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2001 Toyota Tacoma 2WD 2.4L 4cyl. Just under 100k miles.

Looking for guidance on narrowing down the culprit for a misfire. Here's the pattern:

Truck runs fine from a cold start, and continues to run fine once it warms up.

But once you stop, shut the engine off, and come back to restart it while the engine is still warm, it'll misfire/run terribly. Strong vibration, low power, hesitation, flashing CEL.

If you let the engine cool off completely and restart it, it's fine again.

Every once in a while, it'll temporary resolve itself once its started misfiring.

Codes:

  • It's throwing code P0301 (cylinder 1 misfire) consistently.
  • Only once it threw a P0171 (lean) after I had to hobble home on a 35 minute drive after it started misfiring (this was before I realized that if it cooled down it would resolve itself temporarily).

Things I've done so far:

  • Made sure the gas cap was tight
  • Replaced all 4 spark plugs (Denso Iridium 3324)
  • Swapped 1st and 4th coil (no change)
  • About a month ago I cleaned the MAF sensor, and I cleaned it again today
  • Cleaned the throttle body
  • Checked/blew out the engine air filter
  • Visual check for vacuum leaks (didn't identify any). Don't have a smoke machine may pick one up.

Here is a video showing:

  • Fuel trim levels when it's cold and not misfiring
  • Footage of when it is misfiring
  • Fuel trim levels of when its misfiring

In researching misfires P030* codes, it seems like there could be a lot of culprits, so I'm trying to narrow it down.

Questions I'm throwing out there:

  • Based on the symptom of it only misfiring once warm and restarted, where would you look first? (then second, then third?)
  • Are there other routine 100k maintenance things that could possibly resolve the issue that I can try? E.g. the spark plugs - they were due anyway so I didn't mind throwing the money at them to see if it fixed the issue. Looking for other "low hanging fruit"/should be done anyway type things.
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    Try using a different fuel station for a couple of fills... have you checked fuel for water in it? – Solar Mike Jul 8 at 5:07
  • Have you replaced the spark plug cable? – GdD Jul 8 at 7:56
  • It has coil on plugs, so there's no spark plug cable to be replaced as part of routine maintenance. @Solar Mike: You may be right about bad gas - my Toyota Matrix which was filled from the same station was smelling really bad of sulfur - filled up at a different station with super and the smell went away. I've just put a tank of super and can of BG 44k fuel system cleaner in the Tacoma to see if that resolves the issue. Will report back. – sbuck Jul 19 at 18:08
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If you are getting a misfire on cylinder 1 and a lean code to boot, I'd suggest your issue lies in the fuel injector for cylinder number one. Especially since this is inconsistent. A lean misfire would mean injector #1 isn't firing, or only partially firing. It could be because of it sticking or because the electromagnet is faulty. If it's sticking, the BG44k should clean it out, but might not.

Two things you could do to check it. First, when the vehicle is running poorly, pull the injector lead to see if the engine changes how it is running at all. If it continues to run the same, there's a hint this could be the injector. You could also pull the injector lead when it is running well to see if it changes how it's running. Secondly, you might consider changing the injector with another injector (ie: move injector #1 to the #3 cylinder and visa-versa) to see if the problem follows it. This is the same idea as you had with the coil, only you'd be doing it with the injector instead. You have to be careful to relieve pressure on the fuel rail when you do this as well as to be careful with the o-rings on the injectors (top/bottom) as these can easily be damaged in the process.

  • So far BG44k hasn't resolved the issue. I agree with you that the next step is troubleshooting the injector. Will report back. – sbuck Aug 8 at 17:10
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    Issue resolved! I ended up replacing all 4 injectors with a set of refurbished Toyota injectors. Check out the crack that was on the injector I pulled out of cylinder 1 (the misfiring one): i.imgur.com/NRUm1Fd.jpg – sbuck Sep 12 at 18:14
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It may not be completely applicable in your case considering the issue only arises when the engine is warm but I would look at the timing belt. Normally though it doesn't discriminate based on temperature so I'm not so sure.

Another issue that is more related to temperature is checking the spark plug wires. Resistivity rises with temperature and so it seems that this may be affecting the spark. What you can do for that is pull the spark plugs out and leave the coils attached. Go to your fuse box and pull out the fuel pump fuse and get a friend to start the car and check the spark plugs to see the spark of both and see what issues could be arising.

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