: FPR acting up

11-09-09, 06:16 AM
I have a question about the fpr which came about when searchin for a vacuum leak. First, a little history - 2001 SLS bone stock, 89,000 mi. overall, very good mechanical condition. This car runs and idles fine, no complaints except for the SES light indicating some sort of vacuum leak - Codes pulled are; P0171, and P0174. I have done all the preliminary tests, spraying brake kleen even starting fluid at all vacuum connections, removed intake manifold and changed gaskets. new O rings on injectors, plulled the plenum piece, and inspected it. also inspected the over pressure valve on the intake manifold.
Cleared the code, and after less 20 mi light comes back on again. So while troubleshooting this problem I noticed the fuel pressure measured at the fuel rail with pressure tester at 35psi. I disconnected the vacuum, and no gas was present at idle or slight rev. but the mystery is the pressure went up to 44psi with hose still disconnected! I put the hose back on - and back down to 35psi Is this suppose to react this way?? I changed out the fpr and identical results.
35psi with vacuum hose on, 44 with vacuum hose disconnected. After this I replaced the fuel filter with new one, and also replaced the fuel pump with a new one. Same problem, same codes, Any suggestions???
Caddillac T

11-09-09, 11:30 AM
When vacuum is applied at idle (high vacuum) the diaphragm is lifted pulling open the pintle valve and returning most fuel back to the tank as it is not needed, thus lowering the fuel pressure. With no vacuum the valve stays closed, thus higher pressure. That said, I think it should still be around 45 at idle.