DaveW

I have to beg ignorance on exactly how the FPR functions - never studied it or needed to.

scotty305

iTrader: (2)

Here's a good animation and description of how the insides of a fuel pressure regulator work, it's a spring-loaded bypass, the spring sets the desired bypass pressure. The vacuum port pressurizes the spring, which is explained pretty well near the end of the video.

Here's a video showing how to rebuild an aftermarket regulator, the internals of aftermarket units should be similar to how the stock ones are built.

When I was searching for a good demonstration of how fuel pressure regulators work, I found a nice video from a well-known rx7club member. I'm not just including this because I'm a fan, but his demonstration showing why the fuel pressure needs to increase when the manifold is seeing boost pressure is very good.

scotty305

iTrader: (2)

The fuel pressure regulator is almost never completely closed, the bypass is spring-loaded and will open just the right amount so that the fuel pressure is about 40psi higher than the pressure at its reference port. If you're feeling adventurous and have a friend with a fire extinguisher on duty, try dumping the fuel return line into a 5-gallon fuel jug instead of letting it go back into the gas tank so you can see how much fuel is bypassed with the engine off or with the engine idling. On second thought, it's probably safer to just take my word for it.

It's probably important to note that many US-made 'boost/vacuum' gauges will use a different scale like inHg inH2O to display vacuum, because that was a common measurement unit back in the days with hot-rodders in the US were doing carburetor things. I'm going to use use psig units to describe both boost and vacuum because it makes the math much easier. People from outside the US might describe everything in Bar or kPa units, which are both good measurement units as long as you use the same scale for both vacuum and boost. When the reference port is correctly connected to manifold pressure, you can expect to see fuel pressure always be about 40psi higher than manifold pressure. When the engine is idling and pulling vacuum, the manifold pressure will be about -10psig (10psi less than atmospheric), and so the fuel pressure should be about 30psi (which is 40 higher than -10). At full throttle and zero boost, the manifold pressure will be about 0psig (same as atmospheric) and the fuel pressure will be about 40psi. When at full throttle and the turbo is making boost on a stock twins setup, the manifold pressure will be about 10psi (10psi higher than atmospheric) so the fuel pressure should be about 50psi (which is 40psi higher than +10). If you're interested, I can post screenshots from my aftermarket ECU that include manifold pressure and fuel pressure. The fuel pressure changes smoothly when manifold pressure changes, it's nothing like an on/off switch.

If the reference port is disconnected , it will never influence the bypass spring and the fuel pressure will not change as it should. When the engine is pulling vacuum -10psi, the fuel pressure would be 40psi (which is about 50psi higher than -10psi), and the engine will get more fuel than the ECU was programmed to deliver. At full throttle and zero boost, you get lucky and the engine happens to get the same amount of fuel the ECU was programmed to deliver. At full throttle and 10psi boost, the fuel pressure would still be 40psi (which is only 30psi higher than +10psi) and the engine will get less fuel than the ECU was programmed to deliver.

If your PowerFC was tuned when the FPR reference port was not connected to manifold pressure, the tuner would probably have increased the injector duty cycle to compensate. I forget the exact math, but my guess is around 10 or 15% more duty cycle would be needed. If you fix the FPR reference after the ECU was already tuned to compensate, the wideband O2 gauge would probably show the engine running a little too rich in boost.

Redbul

iTrader: (26)

So, Mazda inserted a solenoid into the vacuum line running from the manifold to the FPR, thereby adding an element of AI and potential for trouble. It seems the ECU considers five inputs to decide to tell the solenoid to cause the vacuum to be choked off..


They make it hard to find in the FSM by calling the Solenoid valve the PRC Valve. But it can be found on page F105 of the FSM. Or here:


What could possibly go wrong.

Redbul

iTrader: (26)

Page F-31 of the 1993 "Service Highlights" gives us a little more explanation about what is going on.\

So what happens if your 30 year old PRC Solenoid decides to pack it in?

j9fd3s

iTrader: (3)

nothing, solenoid is only turned on during a hot start. the rest of the time it is off. so if it fails, maybe you have a rough idle after the car starts hot.

its labeled PRC in the sensor check screen of the Power FC commander, you can watch it if you like

scotty305

iTrader: (2)

I deleted the PRC solenoid from my car about 15 years ago, and have an aftermarket fuel pressure regulator set to about 50psi base fuel pressure. 50psi was per the recommendation of an rx7club member who had worked closely with Paul Yaw, he said ID1000 injectors would have a better spray pattern at 50psi than 40psi base fuel pressure. Better spray pattern helps mix the fuel with the air so the air/fuel mixture burns well after the spark ignites it. I haven't noticed problems with hot starts, aside from needing to adjust the ECU temperature-based fuel trims from the default settings in the base map. The odds of an ECU base map being perfect for a daily driver are slim, at least for the brand I have. I've also added a fuel pressure sensor, so the ECU can adjust the injector duty cycle to compensate if the fuel pressure goes higher or lower than expected. Many modern cars have fuel rail pressure sensors, especially turbocharged ones.

Redbul

iTrader: (26)

Here is a table of what each solenoid is doing when. PRC is near the bottom. I suppose the chances of it getting stuck "on" are slight. But broken nipples are more of a possibility.