Just bought this 1999 4runner 3.4 automatic at 247k about a month ago. Was in great condition and about a week ago it started vibrating and I lost power and it got hard to start then check engine light came on. I assumed it was misfiring, read the code (P0301) and I was right. Everything seemed old and beat up on her, so I went ahead and put a new coil pack in. Still threw the code, wires looked bad and I knew spark plugs needed to be changed anyways, so new wires and spark plugs. Still throwing the P0301 so compression test on injectors and they're fine. I've read a handful of forums and it seems those are the main issues. I've talked with some other 3.4 owners and some mechanics and they all recommended that. I don't know what else to check/do at this point because it is still vibrating hard, taking a minute to start and I can easily tell it's losing power. Just got tags on it so I am hopeful that I can get everything to work out. Anyone have any ideas? (ps/ this is my first 3.4 and 3rd gen, I have two 3.0's - a 88 and 92)

 

What's a compression test on injectors? I tinker with engines a fair amount and I'm drawing a blank on that one.

You need three things (at the right time) to fire a cylinder:
- fuel
- air
- spark

Spark: From your post it sounds like you've tested/swapped pretty much everything to do with the spark side. Since this is a wasted spark design, the coil pack also fires a plug on the other side of the motor, so since you're only getting a P0301, that pretty much guarantees the coil pack is working and firing. So probably rule out spark?

Air: Verifiable via a compression test (perhaps what you were calling a compression test on the injectors?) - check that the engine is efficiently pumping and compression air like it should. If it's getting a good compression PSI reading, consider that to be working.

Fuel: In a fuel injected engine, just means that the injector is opening, not leaking, and has proper fuel pressure. A fuel pressure issue would affect all 6 injectors fairly evenly, so probably rule that out. Leaves an injector that's got electrical issues. You could diagnose this by swapping the #1 injector to another spot and see if the problem goes with it.

I had an injector go bad on mine at roughly a similar number of miles. Initially it would occasionally stop working when the engine was fully warmed up. Then it became a bit more prevalent. I tried similar things to diagnose - moved the coil packs around on the cylinders, tried a different set of wires, compression tested it, no change. That only left the injectors. Unfortunately, it's a little bit of work to get down to them on the 5VZ engine. The intake has to come off.

 

If you are doing the work yourself and don’t mind eating the cost of a few gaskets, you can separate the intake and swap your injectors around. Move #1 to #3 and see if your miss moves with it.

that’s usually my go-to suggestion is injectors. I had a few die a couple of years ago. Drove me nuts as it came on from heat soak and would go an hour from cold with no issues then miss non stop on hot days.

 

I would also lean towards injectors, wiring or missing tooth on crank rotor. How did you do a compression test on injectors?. And your compression test numbers?, WOT with fuel pump off.