanner has a page that has the title misfire data when you are in the enhanced OBD2 mode.

CRP123 scanner
When you click on misfire data it opens up a page that shows current misfiring as well as misfire history. Most cheap scanners won't show this data.

 

That scanner is actually on sale right now for $110 on amazon so I'll return this one and purchase that one thanks ~

 

It's actually a pretty good scanner. It does all cars foreign and domestic. It operates mostly like the factory's scanners. So it can read things like the steering wheel position sensor, abs and transmission data. It's only downfall is that it's not bidirectional.

 

Yea I almost want to spend the extra $ on a tech 2 so I can disable my tpms but I think this one will do me well. Nice that you mentiom swps too because mine has gone bad so it will be neat to test it out i think. Thanks for the tip.

 

Okay so the tool works. the results are misfires on all cylinders. in the 100-500 count range.
more testing:
it misfires the most at idle. Almost no misfires at 2k rpm. It also seems to misfire more on odd numbered cylinders but it misfires on even ones too.

 

The MAF also seems to read fine. It goes between 6.0 and 7.0 g/s while at idle.
Sometimes my heated oxygen sensor 1 drops down to 50 mV but the #2 sensor stays consistent between 700 and 800mV

 

Your MAF is reading fine...at idle it should read approximately your liter displacement in grams/second...what are your long and short term trims on both banks at idle and 3000 RPM ??...below is a screen capture of my upstream and downstream O2 sensors at idle and some other data PIDS...the downstream (sensor 2) should be steady at about 700-800 mv’s...upstream should oscillate between 200-800 mv’s !!...if upstream go lean (below 450mv you should see your fuel trims go positive

 

Okay here I recorded a video of all that data. I'm not quite sure what to make of it.
It seems fine based on what you said I think. So this wouldn't point to an issue I don't think?
EDIT:
I watched a video about fuel trim.
Does my long term fuel trim seem a little high? especially at 3000 rpm? That would mean it might be a little lean?
Also your O2 sensor 2 on B1 and B2 are giving high volt but mine give low volts at idle.