Hi all,

I've had a P0301 misfire lately. Ever since I bought my 4Runner I've been fighting it off. I got the injectors cleaned by WitchHunter, changed the spark plug and spark plug wires, and done an inconclusive coil test. P0301 each time.

I usually will swap out my EFI fuse. I swapped a newer EFI fuse (still an old, spare one) in and I had no CEL for about 700 miles I'd guess. Even took it up to the mountains, everything ran smooth as can be.

Today, the CEL came back on while I was driving to school. Got the code read, P0301 is back.

My current solution is to replace the fuse, which I have with a fresh new one, and possibly look towards the fuel pump (I need to change the sensor gauge anyways). I was wondering if it would be a good idea to check the fuel pressure regulator as well, or would this not be a problem?

My reasoning is that since the EFI fuse keeps getting burned and changing it clears my CEL, there must be a bad relay or component in the fuel system - my research has led me to find that a bad fuel pump can cause a misfire.

I hope that anyone with any knowledge of this situation can lend some wisdom, because this is bugging me! I am having fun trying to figure it out though. Hopes this helps someone else in my shoes as well -- I'll make sure to update if I end up finding any more evidence.

Are you actually blowing the EFI fuse, or are you just replacing it and the problem disappears?

Does it misfire all the time, or just when hot, or randomly?

I just went thru he!!, erm, I mean, I just learned a lot trying to sort out my misfire. The problem ended up being 2 of 6 reman 5-O motorsport injectors tested good ~14.4 ohms at ambient temp, once they got hot one was reading ~88ohm, the other ~1ohm.

I ordered the injector harness repair "kit" from Toyota (2 wires and the plastic piece that clips to the injector) which allowed me to make a test pigtail so I could check the resistance of the three passenger side injectors when it's hot.

If you're blowing the fuse then it's possible the fuel pump is going out and pulling too much current - but I'm not sure if the EFI fuse is the same fuse that goes to the pump.

If you haven't tried swapping the location of the coils to see if the problem follows. I ended up replacing all three of mine, the old ones still sparked, but when I tested resistance there was some floating on a couple - meaning the multimeter wouldn't settle on a resistance #

Perseverance is the key!

Good luck

I'm also curious, is the EFI fuse (we are talking about the fuse, not the relay right?) actually showing burns or damage? If yes, you've got a serious electrical gremlin in your system, you should NOT be blowing that fuse.

I think you've conclusively ruled out spark.

Fuel pump failures are rare, but they do not cause just one injector to misfire. You would have loss of power when the engine is under load since the pump cannot keep up with the demands of the engine. If you can floor it and the truck responds well (no CEL on before trying) you can assume the pump is good. I'd replace a potentially clogged fuel filter before even thinking about changing the pump. Not an easy job for one person.

Every time you remove the EFI fuse you are resetting the learned fuel trims as well as clearing the CEL codes. That's actually how you reset any code. So it makes sense why removing/replacing the fuse makes it go away. That's why we are all curious if the fuse is actually damaged before you replace it. Photo would be awesome if you have one.

You could try also replacing the EFI relay in the same junction box if you think it's a relay issue.

I replace it and the problem disappears. How I got to this conclusion is this:

Truck had a misfire. I pulled EFI fuse after I thought I had fixed it (spark plugs, wires, etc.) and it kept coming back after maybe 20-30mi of driving. In a moment of desperation I just tried a spare fuse I had laying around. No CEL for ~700 miles.

Then it came back and I put a whole new fuse in. Little 15's. Blue ones. Here ()() is a photo of the original fuse, I think I threw the 700 mile trooper away after swapping it out with a new one. It doesn't look /too/ bad to me, but I haven't seen any CEL yet and it's been over 100mi.

My only gripe so far is sometimes it feels as if the truck idles strangely at stoplights. It doesn't change the rpm gauge, but it feels shaky to me. Maybe it's just me?

My injectors have been serviced by WitchHunter, so they pass all his tests - they should be fine, no?

I've found online in this post: https://www.tacomaworld.com/threads/...r-loss.374758/

while I don't have EXACTLY the same codes (p0171, 0172, lean that I know of) and my truck as far as I know hasn't been through a flood, his symptoms seem to point me to the fuel pump.

So, your thoughts? Am I headed in the right direction? Is my vehicle an absolute travesty and I'm just plugging my ears going 'la-la-la-la-la'?

pls help i just wanna do rad shit with my 4runner

Both those fuses are OK and changing one for another shouldn't make a difference, other than perhaps resetting a computer thingie or something. Either they look like that (the u-shaped wire in the middle is intact = good) or they don't' (the u-shaped wire is obviously not intact=bad).

As for the injectors, it depends on how/what is tested. Mine tested fine at 95*F, but put them in the oven for 20-30minutes @ 250*F, they definitely are not in spec.

That shake you feel at the stoplight is more than likely a faulty injector and it's running on 5 cylinders, or you have an old weak coil.

While trying to figure out my P0301/0304/0300 I was surprised how well the motor managed to hold steady RPMs while sometimes hitting on only 4 cylinders.

But, the best way to know is find a shop or someone with the right equipment to get the fuel pressure checked, also have them check the Fuel Pressure Regulator. That's got to be cheaper than replacing the pump...

