Why is my fuel pressure dropping when hot?
 

I have my fuel pressure set at 65 PSI base, return style system installed, Walbro 255HP/Walbro 450 fuel pumps that are new, 10 gauge wires to pumps, 6AN lines, Aeromotive FPR in the engine bay with 1:1 vac reference connnected, E85. Wix fuel filter is 2 years old, regulator/lines are 2 years old. Pumps are 1 year old or less.

For whatever reason, sometimes the car will go lean after warming up from say a 30 minute drive. Checking the mechanical fuel pressure gauge, and the datalogs with a fuel pressure sensor both confirm that fuel pressure is dropping anywhere from 10-25 PSI when hot vs when cold. It doesn't always do this, but this is the second time it's happened. It happened months ago, but the problem disappeared before I could install a fuel pressure sensor. But now it's doing it again.

Pinching the return line at idle, the fuel pressure shoots up to ~100 PSI.

I think it's the Aeromotive regulator going out, any thoughts?

I've had very similar symptoms on a e85 wrx, ended up being the o-ring on the pump wasn't sealing completely. FWIW

What O-ring?

Just talked to Aeromotive, they say the regulator is fine. If it's holding 65 PSI at idle, when I shut it down, after 30 minutes it drops to about 45-50. After a day maybe 20, it takes weeks to drop to less than 10 psi. They say that shows the ball/seat is sealing well.

They also said it could be cavitation.

Do you still have logs of it?

I have logs, they show the pressure being at target at cold start, and dropping as the car warms up. Will go from 65 at idle, to the 40's at idle when really hot.

Aeromotive mentioned fuel cavitation, possibly from overheating the fuel. Seems very plausible since this wasn't a problem when it was cooler.

I just rewired the pumps so that the 255 runs at key on, and the 450 stages in at 150kPa. Added bonus, less electrical load at idle/cruise, and that should help put less heat into the fuel and pumps. Will test later today when it gets around 100*F. Staging the second pump on at idle causes about a 1-2 psi spike in fuel pressure, but it quickly goes back to target. So regulator seems to work well!

Yeah sounds like you're boiling the stuff.

The o-ring I was talking about seals the dw65c to the oem Subaru "pumpkin" inside the tank. You don't have this contraption to worry about, I was just throwing out ideas in case maybe you had some O-rings elsewhere in the fuel system that might have gotten weak with high pressure/temp.

Do you have a flex fuel sensor? They measure fuel temp.

I wanted to see the logs to try and pick out something that correlated with the pressure drop. MAP, AIT, CLT, BattV etc.

I'll dig around and find some logs in a bit for you then. It probably does correlate to AIT, since in the winter/spring this wasn't an issue and now it is.

No flex fuel sensor, but I will be installing one soon, like, I need to order one soon. I remember seeing a post here about a cheap (continental I think?) sensor that works.

Yeah. I have a big thread about them. I got mine for less than $50.

https://www.miataturbo.net/ecus-tuni...or-info-88555/

Its recommended that you run a shielded wire for the signal line.

Lol well two things happened. One, rewiring the pumps seems to have fixed it. Gauge shows it holding pressure GREAT and AFRs are back to normal. Will race it tomorrow, so will know for sure after that.

And for whatever reason I have not looked into yet, my fuel pressure sensor just died during the last drive.:vash: I did an oil change earlier, maybe I snagged a wire or something.

Maybe you were leaking like I suspected too ;)

Yeah who knows. I swear I fix one thing, and then something else fails... Airbag light came on too during last drive while ripping it in 3rd. 7 blinks, google says passenger side airbag. Never had an airbag code on this car before. Maybe it was pulling so hard it ripped some wires out of that too.

remove airbags, problem solved
weight redux too:D

After more testing and measuring, the walbro 450 is fine, and the 255HP is dying. 255 is 7 months old, 450 5 months old! The 255 is pulling wayyyy more current than the 450, it's actually heating up 10 gauge wires after running the pump for 5 minutes. And you can hear the pump speed going up/down while this is happening even though the voltage going to the pump is constant.

Probably going to just run the 450 and turn down the boost until my fuel pressure is stable, and race at reduced power till I can fix this. Google suggest the 255's aren't e85 safe? Or at least, walbro doesn't rate them as E85 safe, but I know tons of people run them. Maybe I need dual 450s? Or something else?

Don't get another wally. Get DW which are newer tech and completely fine to run with e85.

Yes, wally's are not made for e85 (despite many people using them for that)

The dw200 is basically a wally255hp but quieter and E compatible

Quick search says the dw300 is a turbine style pump. Is this true? :)

No idea. But I've used plenty of those and all performed great up to about 450 as well

Yeah looks like it is a turbine style, sweet. That makes PWM'ing the pump possible which I've wanted to do to reduce electrical draw anyway. Also turbine pumps hold pressure a lot better at high pressure than gear rotor pumps. Thanks V! Going to buy a DW soon. :)

Np, you'll love it.
A tiny tip: be careful when installing the filter onto the pump: we snapped off the little ear, and then glued it together, but apparently there's a tiny little "breather" hole on the bottom in that area that if you clog it, it will kill the pump. we had to get the pump replaced cause we accidentally glued it shut and the pump burned up.

Both a 13.5 V, 90 PSI

Pump Flowrate

Walbro 450 268 LPH
DW 300 255 LPH

Basically the 300 is as good as a 450 at high pressure since it holds pressure way better being a turbine style pump. At 100 PSI, the DW is killing the 450 with 235LPH vs the 450's 206LPH!

Maybe this will be my 5th last time I upgrade the fuel system?