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FPR vacuum question

jtalton626

Probationary Member
21
0
Jan 18, 2009
Yadkinville, North_Carolina
Good day all,

I don't post on here hardly ever, mainly just read and learn...but I have a question that has stumped me for 3 years.

My car likes to fall on its face right when WOT boost kicks in. You get past 4k on the tach with more than 7 lbs of boost and it falls out. Have tried everything I know of.

This problem has continued through the swap out of the entire motor, including ALL NEW fuel pump, plugs, injectors, rail, filter, cleaned gas tank, ignition coil pack, new ecu, new Power Trans unit, etc.

The only thing I have left that I haven't tried is this.

I noticed a "T" connection connecting the vacuum line from my fuel pressure regulator to my boost gauge. This was installed by the person that owned the car before me. I couldn't help but notice that on the other side of that "T" connection is the VSV, or the Vacuum Switching Valve". Now, correct me if I am wrong but it is my understanding that the VSV is what changes the vacuum pressure going to the FPR and allows your vehicle to get more fuel while at WOT. Am I correct? Because if so, then perhaps when my car hits WOT the the added connection does not allow the vacuum to affect the FPR correctly, which would cause fuel cut.

Now for my secondary question. Assuming my theory is correct, and the vacuum does affect the FPR, then could you not theoretically install a boost controller to the vacuum line of the FPR and effectively make a manual FPR to bypass the VSV altogether? If so, then could you not in turn use a 2 stage boost controller to simply pick when you wanted that extra fuel?
 

steve

DSM Wiseman
14,883
1,727
Feb 3, 2002
Boulder, Colorado
I couldn't help but notice that on the other side of that "T" connection is the VSV, or the Vacuum Switching Valve". Now, correct me if I am wrong but it is my understanding that the VSV is what changes the vacuum pressure going to the FPR and allows your vehicle to get more fuel while at WOT. Am I correct? Because if so, then perhaps when my car hits WOT the the added connection does not allow the vacuum to affect the FPR correctly, which would cause fuel cut.

I'm glad you found your problem but I'll try and explain how this works and correct your misunderstanding.

The Fuel Pressure Solenoid is a two port switch. One port runs to the intake manifold and the other port to atmospheric pressure (the filter on the top). Normally it connected the output to the intake manifold so that the FPR sees manifold pressure (both positive and negative pressure) and causes the fuel pressure to track manifold pressure 1:1. This keeps the pressure across the fuel injectors constant so they always flow the same amount per unit of time that they are open.

During hot starts, the ECU switches the FPS so the the FPR doesn't see manifold vacuum. This raises the fuel pressure and aids in avoiding vapor lock in the fuel rail.

It's the ECU's job to decide how much fuel to inject based on engine load (airflow), air and coolant temps, RPM, and throttle opening. It's what causes the engine to get more fuel at WOT and not the the FPS. If the FPS fails it can cause the FPR not to see boost and not raise the fuel pressure to match which causes the injectors to choke off causing the engine to go lean with boost.
 
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