2¢

Blargh! Mo' money mo' problems! Well, I guess I'll run some more tests (when I get time/the motivation). I've mostly noticed the CEL will pop on when the motor begins to warm up, usually within the first 5-10 minutes of a drive. I can't recall the last time it popped while we were at full bore having some fun. I'll give the coil packs another go and remove the intake manifold and fuel rail at some point and do the oven test. Give me a few days and I'll be back atcha with the results ;)

Thanks!

Fuses are good, that's a relief. Electrical gremlins are possibly the hardest problem to fix.

So you are also getting fuel codes? Here's a few cheap/easy things to do first:

1. Clean your MAF sensor, search in the forum and there's photos on what two spots on the sensor to clean.

2. Replace your air filter if dirt or full of dust/hair/leaves/bugs etc.

3. After a long drive or after the code pops back up and it misfires, just check the resistance at home. No need to take them out and bake them. The #1 cylinder is the one in the front on the passenger's side of the engine, you don't need to rip apart your engine to get to it. At most you'll just need to remove the intake and then you can check the resistance (ohms) with a multimeter. They are about $10 at most stores if you don't have one.

Nah, no fuel codes. Just p0301. I'll probably do that tomorrow - I feel like going up to Mt. Charleston with the pup and taking a load off. Just finished a summer English class ;)

Wonderful that there are no electrical gremlins. Thanks for all the help ya'll, I'll check back in tomorrow with some specs so we can talk shop. I'll bring some e-beers.

Hey ya'll,

an update: I bought a Klein multimeter because I'm broke. My #1 fuel injector reads at 17.3ohms after driving and warming the engine up. I checked the #3 injector and it reads the same. The CEL popped on today after another couple hundred miles of driving.

wat do? Do I have a valve hanging up or something? My coil packs were good last I checked, and I've switched them /all/ around and it still reads p0301. It happens almost randomly - I get 17mpg/avg in Vegas with the a/c crankin'.

bls help i am confus and cleaning my maf (which is different from everyone elses, 22250-20020)

thx lads

Today the CEL popped again. P0300, P0301, P0305. Any ideas...? This is my daily and I have to keep driving it. Thanks.


Sent from my iPhone using Tapatalk

Well, now you do have the P0300, which is multiple cylinder misfire. Something's gone wrong.

A few more easy things to check (tried to read back but might have already asked this):

1. Clean your MAF, there's two spots of wires to hit. The MAF is one of the keys sensors that controls fuel. You can search here on the forums to finds threads with photos, depending on what MAF sensor you have.

2. Fuel pressure might be a factor here after all. Have you ever replaced the fuel filter? It's located under the vehicle right under the driver's side footwell. There's a plastic cover that protects it from road grime. If dirty and clogged it will limit how much fuel is coming it. Filter is roughly $40, Toyota only! If you do this, get a brake line or crow's foot wrench, it's very common to strip the compression nuts and then you'll never get it off.

3. Inspect all wiring coming off the main engine harness. Any kinks, breaks or tears need to be inspected and repaired. This includes your grounds! If you have a bad ground (battery to chassis, engine block to chassis) then your spark will not be at 100%. Make sure wires are tight and not frayed and there's no corrosion or grime that could affect the metal to metal contact.

One last question, is it possible to take out the spark plug in cylinder 1 (passenger's side front) and snap a photo of the electrode end? That will help give us a clue what might be going wrong. Lean cylinders will cause the spark plugs to have a white, powdery coating. A healthy plug will be mocha colored.

I can absolutely do this wen I get home from school.

I have not replaced the filter - I actually tried to gung-ho it and did not use a crows foot. I have a crows foot now, but it will not go on... lol.

I also cleaned the mag the other day. My hoses are old and ratty as well.


Sent from my iPhone using Tapatalk

Ok, here are pictures of cyl 1 spark plug:

And cyl 5 spark plug:

I also checked the coil packs 1.3-1.4 ohms. I also saw this thread which says sometimes they cause misfires in humid condition: https://www.rme4x4.com/threads/how-d...-is-bad.80026/ (4th reply)

It has been humid in Vegas lately... hmmm... anything else to check? Wires look good. Ground is good. I am missing a bolt for the little fuel line that goes to the regulator that bolts to the intake - I just haven’t gotten a new bolt there since it only keeps the line up. Could use a zip tie if I wanted haha

Thanks y’all, hope someone can help me figure this out more! If y’all need more info please let me know and I’m happy to oblige

See how on the cylinder 1 there's a powdery buildup on the two ground straps? That usually indicates a lean condition in the cylinder, not enough fuel.

If you know your hoses are old and cracked, you can perform a vacuum check. It can cause the engine to run lean:



If vacuum check shows up as good, it's really hard to tell from here without just throwing parts at it. If you have a way on your own to read fuel pressure, that would be a huge help (would indicate bad filter or bad pump) but if not, it's most likely the Fuel Pressure Regulator or, your brand new refurbished injectors .

That's about I all I know to do over the internetz, hard to diagnose from here without seeing the engine in person.

Sounds great my man. I’ll spend the weekend getting all the facts for you and report back. Drove her up to the mountains today - she runs great as always. Curiously, the CEL always comes up at the same stoplight! It’s never popped on during a trip across town or up to the mountains... always at the most inconvenient times like going to school or getting lunch.


Sent from my iPhone using Tapatalk