OT: Car help, severe hestitation and stutter at idle

[blusubaru]

1998 Subaru Forester, 161,000 miles. 2.5L, AT.

4k miles ago solved a major hesitation issue during acceleration by replacing knock sensor, plugs and wires (OEM sensor and wires, upgraded plugs). Car had very little power until about 2.75 or 3k rpm, through all gears. Everything ran great.

Suddenly the hesitation has returned, but now accompanied by a severe stutter. At idle it barely stays running, floats back and forth between stalling and 1k rpm. At highway speed, letting up on the accelerator to coast and then getting back on the accelerator causes mass confusion and hesitation. This is almost feeling like the car can't decide which gear it's supposed to be in. Coming to a stop light I'm forced to two-foot it to hold the idle, or shift to neutral and keep the rpm's up.

I changed out the ignition coil this afternoon to another known working part, no change.

Gas is not bad, only a week old from the Shell station on the corner. Wife swears she didn't dump diesel in it! I put some Marvel Mystery Oil in the tank and took it for an hour drive around the city. Took it from a full tank to just over 3/4 and the problem got noticeably worse.

Over the last 4k miles, mileage has been steady at 26mpg with 80% interstate driving at 75mph. That tells me the O2 sensor is good.

Any thoughts?

[August West]

Vacuum leak

[JoeH]

August West wrote:
Vacuum leak

ditto!

[blusubaru]

Where should I start?

[JoeH]

Start with the vacuum hoses connected to the engine, see if any are cracked. With 10+ years on the car the rubber compounds start cracking with age. Used to be they lasted much less, so you replaced the hoses every few years anyways. Check especially at all mounting points and hard bends.

[Racer X]

bad turbo waste gate?

[blusubaru]

Racer X wrote:
bad turbo waste gate?

Non turbo.

[deckeda]

The fluctuation in idle --- is it when fully warmed up or only when cold?

Is it throwing any codes (check engine light)?

[blusubaru]

JoeH wrote:
Start with the vacuum hoses connected to the engine, see if any are cracked. With 10+ years on the car the rubber compounds start cracking with age. Used to be they lasted much less, so you replaced the hoses every few years anyways. Check especially at all mounting points and hard bends.

After researching around for any common fault points in the hoses, I found the fuel filter hoses to be a one to check. I replaced the fuel filter about 6k miles ago, but those hoses are good. I checked all hoses up top to try and track it down, but nothing doing. Poked and prodded, moved around, couldn't hear any extra air whistle. Performance really started to suffer at idle, tripped the check engine light again. I think it's time for a dealer appointment. At least a scan of the code will provide a starting point.

[blusubaru]

deckeda wrote:
The fluctuation in idle --- is it when fully warmed up or only when cold?

Is it throwing any codes (check engine light)?

Check engine light is on, I don't know of a source to scan the code. Does AutoZone really offer something like that? It happens at warm and cold.