Narrowing down cause of misfire when engine is warm

2001 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.

Below is a video showing:

1. Fuel trim levels when it's cold and not misfiring
2. Footage of when it is misfiring
3. 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:

1. Based on the symptom of it only misfiring once warm and restarted, where would you look first? (then second, then third?)
2. 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.

 

A. Did you also change plug wires?
B. I had a truck doing this exact same thing, was a head gasket leak. 100,000 miles seems low for that kind of problem, but I’m putting it out there.

I’m betting plug wires. Had a car do the same thing, plug wires were the issue.

 

It has coil on plugs, so there's no spark plug wires to be replaced as part of routine maintenance.

I'm starting to wonder if it's a bad tank of gas - my Toyota Matrix which probably got filled from the same station started smelling really bad of sulfur. At the next fill-up we put in super from a different station and the smell is gone.

This article supports this theory: Bad Gasoline Causes Performance Problems:

When people use gasoline that does not keep their fuel system clean, their injectors gradually clog up with varnish deposits. Some injectors (mostly the older pintle and nozzle style) are more vulnerable to clogging than others. It is not something most motorists notice right away because it takes time for the deposits to accumulate. But with every drive cycle, the accumulation of deposits gradually restricts the nozzle orifice causing a reduction in fuel delivery.

Deposits also disrupt the normal injector spray pattern, which interferes with fuel atomization and mixing. This results in a lean fuel mixture that may cause the engine to misfire, idle poorly, and hesitate or even stall when accelerating.

It does not take much of a restriction in an injector to lean out the fuel mixture. Only an 8-10% restriction in a single fuel injector can be enough to cause a misfire.

A lean mixture caused by dirty injectors also increases the risk of detonation when the engine is working hard under load.

Lean misfire at any speed increases hydrocarbon emissions that may trigger a misfire code and turn on the Check Engine light on 1996 and newer vehicles with OBD II systems. The code will often be a P0300 random misfire code, or you may find one or more misfire codes for individual cylinders depending on which injectors are affected most.

Use of the proper amount of detergent additive helps wash away the varnish deposits and keeps the injectors flowing normally. Varnish buildup is worse after a hot engine is shut off and undergoes a period of heat soak. During this time, any residual fuel that is in the tips of the injectors evaporates and leaves behind the heavier waxy compounds that form varnish deposits. If the next batch of gasoline that squirts through the injectors when the engine is started does not contain enough cleaner, the deposits will remain and continue to accumulate. Eventually it builds up to the point where it restricts the injector and causes drivability problems. This requires cleaning the fuel injectors.​

 

I had mine do that a long time ago. Turned out it was a coil pack. Mine is a 98 though. I think up to 02 is about the same.

 

I did swap the 1st and 4th coil pack to see if that changed the misfire to the 4th cylinder - no dice.

 

My truck started smelling like sulfur when I cracked the windows a bit. Happened about 5 minutes after I started it for the first time one morning. Then after one stop and about 30 miles, I tanked up and right after that the CEL came on. And the engine pinged with moderate to heavy load. Unfortunately, subsequent tanks of gas resolved the pinging problem but not the sulfur stink. That happened in early June, and last night I noticed the same old stink.... bummer.I'll just have to wait and see what the CEL does.

 

As a test I would swap injectors, cheapest thing to do, or replace the #1 cyl fuel injector. Injectors can be "rebuilt" but the down time IMHO, isnt worth it.

 

Sounds reasonable. In my experience, electronic components work well when cold, marginal ones act up when hot. If it isn’t a coil pack and you’re positive it isn’t an issue with the harness or connector itself, I’d do as advised above and swap an injector.

 

had this exact thing. in addition to the maintenance you've done, I used electrical contact cleaner on EVERYTHING in the ignition and applied new dielectric grease to every connection. so far so good.

 

Update: I pulled the injectors and replaced them with refurbished ones. I didn't get to put everything back together and test before departing on travels, so I don't know yet whether it fixed it (will report back when I know more).

But, I did find a crack on both sides of the cylinder 1 injector (cylinder that's misfiring). Other injectors were fine.

Seems likely this is the culprit? What could have caused this?

But,

 

I'm chasing a P0301 on my '95 with 126K miles.
Hot misfire only. No other codes. Replaced the plugs and coils. Added a can of 44K.
I gotta get in there and check out the injector. Betting it's going to look something like yours.

 

Be sure to check the resistance of the injectors. Should be around 14-16 ohms. When I got the P0301 code I found that the injector on #1 cylinder read 24 ohms and the rest were between 14.2 and 15.7 ohms. I replaced the injector and have not had the problem since.

 

I've got a brand new Toyota injector still in the sealed bag, bought it for my previous '03 Tacoma but didn't need it. PM me a reasonable offer and it's yours.

 

Follow-up: I can now confirm that replacing the injector fixed the issue; no more misfiring and it's running great!