I have a 2001 S40 1.9l turbo with 160,000 miles with a rebuilt top end 22,000 miles ago. I've had no problems until the past few weeks. It started with a slow acceleration from the stop - hardly any power at all. Then it started to rough idle but only intermittently. Codes came up for the MAS and O2 sensor so I cleaned the MAS but the problem persisted and the codes still came up. I replaced the MAS and the bank-1 sensor-1 O2 sensor. The O2 sensor code cleared up but the MAS code is still present even after replacing it. The problem only occurs after driving it and then trying to restart it soon after stopping - but not all of the time! When it does happen it will not idle fast enough to start. I can let it sit for 10-15 minutes and it will restart fine. I have cleaned the throttle body and the IAC and replaced the hoses going to the throttle body and the intake. The current code is PO101 and PO101PD. A lot of posts have mentioned a vacuum leak - would the problem be constant if that is the issue? Any ideas on what to do next before I start throwing more parts at it? Thank you!

 

Check the fuel pressure regulator. It is located on the fuel rail to the right. There is a vacuum hose that tends to crack, also the fuel pressure regulator is known to go bad and may even leak fuel through that vacuum hose.

 

Agreed, I can confirm that FPR can cause these symptoms. It can also cause car to stall altogether at full speed so best to get it checked out without delay.

 

Thank you for your feedback. I suspected the FPR also and debated whether I should replace it as a last resort before taking it to someone. So I did and alas; it wasn't the solution. The technician told me that 10 different issues could cause the code. After a week in their shop and them going through the first 9 possibilities - the diagnosis ended up being low compression in the #3 cylinder. They suggested another rebuild or replacing the engine. Because it is a 2001 - I'm going to park it for now until I decide what to do. BTW; we took it to Tanner Auto because they were recommended in another thread on this forum and we were very happy with their service. I highly recommend them if you live in the Phoenix area.