Help with fuel pressure weirdness and theory

I want to start by thanking everyone who helps out on this forum. I've spent so many hours reading through posts and learning just how bad an idea it was to buy MY MS6

I'm the disappointed dad of an 07 speed6 that wasn't driven for years. Of course the fuel was spoiled and I suspect there's gunk and varnish throughout. The issue is fuel pressure when the engine is hot. Cold starts are hesitant, but it starts and runs. If left to idle for 20+ minutes fuel pressure drops to around 40 psi and the engine stalls. It won't start up until it cools down. When I check fuel pressure by AP, it is extremely slow to rise and only hit 400-600 psi. I have replaced the PRV and cleaned the spill valve. There are no signs of an external fuel leak. The HPFP has CS internals and hits 1950 psi while driving. I can drive it for up 40 minutes before it stalls. The engine misfires under load, in boost, feeling like fuel cut, between 4k and 5k rpms. In fact it seems like there is a consistent misfire as long as it's running and it feels choppy at part throttle. There are no CELs currently, but I've had them come and go.

My main theory is that the check valve(s) in the pump are sticking open when hot allowing fuel pressure to somehow be directed back through the HPFP. Is that possible? And can the check valves be cleaned?

My second theory is that the fuel injectors are sticking open. I'm confident the should be cleaned.

Mods:
CS SRI (what a leaky pos)
CS Pump Internals
Catless Downpipe and Test Pipe that I put a CC on
Other info:
Engine was rebuilt about 20k miles ago
This car is making me sad

 

might be worth cleaning the whole hpfp
https://www.mazdaspeeds.org/index.php?threads/hpfp-o-ring-rebuild-michael-rey-facebook-kit.8069/

odd that it looses pressure when idling almost like something heats up and comes loose.

there's a thread here ended up being some wiring. check the ground near the hpfp. little black wire
https://mazdaspeeds.org/index.php?threads/fuel-pressure-drops-then-stalls.5606/

@Enki and @Easter Bunny have a far better understanding and knowledge than I do have the forming fueling system hope

 

Thoroughly clean the pump and spill valve and report back.

How many miles are on the chassis?

 

Chassis has 96k miles. I'll cleaned the spill valve thoroughly, so pump cleaning is next.

 

I've read the thread leading to wiring issues a few times. The crank position sensor part of that thread is also interesting since it sounds a bit more similar to my issue. My issue is almost clockwork consistent. Pump cleaning seems easy enough so I'll give it a shot. Thanks!

 

It sat for a while. Check the sock filter in tank, and while you're at it, might as well just replace it since you'll be there anyways.

 

My bad. I forgot to mention that I replaced the in tank pump with a DW300c. I also removed the fp resistor just to be sure. Am I wrong to think the AP reading KOEO ranging from 29 to 51 psi seems low?

 

Did you change the sock filter while you were at it?
29 is a bit low but if you saw that right after putting the pump back in it's reasonable. Should be close to 40 KOEO when cold.

 

FWIW I've also checked the wiring at the fuse block and didn't find any corrosion or insulation damage. I swapped relays around with no change. My theory involving the check valves in the HPFP are based on my experience with pressure washer triplex pumps. They are quite similar. The pump shaft moves the plunger(piston) into its sealed cylinder and pushes the water through the check valve. If one of the 3(triplex) check valves is unseated or broken that plunger just churns water. Even though the water hose provides constant pressure it can't rise above inlet pressure.
So if the HPFP check valve's opening pressure drops when hot, say below idle running pressure, then the injectors can't fire and the pump just churns gas. What's weird is that the rail pressure doesn't climb to the PRV opening pressure. I wonder if the fuel cools too quickly for the gas vapor to build up. Tomorrow I'll clean the pump and maybe soak the check valve and post results.

 

I've never heard of the rail check valve, which is the part that the hard line connects to on the bottom of the pump, failing.
You've already checked spill valve and PRV, so there's nothing left other than supply side.

Also good luck getting that check valve out, lol

Edit:
What is your fuel level at when this starts happening?

 

It occurs at any fuel level. I agree that a check valve failure is unlikely, or maybe just more often resolved by HPFP replacements. Either way probably rare. I'm not exactly planning on removing it, just submerging it in a solvent for a day. Thanks!

 

Are you sure the in tank fuel pump is fully seated?

 

I'm pretty sure it is. Maybe worth a look though. After the spill valve cleaning I was seeing better pressure from it. I removed and cleaned the HPFP this morning and cleaned it. I found some bits of debris, not sure what but (maybe scrapings from the o rings that had been replaced) in the piston chamber. I also thoroughly checked the outlet check valve and it seems fine. These pumps are super simple and just don't seem to have many points of failure aside from the spill valve solenoid. I haven't gotten it all back together yet but I seriously doubt the cleaning solved the problem. Still worthwhile to see how it works.

 

What did you clean and lubricate with?

 

I used chlorinated brake cleaner and lubed with Techron.

 

Not sure brake cleaner is good for the orings. The standard is lighter fluid for cleaning, and redline injector cleaner for lube and priming (when you put the pump back on the car, make sure the spill valve is loose; remove the spill valve after everything else is bolted up and fill the pump to the bottom thread with redline).

 

I have seen the odd post about the check valve in the bottom of the pump I

didn't @Rokusek have one fail? or was it he just sent a spare one to someone who's failed. this is the only time I recall hearing of the failure of the check valve on the bottom of the pump.

 

Got it. I was careful with the orings as they're avoidable and maybe I'm wrong but I guessed that Techron is fairly similar to Redline s1 and I had it on hand. Thanks for the priming with cleaner advice. I hadn't really thought about that part at all!

 

Naphtha is the best solvent I've found for both fuel and oil residues (cleaning) and the Redline stuff seems to be immune to the harsh heat and corrosive attributes of fuel/oil mixtures that reside in the pump.

 

Yeah, I recently discovered Naptha vm&p, which I used to remove the nasty blue paint from the wheels, in a thread about cleaning the pump. I've also used it to soak clogged lawn mower carbs. Unfortunately I had to resort to what I had on hand due to my toddler's surprise sick day.