Idle fuel pressure reading vs. outdoor temperature

[Vapour Trails]

For those of us with fuel pressure gauges, at idle (-20 vac) the fuel pressure should theoretically be 29 psi (or so).

Has anybody seen their idle fuel pressure rise on very hot days?

To rewind a bit, I got a custom tune for my car in Sept 2009. The fuel pressure always seemed stable, regardless of temperature.

In Aug 2010 I had the same tune revised for GT500 injectors (by mail). The car never ran correctly, but that's another story. On this revised tune I noticed on hot days the fuel pressure would rise at idle to 35-40 psi and I thought it was something in the tune.

I put the old injectors back in and starting running the Ford Racing/Whipple tune. Again I noticed a rise in idle fuel pressure on hot days, so I concluded it wasn't a tune issue and it might be normal.

Just recently I went back to my first custom tune and now my fuel pressure doesn't seem to rise on hot days in traffic.

It seems to me if the temperature is increasing the pressure in the rail, that the fuel pump would sense it and dial back in response.

Should the idle fuel pressure ever vary?

 

[JeremyH]

Mine has done it in the past, the rails can get heat soaked, at idle it goes off vacuum to maintain pressure, however the rail pressure can go up 5-10psi if the rails are heatsoaked and you sit and idle for awhile. Once i would cruise for a bit then come to a stop and idle it cooled down and the pressure went back to normal. I see around 30psi at idle normaly with -18 vac.

 

[5.0lightning]

My gauge reads the same, it's up about 10-12 psi on hot days. Mine has always displayed low, around 25 psi at idle on a normal day. It will idle at 36 - 38 on a hot day and slowly go back down after driving for 15 minutes or so. I verified it's 39 at idle through the x-cal on a normal day. I'll check it on the next hot day to see what it actually goes up to on the x-cal next time the gauge reads high.

 

[Vapour Trails]

Good input, thanks.

 

[fdjizm]

My fuel pressure goes to 40psi when it is very hot out, I've seen a high of 50psi in traffic with the AC running.
Normal idle is what looks to be 28-30 psi.