P0306 CEL - Where to start

[MacLaddy]

Hello Folks,

My '99 runner has recently begun running extremely rough on "occasion." It usually runs just fine, but once in a while it will run terribly when it first starts up.

My last encounter went something like this. After starting my 4runner I noticed a check engine light was one, and it was running a bit rough. I opened the Torque app on my phone and found three codes. P0300, P0306, and P0120. (the PO120 I am not surprised about, as it is a throttle position sensor issue that I don't believe is related to this problem.)

So I have the P0306, and P0300 codes, plus a semi-rough idle. I drove for about two or three minutes and it becomes much more sever. I start getting the flashing CEL, and a much rougher ride. I drive it to a nearby store, grab what I need, and when I return Gunner is running fine. It still had the codes, but was running fine.

I shut it off again, cleared the codes using the Torque app, and have still not seen this problem resurface days later.

I've read up on this in the forum, and it seems like the fuel injector is a common issue. However, it seems like the injector problems typically act up only after the engine warms up. My case seems to be the opposite.

Any thoughts would be appreciated. We are planning an 800 mile drive this weekend, and I need to have this figured out. I've cleaned the MAF sensor, but that is the extent of my troubleshooting at this point. Plugs & wires seem to be the obvious culprit, but I wanted a second opinion before I started digging into it.

Thanks,
Mac

 

[jeremyc74]

I opened the Torque app on my phone and found three codes. P0300, P0306, and P0120. (the PO120 I am not surprised about, as it is a throttle position sensor issue that I don't believe is related to this problem.)

Why would you think the TPS is unrelated to the misfire codes?! The ECU is giving you one very specific failure, and two symptoms of the failure. The first place to start is the TPS, and go from there. The ECU can't fuel the engine properly if it doesn't know where the throttle plate is at.

 

[MacLaddy]

Why would you think the TPS is unrelated to the misfire codes?! The ECU is giving you one very specific failure, and two symptoms of the failure. The first place to start is the TPS, and go from there. The ECU can't fuel the engine properly if it doesn't know where the throttle plate is at.

I suppose I am taking it for granted that the presence of this code is unrelated, but this particular code has been haunting me for a few months now, and it has- in the past- had no effect other than affecting proper downshift when depressing the accelerator.

I intend on replacing/repairing the position sensor before the upcoming trip, but assuming it isn't related, then what do you think?

I've also had both P0306 and P0300 codes pop up without having P0120 registering. I see where your coming from, but they really don't seem to be related.

*EDIT* Additionally, the P0120 code never came up at all until I cleaned the MAF sensor, which I did entirely because of this issue. This is also when I accidentally ruined my throttle position sensor... but I don't want to go there.

 

[jeremyc74]

I suppose I am taking it for granted that the presence of this code is unrelated, but this particular code has been haunting me for a few months now, and it has- in the past- had no effect other than affecting proper downshift when depressing the accelerator.

I intend on replacing/repairing the position sensor before the upcoming trip, but assuming it isn't related, then what do you think?

I've also had both P0306 and P0300 codes pop up without having P0120 registering. I see where your coming from, but they really don't seem to be related.

*EDIT* Additionally, the P0120 code never came up at all until I cleaned the MAF sensor, which I did entirely because of this issue. This is also when I accidentally ruined my throttle position sensor... but I don't want to go there.

If the TPS has gone bad to the point where it's affecting the ECU's control of the transmission, it is absolutely going to cause problems with fueling. You may have some other issues going on, but you can't be sure until you get the TPS straightened out.

The TPS is easy enough to test, but 9 times out of 10 if the ECU is telling you it's bad, it is.

 

[jgue467]

well the 306 is saying misfire in cylinder#6. Have you replaced the spark plug and/or wire on that cylinder? (I'd do them all but at least that one first to see if the code disappears.) + of course checking the TPS (could be dirtly and/or loose or simply failing). Luck and enjoy that trip! :cowboy:

 

[Brveagle]

Umm, fix the TPS? Cheap simple fix. Start simple, work hard.

 

[skinnie]

Most likely an injector. Mine was having the same problem

Changed wire n injector and now runs smoothly

 

[gamefreakgc]

If it's due for spark plugs and wires you can do them anyway. They are cheap and simple to do. Fix that TPS as well, it affects more systems than you think it does.