P0300

[GSPRunner]

Hey guys, wondering if you could lend any opinions on an intermittent CEL I've been having for awhile now. It pulled up as P0300.

Basically what has been happening for awhile now, is sometimes, and sometimes NOT, when I start my rig whether it's cold or warm, a CEL will come on but then turn off after driving for a couple miles. When it turns off, I feel a slight hesitation from the engine. When it's both on, or off the engine runs fine. No rough idle, fairly normal gas mileage, no loss of power what-so-ever. There are a lot of things that could be causing the P0300, but in the last 10k miles I've replaced all coils, plugs, wires and all 6 injectors.

Any thoughts?

 

[EatonKyleH]

Hey guys, wondering if you could lend any opinions on an intermittent CEL I've been having for awhile now. It pulled up as P0300.

Basically what has been happening for awhile now, is sometimes, and sometimes NOT, when I start my rig whether it's cold or warm, a CEL will come on but then turn off after driving for a couple miles. When it turns off, I feel a slight hesitation from the engine. When it's both on, or off the engine runs fine. No rough idle, fairly normal gas mileage, no loss of power what-so-ever. There are a lot of things that could be causing the P0300, but in the last 10k miles I've replaced all coils, plugs, wires and all 6 injectors.

Any thoughts?

An odd one that's for sure. I like odd haha.

Anyway as you already know a P0300 can be caused by A LOT of things... You could have anything from a barely in spec new injector, to valve timing, valve clearance issues...and the list would continue as far as things you could look into.

Without tearing into all those things, spewing a lot of nerd talk, or just taking wild guesses I pulled this up and figured it would be a non-intrusive way to gather more info on it.

P0300-Random misfire ....very random(not the code description... just how heading describes the customer complaints on a insider-ish dealerish network I use.)

Did you know Toyota's rear oxygen sensors have a maximum of 2.0% fuel trim correction? Officially it is written that the front sensor is used solely to determine fuel trims and that the rear ONLY monitors the cat. That is only sort of true. The rear oxygen sensor has a 2% correction input to in theory adjust for a weakening catalyst.....

Anyway long story shot.... Myself and many others have seen a rear oxygen sensor swing fuel trim just slightly lean enough to cause extremely intermittent random misfires.

With basic scan tool features you can go check this out or have a buddy drive you around while you stare at the fuel trims haha.

Not my words below... I'm sure you put the right plugs in. I just cut an paste.

Tests/Procedures:
1. Monitor the misfire data and fuel trim readings on a scanner. Check to see if the fuel trims change just before or when the misfires happen.

2. Monitor the rear oxygen sensor signal for good cross counts when the problem is happening.

3. Supply the engine with an additional fuel supply when it is misfiring.

4. Check to see if the vehicle has the correct NGK spark plugs installed.

Tech Tips:
This vehicle runs good most of the time. Every once in awhile it will misfire mostly at idle. It would not be uncommon for the rear oxygen sensor to get lazy and drive the fuel mixture slightly lean. This can cause a misfire or slightly rough engine operation.

_____________________________

Just figured I'd share. I'm really kinda trying to pull back from getting too involved with everyone's problems simply because like I said there are SSOOOOOO many things that could cause your issues and proper diagnosis is many times out of peoples comfort level or too time consuming for me to guide. I just thought this information is probably not something you would see elsewhere and definitely a possibility.

 

[GSPRunner]

An odd one that's for sure. I like odd haha.

Anyway as you already know a P0300 can be caused by A LOT of things... You could have anything from a barely in spec new injector, to valve timing, valve clearance issues...and the list would continue as far as things you could look into.

Without tearing into all those things, spewing a lot of nerd talk, or just taking wild guesses I pulled this up and figured it would be a non-intrusive way to gather more info on it.

P0300-Random misfire ....very random(not the code description... just how heading describes the customer complaints on a insider-ish dealerish network I use.)

Did you know Toyota's rear oxygen sensors have a maximum of 2.0% fuel trim correction? Officially it is written that the front sensor is used solely to determine fuel trims and that the rear ONLY monitors the cat. That is only sort of true. The rear oxygen sensor has a 2% correction input to in theory adjust for a weakening catalyst.....

Anyway long story shot.... Myself and many others have seen a rear oxygen sensor swing fuel trim just slightly lean enough to cause extremely intermittent random misfires.

With basic scan tool features you can go check this out or have a buddy drive you around while you stare at the fuel trims haha.

Not my words below... I'm sure you put the right plugs in. I just cut an paste.

Tests/Procedures:
1. Monitor the misfire data and fuel trim readings on a scanner. Check to see if the fuel trims change just before or when the misfires happen.

2. Monitor the rear oxygen sensor signal for good cross counts when the problem is happening.

3. Supply the engine with an additional fuel supply when it is misfiring.

4. Check to see if the vehicle has the correct NGK spark plugs installed.

Tech Tips:
This vehicle runs good most of the time. Every once in awhile it will misfire mostly at idle. It would not be uncommon for the rear oxygen sensor to get lazy and drive the fuel mixture slightly lean. This can cause a misfire or slightly rough engine operation.

