Engine stutters after warming up (99 Impreza)

[yuityjojo]

Hey everyone

I've been having stuttering issues, so I've come here for advice

Precisely what happens is, the moment the temp gauge reaches halfway, the engine starts to stutter during acceleration (and slightly stutters when you let go of the gas)
However, giving it more gas immediately stops the stutter, and it works flawlessly, although it seems to be more sensitive when it happens

It idles fine, otherwise it only fluctuates between +/-50 RPM which is very slightly audible, and sounds similar to the stuttering itself

So far, I've been to service once where they recommended a fuel cleaner additive, which didn't seem to help ultimately, but a few days after, the issue stopped for a while, then returned.

Since then, it has gotten an oil change, and a new air filter, just in case (it looked quite clean) - along with fresh spark plugs, but that didn't remedy stuttering, it did however make the car more responsive in general

Worth mentioning, diagnostics were ran, result was the knock sensor being shorted / or having a cut wire, and the same goes for the speed sensor (which I do know about, I've lost it a while ago randomly while driving - but the stutter came much later)

Fast forward to today, I went to fix some radio wire connections, and therefore I disconnected the Battery, numerous times in the process - when I went for a small trip later, there was no stuttering whatsoever, not even after reaching operating temps - nothing at all, throughout the whole time

The last time something similar happened was after I slightly slammed the hood after checking something, the stuttering went away for about a week or so.

It really puzzles me overall as to what is causing this, because during my research, I've found quite a few posts, and solutions - some people fixed it with new HT wires, some had to replace the coil packs, and some were just down to the spark plugs.
But in my case, it goes away either randomly on it's own, or after re-connecting the Battery - temporarily in both cases, although it's yet to be seen if the issue comes back this time

So my question is, what could be causing this?

In case it helps, the engine is EJ16

 

Thank you in advance

[David Wilkinson]

Sorry to hear all the trouble your having mate.

My 96 WRX has had similar issues. Faulty wiring was a lot of my woes.

A wire to the maf sensor was split, my coil pack was shorting in the colder weather and the last time I unplugged and replugged my maf sensor and it worked.

So from my own experience I would recommend checking the condition of all your wires to the sensors.

I hope that’s some help and I’m sure you will get some more advice 👍

[Linx]

Have you replaced the knock sensor?

[yuityjojo]

On 6/5/2021 at 10:36 AM, Linx said:

Have you replaced the knock sensor?

Not yet, last time I checked it wasn't in stock around here

[yuityjojo]

On 6/4/2021 at 11:32 AM, David Wilkinson said:

Sorry to hear all the trouble your having mate.

My 96 WRX has had similar issues. Faulty wiring was a lot of my woes.

A wire to the maf sensor was split, my coil pack was shorting in the colder weather and the last time I unplugged and replugged my maf sensor and it worked.

So from my own experience I would recommend checking the condition of all your wires to the sensors.

I hope that’s some help and I’m sure you will get some more advice 👍

Thanks for replying

That seems like a good starting point, I'll check them and see if there's anything weird