The VDI operates as you described (and moves freely by hand), the ssv also moves freely by hand, and opens at around 6k. When is it supposed to open? I haven't checked the APV yet because I couldn't find it

The APV is inside the motor controlled by an electric motor at the bottom of the LIM.

It looks too complicated for me to test myself. When is the ssv supposed to open?

SSV opens with your secondaries ... typically around 3500-4500 RPM depending on load and TPS mostly.

So then it should open before 6250, even if revving in neutral?

doubtful.

shine a light on it with someone else in the driver's seat with the car running. turn the car off and I believe the SSV will cycle.

ssv should throw a code if it isn't working...

I will have to do that

Pulled a leading and a trailing plug and they both looked like this, after ~1000 miles or less

Ok, I'd say you are running on the richer side rather than the lean side. Either A) you aren't burning the fuel you have (misfires) or B) The ECU is seeing that you are running leaner than you actually are, and is adding too much fuel. Both perspectives can cause power loss.

I'd rule out fueling problems (pump and filter).

Still leaves a spark problem or a sensor problem that is making the ECU act incorrectly.

I have the P0301 misfire code. Is there anything else that could cause spark issues besides coils, wires, and plugs? I've already replaced all that. The ECU adding too much fuel would also cause the misfire code, wouldn't it?

Technically, yes, if there was too much fuel to make a proper spark.

Pretty much anything that can interfere with fuel or spark, or provide the ECU with a false reading of what the engine is doing. I crossed out anything that we can probably rule out from the info you provided so far.

The possible causes of misfires get pretty big:
- Coil failure
- Plug failure or fouling
- Plug wire failure
- plug wires on in the incorrect order
- plug wires loose or not seated right (on either end)
- MAF fouled or failing
- Front O2 failing
- ESS fouled or failing
- vacuum leak
- compression failure
- grounding problem
- intake screens out of place or missing
- aftermarket intake
- oil being burped into the intake
- bad gas
- fuel filter clogging
- fuel pump failure
- Fuel trims missing or needing reset
- cat failure

You said there is an additional engine block ground, but I'm leaving the grounding problem in that list because failure of a different ground could be messing up something like a sensor. Some of the other ones to check are the ground to the PPF, ground to the transmission, ground to the crossbrace under the battery box and air box that they are bolted to, and the ECU grounds. Completely clean and secure.

The Plug wires are all correct in order and seated correctly on both ends.
The MAF is clean as can be, but I guess it could be failing.
The front O2 could be failing, but I'm not too sure on how to determine that, or where it is, for that matter.
If you mean by "burping" that oil is coming in through the rubber accordion hose from the airbox, then that's not happening, as it is oil-free, and I do have a stock intake/airbox.
I'm checking the grounds and vacuum hoses, but I probably wouldn't notice a leak
Not bad gas, has been doing this through multiple tanks of V-Power from different stations
I have done the brake stomp successfully multiple times, so I don't think it would be the fuel trims.


Edit: All the vacuum lines I can find look good, and I don't see anywhere that looks like a line or hose is missing.
I checked the ground under the battery box, and it's A-ok. I can't find any threads indicating where the transmission or PPF grounds are darn it :/
As far as I can tell the ECU grounds are good. Both of the intake screens look good as well.

Well, if everything you note is correct, it sounds like an O2 failure might be the cause. Might. Process of elimination always has the nasty habit of eliminating all options :lol:

The front O2 is threaded into the header, from the top just forward of the end of the header where it meets the cat. A complete PITA to get to if the car isn't on a lift, and usually stuck in there pretty good. Recommend if you pursue replacing the front O2 to buy it and bring it to a corner shop to do the replacement for $20-$30 or less labor.

I don't think a vacuum leak would be causing your problem though. Typically vacuum leaks are drawing a relatively fixed amount of air, regardless of engine load. So lower engine loads/speeds will have a greater proportion of air being drawn through the leak (and not past the MAF sensor) and have the biggest impact on performance. Higher engine loads typically "drown out" the difference as it is such a small percentage of the whole.

Could a failing MAF cause issues at high RPM, or is that just at idle? Another member was having trouble with her 8 and idling, and I replaced her MAF (after cleaning had no effect, and we figured out what is was by swapping the MAFs between our cars, and then mine wouldn't idle lol :yelrotflm) and it fixed her idle

Depending on how the MAF failed, it could manifest issues pretty much anywhere in the power band.

MAF failure isn't common. Not rare, but not common. Usually just gets dirty, mostly from oil residue.

I could always go pick one up at advance auto (they had one :Eyecrazy:) and try it, and if it doesn't do any good, I'll just take it back.

Also, keep in mind that any "sensor failure" could technically also be a failure in the electrical wiring for that sensor. Technically up to and including the ECU. Though ECU damage/failure is exceedingly rare, and even when it does happen, it's usually from an alternator over-volting.

So, for example, if you did a clutch replacement and pinched the wiring harness that runs along side the transmission between the bellhousing and the block on re-install, you could have sensor problems with either O2 sensor and/or the neutral and reverse switches.

So if you keep coming up to a dead end, trace wiring as much as possible from a suspected sensor to make sure that the wiring is still solid, protected, not cut, etc...

Good point, though it seems the ECU would be saying there was a sensor failure if it were a wiring/communication issue

Generally, yes, the ECU should. My cat failure and subsequent O2 failure never threw a CEL. Either of them. The whole ODB2 / CEL system is focused on emissions still. Within the next few years new cars will be monitoring and reporting on errors or faults farther beyond emisisons. There has been lots of legal back and forth over it between the repair industry and manufacturers, and they agreed to a out-of-court compromise recently.

So the cat had failed on the previous owner, and there was a downstream oxygen sensor failure code at one time, but it went away. Even though I'm running catless, could that earlier cat failure have affected the upstream sensor as well? And are both oxygen sensors the same or are they different designs?

Running catless can tend to fry the O2 sensors a bit faster, though usually from a tune that isn't trying to save the cat. The rear O2 sensor is primarily there for emissions, though the ECU uses it for fine tuning mileage when you settle into a steady-state cruise. It can not cause this level of issues, and there is no direct cross-over where a failure of the rear O2 can damage the front. A failure of the front can damage the rear, by affecting the AFRs in the wrong way, but not a direct effect linking them together.

Well, thats good. I plan on ordering a new front O2. Are the front and rear O2 sensors the same?

No, they are not. The front O2 is wideband and the rear is narrow band. They have the same harness, but are sending and looking for different voltages. The front is ~$200-$220, the rear is $150-$180 (I think those are dealer prices, Rock Auto might be your cheapest option but not all O2 sensors are created at the same quality, Mazmart would be your for-sure option that doesn't gouge). So if it's not clear between two listed O2 sensors which is front and which is back, it's typical that the front is the more expensive of the two.

Ok, thanks for all the help!

If the 02 sensor was bad that wouldn't cause an issue at 5k and beyond the car would be messed up all over the place. A significant power loss above a certain RPM generally says a failed valve.