Modded Mustang Forums banner

1 - 16 of 16 Posts

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #1
I just finished wiring up my prosport gauges and the fuel pressure at idle is roughly 40 psi and at WOT its at 50-55 psi. The sender is mounted on the shrader valve and my car is N/A

I thought I read somewhere that at WOT it should be 40 psi? Is my gauge off?

Can anyone enlighten me?

tanks
 

·
Registered
Joined
·
2,936 Posts
The rail pressure will vary as the PCM uses feedback from the Fuel Rail Pressure (FRP, connected to both the fuel rail and manifold vacuum) sensor so as to keep the pressure drop "across" the injectors at 40 (or 55, more about this below) psi.

This means that at idle when manifold vacuum is high--say 20 inHg. which is -9.8 psi--the fuel pressure only needs to be 30.2 psi to achieve the 40 psi differential. At WOT however when manifold vacuum is low (2-3 inHg/1-1.5 psi) the rail pressure will be 39 psi give or take.

So--this means 40 psi pressure at the fuel rail, at idle, is too high--but before taking action you need to verify that the ProSport gauge is accurate.

Re: 50+ psi:

There is also a temperature sensor in the FRP. If the fuel rail temperature rises above 150°F, and other conditions warrant (ECT, IAT, load, etc), the PCM will command the differential fuel pressure target to be 50-55 psi to prevent vapourization of the fuel in the rail.

If your AFRs are behaving at WOT there is nothing wrong, it's just the way the returnless system works...
 
  • Like
Reactions: justAbullitt

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #5
The rail pressure will vary as the PCM uses feedback from the Fuel Rail Pressure (FRP, connected to both the fuel rail and manifold vacuum) sensor so as to keep the pressure drop "across" the injectors at 40 (or 55, more about this below) psi.

This means that at idle when manifold vacuum is high--say 20 inHg. which is -9.8 psi--the fuel pressure only needs to be 30.2 psi to achieve the 40 psi differential. At WOT however when manifold vacuum is low (2-3 inHg/1-1.5 psi) the rail pressure will be 39 psi give or take.

So--this means 40 psi pressure at the fuel rail, at idle, is too high--but before taking action you need to verify that the ProSport gauge is accurate.

Re: 50+ psi:

There is also a temperature sensor in the FRP. If the fuel rail temperature rises above 150°F, and other conditions warrant (ECT, IAT, load, etc), the PCM will command the differential fuel pressure target to be 50-55 psi to prevent vapourization of the fuel in the rail.

If your AFRs are behaving at WOT there is nothing wrong, it's just the way the returnless system works...
thanks cliffy,

the prosport gauges should be pretty damn accurate, I've seen in some magazines where they were more accurate than some autometer gauges.

I contacted prosport and they told me to check the resistance across the posts which ill do when i get home. Also I kind of Hodge podged the ground connection on the sending unit to the headlight ground just to verify that it worked and it wasn't very clean so there might be enough resistance there to screw up the reading on the gauge. Ill mess around with it later tonight and see if I can fix it, if not its under warranty so it should be good.

thanks!
 

·
Registered
Joined
·
2,936 Posts
thanks cliffy,

the prosport gauges should be pretty damn accurate, I've seen in some magazines where they were more accurate than some autometer gauges.

I contacted prosport and they told me to check the resistance across the posts which ill do when i get home. Also I kind of Hodge podged the ground connection on the sending unit to the headlight ground just to verify that it worked and it wasn't very clean so there might be enough resistance there to screw up the reading on the gauge. Ill mess around with it later tonight and see if I can fix it, if not its under warranty so it should be good.

thanks!
Any questionable connections will mess up the sensor signals, the best way to wire sensors and electronic gauges is to use a common grounding point and wire all grounds back to that point...
 

·
US Air Force (retired)
Joined
·
13,542 Posts
An easy way to test the guage is to turn the key on and not start the motor with the engine cold. The gauge should swing up and level out at 40 psi because there is zero vacuum. This approximates a WOT condition when vacuum is very low. When you start the motor it should drop down to around 30 psi.
 

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #8
Any questionable connections will mess up the sensor signals, the best way to wire sensors and electronic gauges is to use a common grounding point and wire all grounds back to that point...
I called the manf. and they said that it shouldn't effect anything, if all else fails i can try a common ground and see if this helps.

Eagle2000GT said:
An easy way to test the guage is to turn the key on and not start the motor with the engine cold. The gauge should swing up and level out at 40 psi because there is zero vacuum. This approximates a WOT condition when vacuum is very low. When you start the motor it should drop down to around 30 psi.
its funny you mention that because when i turn the key it goes to 40 and once it turns on it doesn't really move from 40, maybe its the gauge?
 

·
Registered
Joined
·
2,936 Posts
An electrical/electronic engineer would have told you otherwise. This is why there is very little in a modern automobile that is grounded through the chassis, most everything has a dedicated ground tied back to a common point.

Check out G104 on our cars sometime, five grounds from the PCM, the MAF and the CCRM all run back to G104. The instrument cluster ties to G204 which is hardwired to G104...
 

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #10
An electrical/electronic engineer would have told you otherwise. This is why there is very little in a modern automobile that is grounded through the chassis, most everything has a dedicated ground tied back to a common point.

Check out G104 on our cars sometime, five grounds from the PCM, the MAF and the CCRM all run back to G104. The instrument cluster ties to G204 which is hardwired to G104...
I know its good practice to have a common ground when making multiple connections, I'll most likely end up having a common GND for all my gauges to the inside fuse panel.

Also I just got off the phone with tech and seems as though I have a faulty servo on the gauge itself so i am sending it back.
 

·
Registered
Joined
·
2,936 Posts
I will have to keep ProSport in mind as that is very good customer service--too may vendors would make it be your fault for at least a couple weeks, and then grudgingly if at all send you a replacement...
 

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #12
I will have to keep ProSport in mind as that is very good customer service--too may vendors would make it be your fault for at least a couple weeks, and then grudgingly if at all send you a replacement...
I have the performance series which are the cheapest ones and quality wise it far surpasses my autometer z series boost gauge. It's very beautiful gauge the lighting is very vivid. They have peak and hold gauges for 80.00. My feul pressure gauge has electric sender and it cost 50.00 autometer wants over 100 for their electric feul pressure gauge. Bang for buck they are really nice gauges. And customer service so far has been great, also they are located in Florida.

The only con is the harness they give you isn't very long about 3ft at the most. So I made my own harness with Molex c grid sl connector with about 7 ft of 22 gauge wire.
 

·
Registered
Joined
·
2,936 Posts
Autometer's stuff used to be junk (like AudioVox) sold in the cheapest places like Western Auto and other local "Home & Auto" stores, and have been overrated for years. Their website sucks too, especially if you are looking for a specific item...
 

·
Carving corners
Joined
·
3,328 Posts
Discussion Starter #14
They used to be junk (like AudioVox) sold in the cheapest places like Western Auto and other local "Home & Auto" stores, and have been overrated for years. Their website sucks too, especially if you are looking for a specific item...
prosports website? really? I found it quite easy to navigate through it found everything i needed within minutes. Prosport Gauges - Boost gauges, Wideband Air Fuel Ratio, 80mm tachometers, 80mm Boost gauges,Oil pressure gauges, peak warning Gauges. Wideband Air Fuel Ratio Gauges, 45mm gauges
 

·
Registered
Joined
·
2,936 Posts
No, no, no--I meant Autometer's products and website--sorry 'bout that, I knew what I meant...
 
1 - 16 of 16 Posts
Top