I do not recall ever having this problem on any car I've had a fuel pressure gauge on and a fuel system in, but I've never run a return system that returns after the rails either. Wondering if this is just normal with cars that run the fuel through the rails before returning, but don't believe it is.

Base atmosphere fuel pressure @ 40psi. With the car cold and off running the fuel pump it obviously reads 40psi. Just started and cold, it idles around 35 psi (only about 8 inches of vacuum so this is correct).

Once its been running for 10-20 minutes and is warmed up I lose about 5-10PSI of fuel pressure. With the car off and hot, run the fuel pump and I get around 32psi (should be 40), car on and idling around 28 (should be 35).

Wait an hour for car to cool and test again, pressure is back.


Things I know its not;

Not a lack of flow or dying pump, in all these examples the external wasn't running, switching it on and tripling/quadrupling the flow and there's no change in pressure. There are check valves in place. Also voltage is appropriate.

Not a rip in the regulator diaphragm, that would cause an increase in pressure at idle and not explain the loss at atmospheric with the car off where the diaphragm doesn't do anything.

I don't believe it is the fuel getting hot and vaporizing either, only the oem pump was run while the car warmed up, so there was very low flow and low heat absorption. I am waiting on a fitting that will let me siphon some fuel off the car while running to test the temperature though.

 

what brand gauge and sensor? and where is the sensor mounted?

 

Not the sensor either, but it's directly on the FPR, it's not effected by heat.

 

What does "voltage is appropriate" mean? I ask because I've never seen a street car that didn't drop 1/2 a volt + from cold idle to hot idle. That causes some but I've never seen it cause a 10 psi drop. More like 3-4 psi.

Where are you getting your vacuum/boost reference? Does your vacuum change from cold to hot?

 

I think you saw my old thread, but I'm having a similar issue.

Runs normal around 50psi w/ vacuum on regulator, then suddently drops 20psi of pressure after a while. Restarting the car resets it temporarily. This is on E85 with 1500* heat wrap on all engine bay fuel lines, but with regulator after the stock fuel rail.

I first noticed my issue after making several WOT pulls for tuning and testing (hobbs turned on external pump), the previous ~300 break in miles never saw the issue (but some of those miles were hood off, and it was winter up here). This lead me to believe I damaged the stock pump by back flowing into it w/ my external and no check valve.

For now I just run my external on a switch and it works fine to supply all the fuel my car needs, and when I need to do some other maintenance I will drop the driver tank and replace the pump and add a check valve.

It sounds like you have check valves in place already though, so I'm interested to see what you find the issue to be since I may be going down the wrong path and may still have the issue when I'm done

*ah, I just saw the part where turning on your external doesn't change the pressure... for me this jumps my pressure up to ~58psi, actually 8psi more than when just the stock pump is running. That probably points more to a dying stock pump for me and for you, not sure...

 

ok, mine is, it fooled me enough to replace the stock pump with a Z06 pump. I put a mechanical gauge on and it is rock solid. with the electric sensor it will show up to 5psi loss when hot.

Im thinking of remote mounting the sensor somewhere cool.

 

Just means not low voltage, but it's irrelevant anyway sine increasing flow from the second pump does not increase pressure.

When I say 5-10 psi drop I'm talking about effective pressure, not actual pressure. You SHOULD have a drop in actual pressure as the car warms up and vacuum increases, but your effective pressure should remain the same.

Yea, sounds more like you have volume supply issues. Don't think that running the external without a check valve damaged your pump though. If it was on it definitely wouldn't, and I don't think it would even if it was off.

 

I see, thanks for the tip, I had checked with the manufacturer and he said it definitely wouldn't be effected. Mine's from vei, who made yours? I will add that to the checklist anyway, but I previously had both a mechanical and that one on at the same time with the same reading (don't rem if the car got hot though).

 

ok, mine is a prosport on the corvette, I have a autometer on another car that will drop with heat as well.

