2003 Volvo S40 1.9T 86k miles
My car was running fine until this problem popped up this week. Turned it off when I went into work and it was running fine. Started it after work and it ran like crap.

The car nearly stalls when dropping from high revs to idle, lights dim a little, and low gear light (sometimes) comes on momentarily. The engine misfires when going from an idle to accelerating. At the same time that happened, a whirring sound appeared when driving and the engine revs are up. (I don't know if the whirring and misfires are related or just unlucky timing.) Driving at a content speed no whirring. Parked (idling or revving) no whirring so I can't tell where it's coming from. Got the codes read at AutoZone: Codes: P0172 (running too rich) P0301 P0302 P0303 P0304 (misfires all cylinders). AutoZone told me it was the MAF. I cleaned the MAF and it made no difference. I then tested by driving the car for a few minutes without the MAF attached and the car drove fine (whirring was still there). I reattached the MAF and it ran like crap again. So I bought a new (not re-manufactured) MAF and installed it. Still runs like crap.

AutoZone tested my battery and alternator and said they are both good.

FYI in case it helps:
Timing belt & serpentine belt replaced 15k miles ago
FPR replaced 15k miles ago
Fuel filter replaced 15k miles ago
Upstream O2 sensor replaced 9k miles ago
Negative battery cable replaced 9k miles ago
Upper vacuum lines replaced 8k miles ago
Throttle body cleaned and gasket replaced 8k miles ago
Performed dry compression test 7k miles ago. 165 on all cylinders. 90 on first compression.
New battery 5k miles ago (March 2017)

 

inspect your coil packs for tiny cracks. Pretty common to have these go over time.

 

Normally running poorly is as pierremcalpine wrote, the coils developing hairline cracks that allow the spark to jump to the head. The other two spark plug connectors are also susceptible to cracking, same as the coils. Give her a tuneup with coils, wires, and plugs and then see if it continues. A less expensive test is to coat the spark plug connectors and coils with RTV on the portion that goes down into the head. You are trying to seal the hairline cracks long enough to test if that is the problem.

Another issue is the 1" hose that runs from the front intercooler to the throttle body. It can harden and crack in half causing a problem for the ECM. On a MAF car ALL air must be accounted for as that is what a MAF measures. However, turbo cars have a re-circulation system and the cracked hose causes a loss of air that has been accounted for with the appropriate fuel already.

 

YOU ARE MY HERO! I don't know how I missed that but it was broke clean off! I got a new one from the Volvo dealer here for $19! Solved the problem instantly. Plus now my car has WAY more power than it did even before the total blowout of the hose so I'm guessing it was leaking pretty good for a long time. Also that "whirring" sound was the massive boost leak caused by the hose. Engine purrs like a kitten again and rages like a tiger when you hit the gas. Seriously I haven't had this much power since I bought it beginning of 2016.

 

Thank you for the update!