vacuum reference on regulator
 

Quick question, the vacuum reference hooked up to my pressure regulator is dropping the fuel pressure to39@idle and 35 psi when I hammer the throttle(it used to go up when I hit the throttle to 42/44. I noticed this after coming in from some cruising, the engine bay was very hot and water temp was good @ 185. As soon as I pulled the vacuum line the pressure sticks to 42 and stays there no matter how much throttle I give it. Any idea whats happening, it almost seems like theres a higher vacuum signal in the throttle body when the blades are opened, dont get it, thought it goes the other way.
The engine seems to run well, seems a little down on power but it was extremely hot out today- Your thoughts? Thanks Rich
http://i168.photobucket.com/albums/u...Picture081.jpg

sounds like the vacuum you have it hooked to is ported vacuum; picks up the source above the throttle blades instead of below. But you say it didn't used to be that way, so that shouldn't be the issue.

The only other thing is if the fuel is too hot the pressure will drop. Also a dirty filter will drop pressure, but usually it will still be better when the vacuum is low, not vice-versa.



Jody

Jody,

The vacuum port Im using is the big one at the bottom of the throttle plate. Its got a small piece of hose then a vacuum "Y" fitting is connected to it, one side goes right to the MAP sensor and the other goes up to the regulator.

If it were a fuel filter limiting the pressure it would still be low when I disconnected the vacuum line from the regulator- is that right?

The under hood temp was very high, could very high fuel temps cause this?

Could I run the car with out the vacuum reference, it was tuned this way, would the idle A/F be richer with the small additional pressure running with out the vacuum line?

Thanks Rich

when the fuel temp gets high enough the pump starts to cavitate and fuel pressure drops, until it won't run at all if it escalates enough. You'd know in advance if you pay attention as the pump gets louder.

If the car was tuned without it you would have thrown the idle and cruise tuning off some by adding it, unless you re-tuned for the reference line. The line would have dropped pressure in those situations.

You might try no line and see how the fuel pressure works out when the fuel temps get high. If it has the same issues (which I think it might) then you need to do something to control fuel temps.

Jody

Jody,

The car was tuned with the vacuum reference line connected, I will try it with out the line and see how it reacts.

Thanks for your help- really appreciate it- Rich

should be rich at idle and cruise without it. I read your last post as it was tuned without it, not with it.

Jody

still has me stumped, the A/F at idle goes from 14.0 down to 13.7 at idle with the vacuum hose plugged and removed from the regulator. I waited until it was dead cold and it still seems to drop to 35 psi when I hit the throttle and it takes its time to return to about 40. Both filters are clear.

This may sound nuts, but is it possible for a new engine after about 750 miles to actually improve its vacuum response at the manifold? This thing seems like its pulling more vacuum now, even my old data logs show a lower KPA value at idle. When we did the tune on the engine it had maybe 10 miles on the engine.

I pulled the hose, plugged the line and have a solid 44psi always, as soon as I can get out I will se how it runs. Is it prefered to run a vacuum reference?

Thanks Rich

most prefer to run it but I've never had a problem without on a naturally aspirated setup. If your tune is made to have it I'd run it.

It could be that you're making more vacuum. If the tune improves, more timing at idle, as the engine breaks in, etc. it can and will improve. Put a gauge on it and see. I just don't remember any regulators that change the psi more than 3-5 psi. Plus, it should go up under load, not down.

What size return line on the regulator, and what pump are you running? What filter are you running before the pump?

Jody

http://i168.photobucket.com/albums/u...Picture073.jpgThe small piece feeding the pump is #10 into a 100micron/10 micron out and #10 all the way to the "Y" under the hood then #6 coming out of the "Y" feeding the rails and # 6 going into the regulator. The return coming out of the regulator is #6 for 30 inches(just behind the upper control arm) then increases to #8 braided line all the way back to the tank. A-1000 pump.

I think I might be chasing it down, the 10 micron filter didn't look to bad although it was a bit dark, so I figured let me replace it today. I think the pump is flowing a bit better. When I snap the throttle now it goes right up to 45 psi from 40 at idle with the vacuum line connected, but the the second the throttle blades shut it dips down to 36 and settles at 40. Im starting to wonder now if the pump is taking a dump. what do you think?

Thanks Rich