I have an XKR convertible that under normal operation is a joy to drive, but intermittently has the check engine light and "Restricted Performance" kick in. This used to happen once a year but has become much more frequent, it seems especially prone with firm braking followed by firm acceleration. The intake system has been virtually rebuilt; resurfaced supercharger connection, throttle body, O2 senses left and right, mass air flow sensor, new air flow tube, and of course, air filter. Today after lots of mechanic's mile, the event occurred; the scanner code was PO 174? Has anyone had experiences like this? How can this be fixed???

 

Use Search on that code and on fuel trims

Then get a tool as mentioned and see what they are in the circumstances stated.

 

Well, you’re chasing down the problem, and seem well into digging it out.

Next on on my list would be to check the MAP sensor

 

P0174 is very likely air leaks, i.e. air making it into the engine without being measured by the air flow meter). As you mention the throttle body was replaced, and it occurs after braking, the likely culprit is the funky brass/o-ring/retainer arrangement for the brake booster vacuum line at the throttle body end. check part# C2S15816.




The thing with this code is that it represents the sum total of all air leaks, so it can have multiple sources..Best by far is to do a smoke test to reveal all leaks contributing to this code and fix them all.

Best of luck, keep us posted.