Need help understanding fuel pressure regulator

[Spook50]

I was working on refining my desmog job in my 62 today in hopes of getting rid of more vacuum lines (got rid of a ton, but need to make sure it's not going to cause problems in the long run), and noticed that the vacuum line that goes from the fuel pressure regulator to the VSV that's mounted to the driver's side fender has long been capped off. I looked all through the EFI and fuel control sections of the FSM but can't find any description of how/when the regulator is supposed to have vacuum applied to it. Right now (actually, ever since I desmogged it five years ago) there is no vacuum going to it, but I'm wondering if it's supposed to have either manifold vacuum (which, when stock, it gets when the VSV is open), intake plenum (ie: from the throttle body) vacuum, or just no vacuum going to it like I have it now.

Any thoughts from you EFI gurus?

It wouldn't be too hard to just get the single VSV from a junk yard and wire it back in and plumb it into manifold vacuum if need be.

 

[lehiguy]

Fuel pressure regulators on naturally aspirated engines use manifold vacuum to decrease fuel pressure in many cases. When the throttle is open and vacuum drops, the fuel requirement goes up so the increase in fuel pressure helps the injectors keep up with demand. The same is true of forced induction, when manifold pressure goes up, so does the fuel pressure, the line to the FPR seeing a positive pressure instead of a vacuum signal, but the effect is essentially the same. If your FPR had a vacuum line, it was most likely helping to lean things out at throttle positions that resulted in high manifold vacuum, like idle, high RPM, low throttle angle situations, going down hill, etc.

 

[Spook50]

Fuel pressure regulators on naturally aspirated engines use manifold vacuum to decrease fuel pressure in many cases. When the throttle is open and vacuum drops, the fuel requirement goes up so the increase in fuel pressure helps the injectors keep up with demand. The same is true of forced induction, when manifold pressure goes up, so does the fuel pressure, the line to the FPR seeing a positive pressure instead of a vacuum signal, but the effect is essentially the same the same. If your FPR had a vacuum line, it was most likely helping to lean things out at throttle positions that resulted in high manifold vacuum, like idle, high RPM, low throttle angle situations, going down hill, etc.

Interesting. So from the sounds of it I could easily control it with just manifold vacuum and not worry about that VSV. I'm curious as to what the difference is between direct manifold vacuum and having the computer control the VSV to supply manifold vacuum.

 

[lehiguy]

Interesting. So from the sounds of it I could easily control it with just manifold vacuum and not worry about that VSV. I'm curious as to what the difference is between direct manifold vacuum and having the computer control the VSV to supply manifold vacuum.

I am not familiar with the details of a 3FE, just the principle in general.

 

[Spook50]

I am not familiar with the details of a 3FE, just the principle in general.

I'm not sure what parameters the computer needs to open it, but there's a VSV mounted on the driver's side wheel well that connects the FPR to manifold vacuum. That VSV is also connected to the computer, but again, I'm not sure what it takes for the computer to open or close that VSV.

 

[lehiguy]

I'm not sure what parameters the computer needs to open it, but there's a VSV mounted on the driver's side wheel well that connects the FPR to manifold vacuum. That VSV is also connected to the computer, but again, I'm not sure what it takes for the computer to open or close that VSV.

Some fancy way of controling the fuel pressure. It probably has to modify the fuel pressure with EGR actuation or something like that. I would think that simple plumbing straight to the manifold vacuum signal, in the absence of other smog stuff, would be alright. That's how my Jetta was, no vlaves, just a vacuum line to the FPR. Freaky simple especially for Germans who seem to operate by the Rube Goldberg rule book most of the time.

 

[Spook50]

Some fancy way of controling the fuel pressure. It probably has to modify the fuel pressure with EGR actuation or something like that. I would think that simple plumbing straight to the manifold vacuum signal, in the absence of other smog stuff, would be alright. That's how my Jetta was, no vlaves, just a vacuum line to the FPR. Freaky simple especially for Germans who seem to operate by the Rube Goldberg rule book most of the time.

This is true. I'll just plan on straight manifold vacuum and see how it does with that.

Just for gee-whiz info, with manifold vacuum at idle, the regulator should be putting out 33-37 PSI, and with no vacuum (the way it's been the last five years), it's supposed to be 37-46 PSI. Sounds like I can end up with alot less wasted fuel by reconnecting that vacuum line.

 

[lehiguy]

This is true. I'll just plan on straight manifold vacuum and see how it does with that.

Just for gee-whiz info, with manifold vacuum at idle, the regulator should be putting out 33-37 PSI, and with no vacuum (the way it's been the last five years), it's supposed to be 37-46 PSI. Sounds like I can end up with alot less wasted fuel by reconnecting that vacuum line.

It certainly can't hurt to try. I don't know anything about the Toyota ECM, if its adaptive or not, but you might want to disconnect the battery for a night after connecting the vacuum line to get a fresh start with the correct pulse widths for the injectors. Might not matter, but it does work that way with other vehicles.

 

[RockDoc]

FPR keeps the pressure difference across the injector (fuel rail to manifold) constant for proper metering. In normal operation the FPR sees manifold pressure (vacuum). In one (or two?) conditions the FPR see atmosphere to richen the mixture. The one I can think of is excessive coolant temp, to drop the combustion temp, but it may be used in cold/start up enrichment as well..... not sure.

 

[Spook50]

It certainly can't hurt to try. I don't know anything about the Toyota ECM, if its adaptive or not, but you might want to disconnect the battery for a night after connecting the vacuum line to get a fresh start with the correct pulse widths for the injectors. Might not matter, but it does work that way with other vehicles.

Yeah I reset my ECU before starting it all up.

Seems to be running great for now. We'll see what my next tank of gas nets me for economy.