I have a 99 4runner 5vz-fe, and its had random very weird misfires recently. Misfire NEVER happens when engine is cold.

It ONLY misfires after the car has warmed up, then sat for at least 15-20 min (or longer), then re-started, then driven for a few miles, and then usually starts to misfire if i idle, then randomly after that, even while driving.

The misfire will also just go away by itself randomly, even if its a hot restart, sometimes it just wont misfire and will drive fine.

The other day I had it misfire so bad that the engine stalled (while in traffic, stopped). I tried cranking, and NOTHING happened. All electronics were on, but zero crank when i turned the key, not even a click, battery is OK. After 30-40 seconds, the engine cranked, started up, and was misfiring, struggling to stay alive, I kept the RPMs till traffic was moving, went to a parking lot and then it just all went away and acted normal.

Plugs and wires have 10k on them. To try and cure the misfire, i changed fuel injectors (to see if they would fix the misfire), fuel filter has 10k on it. Coil packs have good resistance.

What could it be? Bad MAF? Bad Crankshaft position sensor? Im leaning towards a bad CPS, but i want to get opinions.

Codes: P0300, P0301, P0306. Cylinder 1 & 6 misfire.


I have new coils on the way as a precaution, since my truck as 232k miles on it, may order a new CPS soon based on everyones thoughts.

I would think coils too based on what you're saying, except for the part about no cranking at all when it died in traffic. That points to a larger issue. How is the general condition of the wiring on the truck? I'd start with grounds and battery terminals and testing the coils (both ways if you haven't already) and CPS before throwing parts at it. I'd also look "upstream" of the coils as its not very likely that two would fail at the same time and start giving you problems.

Have you pulled the plugs from 1 and 6? That will give you a bigger clue as to what is going on.

There’s a procedure for this in the FSM.

Possible issues:
• Ignition system
• Injector
• Fuel line pressure
• EGR
• Compression pressure
• Valve clearance not to specification
• Valve timing
• Mass air flow meter
• Engine coolant temp. sensor

I’m leaning to a bad injector. Can you check the resistance on them.

He said he replaced the injectors. If that information is not accurate… You could swap injector locations and see if the misfire follows the injector or not.

You could also try giving the MAF a small whack with a screwdriver handle, if it is intermittently faulty (like you describe) sometimes the vibration will get you in immediate fault or engine stumble.

Based on symptom of intermittent failure when hot, I like the idea of looking more closely at ignition coils and injectors. See following thread for more info....

Maddening Misfire Mystery

Buckaroo

I have not, but i'll go ahead and do that today to see what it looks like, I will post pics.

I did test the coils both ways, but i'll double check, i dont think I did it correctly last time. Wiring overall looks good. Do you know the location of the grounds? I know of the battery negative and the ground behind the battery on the body.

Small whack on the MAF, never heard of that but ill give it a shot!

Coils would be a nice and easy fix, a new set is arriving this Friday.

Does anyone recommend throwing the coils in the oven to warm them up to operating temp, then measuring resistance?

By the way, thank you everyone for responding, very helpful suggestions! I'm pretty mechanically inclined, but this damn misfire has me pulling my hair out of my head!



P.s. The CEL only came on once, i cleared the codes after the injectors, and it never came back, even after it misfired again multiple times and stalled.

I had similar issues with my 2002 3rd gen. Some of my coil connections had worn out and had wiggled loose. after replacing the ignition coils, wiring and plugs I haven't had any issues.

I hope im in the same boat, I just replaced all my coil packs, and took it on a test drive. Went to a friends house, drove 25 min, let it sit for 10, went elsewhere, let it sit for 2 hours, and just drove 25 min home. No misfires as of yet, but I won't call it fixed until It has no misfires for a week at least, just to be sure!

Heres plugs 1, 3, and 5. They look good to me, I didn't get a chance to check the other plugs (mainly #6), but I imagine it will look the same as plug #1.

As of now, I changed all 3 coil packs, and gave the MAF Sensor a nice cleaning, double checked my vacuum lines, and sanded down the ground next to the battery so its nice and clean (wasn't really bad before).

As of now, no misfires (look at post above), but we'll see if this holds true given that its an intermittent problem and can randomly come back.

Attached Images

     

That last photo looks like it's starting to build up soot, meaning the plug was running rich. Makes sense if the spark was not firing correctly, since it'd be flooded with fuel. None look lean (brown toasted color) which would indicate injectors not firing.

My diagnosis of what you've posted is that you had unburnt fuel in the cylinders due to bad spark. I think your idea to change the coil packs & spark plug wires will most definitely fix the issue. Good luck and I hope we don't see you post back in this thread!

I think you’re right! So far it’s been 3 days an no misfires at all! I’ll report back in 4 more days to see I’ll have gone a week without any misfires.

As of now, I’m ecstatic, easy fixes for a misfire are sweet!

Thank you all for your help and insight!


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Just an FYI, never misfired after the new coils.