Well - I finally sorted out why my MPG had deteriorated on Miss Purdy. The issue turned into more of a project than necessary because I made the mistake of not thoroughly checking the simple things first. Assuming the worst, my troubleshooting process started off by wringing out the L-Jet computer inputs, wiring, plugs, connectors, throttle position signal, air/water temp sensors and all the grounds, etc.... Although I had already found and replaced a seeping cold start injector, it didn't completely solve the problem. I chased a few more rabbits while measuring the pin out resistance of the Air Flow Metering Unit and then disassembled it to examine the internals for wear, contamination or damage. I also adjusted the wiper arm to a new tracking path, cleaned the other contact points and made a couple of updates for reliability.
During several of my earlier quick fuel pressure tests the regulators/dampener had been eliminated as a potential issue since they were found to be producing about 33 psi with the car at idle and 38 psi when lightly rev-ed. Each of the regulator/dampener spring diaphragms had statically held a good vacuum when individually checked with a small hand held vacuum pump. (Vacuum Gauge Holding Steady)
During the third round of head scratching, I discovered that the fuel pressure became unstable at the slightest sideways movement of the vacuum line plumbing and would cause my fuel pressure to fluctuate or spike up toward 42 psi. It turned out that the origin of my problem was a simple split in one of the 3 legged vacuum fittings attached near the top rear side of the throttle control valve. With the Air Filter Assembly and Air Flow Metering Unit removed, there is just enough access room under the spider to replace the leaking rubber elbow. Note: There are actually two small metal vacuum sources side by side in this location. On Miss Purdy the left one provides a vacuum source to both pressure regulators, the pulse dampener and a small solenoid valve looking gizmo mounted along the middle of the passenger fender well. The right one goes to the distributor advance, a thermal control valve and the charcoal canister control. Once I had narrowed down the source of my problem, it seemed like the more vacuum lines I disassembled, inspected and replaced - the more swollen rubber connections, deteriorated fittings and questionable joints I found. Replaced them all.......
Each fuel rail pressure regulator is internally set through a balancing act between an internal spring loaded diaphragm and manifold vacuum. Typically, a high vacuum, low load condition, will see the fuel pressure maintained at a steady (Factory Spec) 36 psi. At idle my regulators appear to hold things a few psi low but I'm also not sure if my test gauge is properly calibrated. This shows the final idle fuel pressure after replacing the vacuum lines and fittings.
If the throttle is suddenly opened wider, the manifold vacuum drops and the regulators will automatically increases the rail pressure. At wide-open-throttle the fuel pressure will temporarily climb toward the 42 psi mark and then taper back down during acceleration. When everything is working properly, the correct quantity of fuel injected at any given instant is the result of a computer calculated pulse width plus an assumed correct fuel rail pressure that is proportionally adjusted according to the engine load. Since I had a loss of manifold vacuum felt to the regulators - my fuel rail pressure was being constantly maxed out in error. When the computer sees the resulting rich exhaust signal being sent back from the Oxygen Sensor, it immediately reduces the injector duty cycle to a minimum - trying to correct the air/fuel mixture. My high fuel consumption was the result of the computer not being able to sufficiently cut back enough on the injector pulse width to cancel out the abnormally high (42 psi) fuel pressure. Each time the injectors were triggered open, excessive fuel was being forced into the cylinders.
Looking back I realize that my problem should have been a fairly simple one to figure out if I'd been paying attention to the basics and had remembered the old saying that - "The Universe Hates a Vacuum and Will Eliminate It At Every Opportunity".
Drive Safe, Michael