Hey Y'all,

I'm helping this guy Greg with a misfire problem. It's the #1 cylinder. He's getting a P0301 CEL. He clears it out and it usually pops back up after 20-30 miles of driving.

Here's what Greg has done on his own:
1) Replaced Spark Plugs
2) Replaced Spark Plug Wires
3) Switched Coil Packs and the misfire didn't change.

He hit me up for help and this is what we did yesterday:
1) Performed a compression test of all 6 cylinders. All the numbers were good. 190's to 210. Cylinder #1 checked out at 195.
2) We did an Ohms resistant test on the #1 injector. It checked out. We replaced all 6 injectors with remanned ones because I had heard an injector can check out with ohms resistance but still be the cause of the misfire.

Greg says the truck runs much better now with more power and better throttle response. But, after 18 miles of driving, the P0301 came back. Listening to his engine, it does not sound like it's running on 5 cylinders. If that were the case, I think we'd know because the engine would be running pretty rough. The idle is smooth, there's no hesitation when he accelerates, yada yada.

I've looked at the diagnosing procedures in the factory service manual and the reasons for the misfire are pretty extensive. It could be improper voltage from the ECM to the affected cylinder fuel injector. It could be the MAF sensor. It could ECT sensor. And, other reasons.

What would you guys check next? If you had this issue, what ended up being your fix that hasn't been performed by Greg or myself?

Thanks!

Maybe there is no misfire. If this was the case, an ECU swap may cure. That would be easy.

Maybe misfire is very intermittent. If this were the case, I'd suspect wiring. Not so easy.

I don't think the ECU sends voltage, it's probably the ground signal it sends?

I read his other threads on this, and considering the fixes he already tried, this is a weird one...

Tim, not sure if you have scope? Maybe a toy worth considering. I have a very inexpensive 2 channel laptop scope, and it is a very powerful tool. I got this when I built a Megasquirt waaay back when you actually had to solder together a PCB. For what Greg has going on, I would try recording two channels: the power and ground signal to #1 injector, and also the 12v constant power somewhere near the coil power like at the fuse box. If there is a misfire, you'll see it and can chase it with the scope. If it's intermittent, you can record a time series and review it at the time that the CEL comes on. On the 12v constant channel, zoom in to a small voltage range around the running voltage on Y axis, (+/- 0.1v) and a frequency on the X axis to see the coil packs dwell and release. If it's ignition, you will see it there.

Thanks for the response. I don't own a scope but maybe it's time I get one. I would really like to help this guy figure out the source of the problem. The good thing is his engine runs just fine but it would be nice to finally figure out what's causing the misfire code.

My 97 was getting a random misfire code about a year ago. It was completely random and the truck otherwise ran perfectly fine.

I ignored it. When I was trying to figure out why my transmission wasn't shifting out of first gear when cold, one of the things I tried was swapping ECU's. I went to a local yard and grabbed one and swapped it in. Although it never fixed my tranny shifting problem, the random misfire code has never popped up again since.

I'm not saying that is what is going on here, but it's a possibility.

Tim,

I've been battling an intermittent P0301 misfire code similar to this since I got my 4Runner - sometimes it doesn't come on for ~300/400 miles, sometimes it comes on after ~30/40.

I've done the same things Greg has done along with getting my fuel injectors serviced. Putting a new EFI relay fuse in seemed to do the trick for a bit. It also /mostly/ happens when it's cooler outside (I live in Vegas, when it was hot, it was totally fine).

Curious to see what your methods to solve this are, watching intently. Hope we can all resolve this soon!

1788xx at the time of this post for mileage, if that helps at all.

Hey Mike, I was just searching this morning the inventory of the PickNPulls in my area if they had a 2004 Tacoma on their lots that I could pull the ECU from. There isn't any. I wonder what other years would work for that year Tacoma. I wonder if a 3rd Gen 4runner ECU from a similar year would work?

The ECU does seem like a likely culprit because Greg's engine runs really well. There's no way he's running on 5 cylinders.

I did find a website that sells refurbished ones buy they want around $200 for them.

Greg's misfire seems to reproduce around 20-30 miles after he resets it and it misfires when the engine is at operating temp.

The next step is to get Greg back over and test a few things. We can test the MAF sensor, the ECT sensor and check the ECU for proper voltage at certain pins. Checking the ECT sensor is going to be the hard one. We might have to redo all the same labor we did to replace the fuel injectors because the ECT is right near the passenger side fuel rail.

I had the same issue few years back p301 and I replaced the injecter with. remanufactured one but still had the misfire swapped the injector with a different cylinder and the misfired jumped with the injector. Went with oem and all was well.

We replaced all 6 injectors that were supplied by the same company. They are supposedly OEM remanned. We'd have to be pretty unlucky to pick the one bad injector out the six and place it on the #1 cylinder.

Since his engine runs really well, the problem has to be somewhere else other than the injector.

Yep, I'll have no CEL until it reaches operating temp - before I fixed the sparks, wires, and had the fuel injectors serviced and replaced the 15A EFI fuse along with clearing the fuel map, the misfire wouldn't come on until a few hundred miles. I have checked oil and coolant, no loss. After OT is reached, CEL pops on or it doesn't. Sometimes it has a slight shake, sometimes it idles normally. I've also had people check it and tell me it doesn't seem like it's running on only 5 cylinders - I've also been checking my MPG and I've gone from ~15-19.5 MPG even while battling this misfire.

Also have replaced my ECT sensor as well, it cleared the faulty ECT sensor code (it was broken off...) but I have noticed no other changes. My next look is to either get a new intake hose (the big one that connects to the MAF and air box) and when I get home I'll check my MAF to see if theres anything I can find.

Lastly, sometimes my CEL will come up and read P0300 - 1, 3, 5 - has Greg's ever done this? It does not do it all the time - mostly just the P0301, but I've seen it!

You will love it. It’s like having x-ray vision lol.

Very nice add-on is a low amp current clamp. Just put it around the power wire to injector and you will see the amperage pulses instead of voltage that actually needs to be probed. If it’s the driver went bad on the pcb of the ecu, sometimes voltage wave looks good, but the current actually ramps down instead of instant switch of current on off.

FWIW - don’t get the scope I linked, doesn’t look like it’s supported anymore but there are several in that easy price range to choose from.

Ok, thanks for the update.

No, Greg has never gotten the P0300 code, only the P0301.

I'm not sure what other ECU's would work. I do know that even in our 3rd gens, the ECUs are different. For example, the 96-98 ECU is the same although if you have 4wd or not, then it is different.

The pins/connectors on the 99+ ECU are completely different which is why I had to swap the engine harness over when I replaced my engine (99 engine into my 97). It was easier for me to swap the harness than it would have been to repin everything.

Moral of the story, your probably going to need to find an ECU from the same year (or very close) and make sure it matches the drivetrain (engine size, 4wd/2wd etc).

When is it throwing the code?

I had a similar issue last year where I'd get misfire codes while cruising at low RPM and no actual misfire or no apparent performance reduction at any time.

Believe it or not, it turned out to be the alternator! What's your charging voltage?