I have been reading about fuel pressure at idle and at WOT, but I havn't seen anything about how it is supposed to be at say 1/2 throttle. I am beaking in a new motor and do not want to run it at WOT. The fuel pressure at idle runs around 38 psi and when I pull the vacume line off the regulator the pressure rises to 45 psi (even when I put a load on the engine). The problem is when I give it gas say 1/2 throttle the pressure drops to around 30 psi.

Maybe I have this wrong, but as the throttle is opened the vacume should decrease, so the more the throttle opens the less vacume you have all the way to WOT. So if this holds true, the fuel pressure should increase as the throttle is opened. Any ideas here? Could it be a bad regulator?

Also I wanted to know if I could just disconect the vacume line from the regulator and run 45 psi all the time. Is this something my programer could get around?

Thanks

 

sound like the fuel pump is not performing very well.

 

With the vacumme line disconected though it runs steady at 45 psi even when it is reved up. To me it would seem as if the fuel pump is getting the fuel to the system.

 

reved up and WOT on the road are two different things.

 

You've analyzed it correctly, more throttle = less vacuum = more pressure. But if the fuel pressure drops to 30psi under part throttle, there is a problem. If it can't supply the required fuel when you push the throttle open, you run a risk of damage at WOT.

Don't confuse this with "cruising" at part throttle. Under those conditions, you may see a high vacuum = lower pressure.

Running without the vacuum will not "solve" the problem. It will still run short of fuel it the pump is weak, or the regulator is faulty. You will only know by watching the pressure as the engine comes up to full load, not by revving it at idle.

Without the vacuum connection, the injectors are going to flow a different flow rate at each combination of manifold vacuum and the fixed 45psi fuel rail pressure. It will only make your tuner's work harder. The vacuum compensation line is there to insure a constant differential pressure between the rail and the manifold... not to hold a constant pressure in the rail.

It is possible to tune without vacuum compensation using an aftermarket ECU, with more friendly fuel tables, but the stock PCM uses the PE mode enrichment at some times and the lack of a vacuum signal might throw it off. I don't run vacuum to my FPR, but I also use a high-end MoTeC M48Pro engine management computer.

 

[QUOTE=Injuneer]
Don't confuse this with "cruising" at part throttle. Under those conditions, you may see a high vacuum = lower pressure.
QUOTE]


Does this mean that vaccume pressure could be higher at part throttle conditions than at idle. I aways thought that the highest vaccume conditions were at idle. I'm basicly tring to figure out why when I goose the throttle with no load why the fuel pressure drops from 34 psi (at idle) to 30 psi and then it rises to 42 psi (with the new regulator).

Now this is with no load, but does this mean that Higher vaccume pressures are present once I goose it than they are at idle, cuasing this sudden drop of pressure. Or does this mean that my fuel pump is not able to supple enough fuel?

When this is happening on the road, I goose it and the motor stumbles a bit and then picks up power like a S.O.B. Now this seemed to fix it self the farther I drove it, but I replaced the fuel pressure regulator and the problem is worse than before. This regulator runs slightly lower pressure than my original one. So is this just a case of the computer neading time to adust to it? Or does this whole problem still go back to not enough fuel from the pump?


Thanks for your time and pateince...

 

If you end up with even lower pressures after a new fuel pressure regulator, it may be that the pump is not in good shape.

At "cruise", vacuum can go above the idle vacuum as you back of slightly on the throttle, but as soon as you move the throttle toward open, the vacuum drops and the fuel pressure should rise. I can't see what the PCM would need to adjust to, it can't "see" the fuel pressure. But it can see the effect of low fuel pressure on the A/F ratio, and start adding extra fuel to the calculations. But if the pressure is low, its a pressure problem, not the PCM "adjusting".

Not sure why you are revving it at idle, since that does not simulate any sort of real world driving condition. But if the fuel system can't even supply the sudden increased fuel demand under no-load conditions, its not surprising that it can't respond to moving the throttle under load.