My Boxster is acting like an idiot
 

After an engine rebuild, the idle was at 800 rpm after the fuel trims caught up to the rebuild. The idle has slowly fallen since then and now sits at a stable 650. A couple of day ago it stalled in a parking lot at closed throttle. Now it has stalled while driving, always at closed throttle. At closed throttle in a parking spot the idle began swinging wildly between 400 and 2000 rpm until the engine died. That took about 10 seconds. Driving after that it began choking and at ~4500 rpm with the engine stuttering and essentially shutting off and then restarting at lower RPM. It has begun to surge almost imperceptibly at 3000 rpm while cruising. I put 5 gallons of gas in it and it ran like a champ to 7K rpm, except for the slight surging. Other than exorcism, any ideas what the problem might be?

The car sat for ~ 3 years after an IMS failure. Complete engine rebuild and it's been on the road ever since.

round up the usual suspects:

fuel pressure

ignition switch

vacuum leaks

loose wiring

I was hoping someone might narrow it down with the following symptoms/clues:

1) idle (low, and now surging and falling until the car stalls)

2) now occasionally crapping out at high RPM (right around Variocam actuation - ~4500 RPM)

3) a little bit of surging under load at any RPM at all times. (barely noticeable)

4) Put in 5 gallons of gas and the car is magically "fixed", Coincidence?

5) and its getting worse as time progresses :eek:

Anyone have insight or a good guess at what problem or failing component would account for all of the above?.

No check engine light

2003 S manual transmission

Fuel pump (See list: fuel pressure)

Sounds like fuel delivery problem, wouldn't hurt to do the throttle initalization process: Turn key to on for 1 minute without touching gas pedal, turn key off for at least 10seconds, then you can start engine as normally.

Check for pending trouble codes, use drive links to disable fuel injectors 1 at a time.
Lot's of 986 fuel pumps aging out these days, also might try a simulated fueling procedure without actually adding any fuel to see if you get the same positive results.

I feel your pain, as I'm having similar problems. -how-diagnose-resolve-engine-issues-using-durametric

It could be the MAF. Try unplugging it and go for a short drive.

If everything has settled down then you know its a MAF problem,
and a very easy fix.

Try cleaning it first ONLY with MAF cleaner spray as anything else tends to make matters worse.
Many folks get more time after cleaning but it seems that only a replacement fixes it for the longer term.

I have to admit that is sounds a lot like a fuel delivery or regulator issue but start with the easy cheap fixes first.

PS: Auto Zone will loan you a fuel pressure tester for free and its easy to test with the fuel rail Schroeder cap being on the passenger side.