I just hooked a mechanical gauge to the fuel rail with a hose and ziptied it to the wiper. so i had both working, went for a drive and it showed 5-6 psi diff.

Im glad i now have a Z06 pump but wish i had checked it first with the mechanical gauge.

 

What regulator are you using? I see MAYBE 1-2psi drop from cold to hot (and I mean HOT).

 

Is that actual, or effective pressure (assuming effective)? It's a pro series aeromotive.

 

It doesn't matter if its actual or effective. They'd be the same number. If rail pressure drops to 48psi with 8psi vacuum and is normally 50psi at 8psi vacuum, its the same no matter what you measure (rail drops by 2 which means effective drops by 2).

Raise your base pressure. That regulator is probably not happy there. The spring is probably barely doing anything at that point and is so extended it is relaxing too much with heat.

 

It does matter if you're just reading the gauge, because as mentioned before, vacuum changes when the car warms up. (but still not by that much).

When you say the spring is "barely doing anything" Are you talking about that pressure setting or at that flow rate? Because even after switching the eliminator on the pressure stays low. At that point, presumably, the plunger would have to open and compress the spring more.

I have an a1000 FPR I can try as well.

 

Fine. Effective drops. I built a microcontroller to make my gauge show effective instead of rail, but ultimately the pressure drops just the same. Engine running or not (and no, the air pressure does not change on a hot engine that isn't running compared to when it is cold).

What I mean is for lower pressures, you're extending the spring inside of the regulator. If it extends too far, it can easily fall out of an effective range and become softer with heat to the point it allows fuel to pass through at a lower pressure.

 

This is good info because mine is doing the exact same thing. I have dual intank walbro pumps and a external Bosch pump. When I drive a half of tank and its hot, I loose FP as well I noticed the last time out. Not sure why but I have seen the same thing. I have a fuel lab Reg on the driver side aeromotive fuel rail. I have the 2 Hobbs switches install to make the external pump come on under boost.

 

Are you trying to say the base fuel pressure needs to be higher to hold a stable pressure, or the regulator orifice needs to be smaller (like the "duty cycle" of the reg is very low)? Besides those, I don't get what you're trying to say.

The problem I have with the orifice size is that that would also be affected when the extra pump is turned on and volume increases, as the regulator then has to open more to allow it to bypass (thus compressing the spring more). So if that were the case you would expect a change in pressure.

If you're saying the base fuel pressure needs to be higher that makes more sense, but it's rated from 30-60psi and the rest are 40-75, so switching wont help. So I'm basically screwed, and that doesn't really help. And also again, you would expect when the flow increases for the pressure to change as the spring is compressed to bypass the extra fuel.

Not trying to shoot you down, I just can't isolate it as the problem without understanding why you say that.

 

I don't get what the confusion is. Run higher base pressure. I said it plain as day in my second post here. Why are you trying to run such low pressure anyway?

 

To be easier on the pump, 75psi of pressure at peak flow requirements isn't too great, best to keep it as low as possible.

Like I said I will consider it and try it, but I would have like to isolate it another way other than raising the pressure as that will **** off the tune.

 

AH. the 3 V. 4 bar. fuel system debate. both work fine and the regulators dont seem to mind. " i use Fuel Lab" for my boost referenced return system i run base of 44psi "vac line disconnected" on 95 lb.@ 3bar. injectors with a OLSD tune. "Not a Vette" I think it helps calm the injectors down at idle and extends duty cycle up top.

Do you want me to mail you my mechanical gauge?

Or you could duplicate the problem then wrap an ice cold towel around the regulator to see if the pressure will rise back up.

 

Thanks for the offer, but I have some.

Tomorrow or early next week I'm going to test the hell out of it, I'm going to hook in, along with the electronic, 2 additional (different style) mechanical gauges, siphon fuel when hot and measure it's temp, try raising base to 50 when hot then let it cool down and test again (that way I don't have to drive it @ 50), then try the a1000 regulator I have. Throw the cold pack idea in there somewhere too.

If none of that manages to resolve it, I will go hang myself.