2001 Forest, 166K, recent head gaskets. Car started stumbling pretty bad under load at part throttle. Checked fuel pressure and found 35lbs at idle. Alldata says I should see 43PSI at idle with an acceptable range of 41-47PSI. Swapped the fuel pump and pressure regulator. The car behaves a bit better than it did, but the problem hasn't gone away. I still see just under 40psi with the new regulator connected to vacuum. If I remove vacuum, fuel pressure comes up to about 46PSI.

 

I did find that if I shift it manually and keep it wound up just a bit, the car runs just fine. WTFO?

how recent on the head gaskets? i would suspect a vacuum leak somewhere - think it might be worth it to shoot some starter/carb spray all around the hoses/top of the engine?

 

i doubt it's the fuel pressure here unless you see a notable dip at part throttle to match the symptoms?

 

no check engine light?

Well first thing is first, got to do the basics, a FULL tuneup.

Head gaskets were done less than 1000 miles ago. New plugs and wires, too. So far as it goes, that IS a tuneup on these engines.

Found the problem. Got the car over to the dealership near me and talked them into plugging in the super-ultra-awesome Subaru Factory Scantool. Showed a miss-fire on #2. Went over it with a fine-tooth comb and found a couple of small cracks in the ceramic casing on that spark plug. Replaced it and voila...no more problem.