_____________________________

Just figured I'd share. I'm really kinda trying to pull back from getting too involved with everyone's problems simply because like I said there are SSOOOOOO many things that could cause your issues and proper diagnosis is many times out of peoples comfort level or too time consuming for me to guide. I just thought this information is probably not something you would see elsewhere and definitely a possibility.

Thanks Sir. I was kind of thinking O2 sensor anyway...figure I'll start with the cheapest easiest thing...

 

[runner823]

Tests/Procedures:
1. Monitor the misfire data and fuel trim readings on a scanner. Check to see if the fuel trims change just before or when the misfires happen.

2. Monitor the rear oxygen sensor signal for good cross counts when the problem is happening.

3. Supply the engine with an additional fuel supply when it is misfiring.

4. Check to see if the vehicle has the correct NGK spark plugs installed.

Tech Tips:
This vehicle runs good most of the time. Every once in awhile it will misfire
mostly at idle. It would not be uncommon for the rear oxygen sensor to get lazy and drive the fuel mixture slightly lean. This can cause a misfire or slightly rough engine operation.

_____________________________

My tip 4 u:

Just monitor short and long terms, add those numbers, and then take your total. If rear is making any adjusts your total will higher or less with the combine short and long. The rear will adjust 1-3% +\- which is normal.
It is very, very, very really very rare that it will cause a misfire condition --- without throwing of long and short. A good way to test pre. & post sensors is by doing a snap test with a propane and watch pre & post sensors response.

So my two cents on the misfire: assuming that the replaced coils, plugs, wires, and injectors are good. But every unlikely a coil and plugs would cause a intermittent misfire. Will the history on misfires, I would be looking at valve train seating issues. I would highly recommend taking it to a dealer or shop to a qualified diagnostic technician.

Or you could do what everyone else do in here and help the parts department make more money,lol.

 

[jgue467]

...Or you could do what everyone else do in here and help the parts department make more money,lol.

Why you throw us ALL under the bus?? :roadkill:

 

[BigRig]

Coolant in cylinder?

Hey guys, wondering if you could lend any opinions on an intermittent CEL I've been having for awhile now. It pulled up as P0300.

Basically what has been happening for awhile now, is sometimes, and sometimes NOT, when I start my rig whether it's cold or warm, a CEL will come on but then turn off after driving for a couple miles. When it turns off, I feel a slight hesitation from the engine. When it's both on, or off the engine runs fine. No rough idle, fairly normal gas mileage, no loss of power what-so-ever. There are a lot of things that could be causing the P0300, but in the last 10k miles I've replaced all coils, plugs, wires and all 6 injectors.

Any thoughts?

Are you losing any coolant or does the coolant in the reservoir look like its getting discolored at all? Could be coolant leaking into cylinder via a small internal head gasket leak, then burning off when you start the car. I had a previous car that ran for awhile like this, didn't overheat or put out a lot of white smoke or anything but ran rough for 5-10 seconds at startup intermittently, and drank a good amount of coolant.

 

[hawaiian05]

Resurrecting a very old thread....
[MENTION=82154]GSPRunner[/MENTION]- What ended up being the cause of your issue? Only reason I ask is because I'm popping a P0300 DTC and experiencing nearly identical symptoms (hesitation/very faint miss upon startup), but no drivability issues whatsoever. I did notice my coolant reservoir low, but also admit I can't remember the last time I checked.

Any input is appreciated.

 

[Pot Hole]

Resurrecting a very old thread....

[MENTION=82154]GSPRunner[/MENTION]- What ended up being the cause of your issue? Only reason I ask is because I'm popping a P0300 DTC and experiencing nearly identical symptoms (hesitation/very faint miss upon startup), but no drivability issues whatsoever. I did notice my coolant reservoir low, but also admit I can't remember the last time I checked.

Any input is appreciated.

My son's 4Runner is doing essentially the same thing - but his doesn't do it generally when it's cold, but starting up after a short drive. It generally clears itself out quickly, but the CEL stays on.

Friend of mine who works on cars says it may be the aftermarket plugs and spark plug wires we put on it. He says Toyota's like factory wiring and plugs and may misfire with other brands. Thoughts?

 

[808HI4Runner]

Did you fix the issue? I am getting same code.

 

[Riceye]

Did you fix the issue? I am getting same code.

First, welcome!

I see that you have posted in four different threads trying to solve your problem. That's fine as you will run up your post count until you can start your own thread.

Next, when seeking help, always indicate the year, engine, transmission, etc. of your T4R. It really helps narrowing down problems/solutions. Even better, add that info on your signature line, which you can edit via control panel.

Anyway, for this specific issue, p0300 is random misfire not specific to any particular cylinder. How old are the spark plugs? Are they dual electrode NGK or Denso plugs? How old are your spark plug wires?

If your answer to any of these is "I don't know", there's your starting point.

Good luck!