FYI- hard troubleshoot resolved

[Dubesa05]

Hey Guys,

just want to put this on here in case anyone has a similar issue. It could be more prevalent since our cars don’t have a fuel pressure sensor for the low side.

Symptoms- I would get a check engine light for reflex low fuel pressure. It would be around 40 ish. After some driving around the pressure would bump up to 80 no issues. Found this weird but thought the 2nd LPFP was helping out, wasn’t causing any issues driving around codes would clear and no issues until the next morning.

I went through a lot, maybe I missed a clamp in the fuel basket, maybe the pump was leaking by. Maybe the fuel filter was clogged.

it ended up being the fuel controller above that battery, when it’s cold it causes cavitation in the lpfp lines and only hit 40psi. When it’s warmed up it will hit 80 psi, I tested it this morning and the car started at 40 psi, I took the controller and put it in the oven for a bit to warm it up, slightly above room temp and put it back in the car. Car fired up with 80 psi no problem. Save yourself the trip back to the fuel tank and try this.

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[Thraxbert]

I also have the same issue. Car sits a long time and the PI rail sags to 40 until the car warms up, then everything is fine. Was going to experimentally install a fuel-rated check valve to stop any potential drainback, and I ALSO wasted a ton of time looking for non-existent leaks. Nice find!

But it seems like there's no true fix as we cannot keep this part warm overnight.

 

[Dubesa05]

I also have the same issue. Car sits a long time and the PI rail sags to 40 until the car warms up, then everything is fine. Was going to experimentally install a fuel-rated check valve to stop any potential drainback, and I ALSO wasted a ton of time looking for non-existent leaks. Nice find!

But it seems like there's no true fix as we cannot keep this part warm overnight.

You don’t think it’s a bad control module? I know older BMW’s had issues with this controller getting too hot?

 

[Thraxbert]

I, personally, have no reason to believe this. The DI rail receives the correct pressure every time, so I'd wager a faulty module would also cause performance issues on the DI rail. My Supra only has about 10,000 miles, so no reason to expect faults from age or use either.

My current theory is that this is a drainback problem, not a module problem. Or a module "problem" that can be overcome with a check valve.

But now you definitely have me thinking: it is true that I have no way to measure the low side of the LPFP right before the DI pump. My PI rail is, theoretically, measuring the PSI of the low side but it's doing so in a place that may or may not relate to what the DI system is receiving. I don't know if the flow into the DI pump is also low until the car warms up, but it is possible. I'll admit that this blind spot makes it possible that the controller is misbehaving, and I wouldn't know it.

 

[Dubesa05]

I, personally, have no reason to believe this. The DI rail receives the correct pressure every time, so I'd wager a faulty module would also cause performance issues on the DI rail. My Supra only has about 10,000 miles, so no reason to expect faults from age or use either.

My current theory is that this is a drainback problem, not a module problem. Or a module "problem" that can be overcome with a check valve.

But now you definitely have me thinking: it is true that I have no way to measure the low side of the LPFP right before the DI pump. My PI rail is, theoretically, measuring the PSI of the low side but it's doing so in a place that may or may not relate to what the DI system is receiving. I don't know if the flow into the DI pump is also low until the car warms up, but it is possible. I'll admit that this blind spot makes it possible that the controller is misbehaving, and I wouldn't know it.

I tested fuel pressure before the DI pump when only the stock pump was pumping the cavitation was wild. You could not see gauge needle only stop and end point of the bouncing. Which was 30-50 psi. If I kicked on the LPFP while doing this it would even out to 80 with no bounce, once off, the pressure would slowly drop back to 40 then bounce. But if I heat up the controller to room temp. It holds 80 with no issues on strictly the stock pump when idling, as if the temp is causing voltage issues to the pump itself when cold.

this correlates with my driving as I’ve watched the fuel pressure slowly rise from 40 psi to 80 when driving. Which would be this controller heating up over time.

also isn’t the DI rail a mechanical pump? So there would be no interaction with this module? The lpfp only feeds the HPFP.

 

[Thraxbert]

Yes, DI rail is a mechanical pump. To rephrase: my low side fuel line is teed, with one line going to DI and the other going to PI. I have a pressure sensor on the line going to the PI system, but no sensor on the line going to the DI system. This makes it possible that both lines are low from a cold start, but I'm only seeing it on the PI line because I can actually measure it with a sensor. This also makes it possible for the car to not care, even if something is actually wrong, because MHD will throw a "low fuel pressure" code for the PI system but the DI system might be receiving sufficient flow to hide any errors. The car is cold and under low demand, so 40 PSI may be just fine, rendering the nature of the problem invisible.

 

[Dubesa05]

Yes, DI rail is a mechanical pump. To rephrase: my low side fuel line is teed, with one line going to DI and the other going to PI. I have a pressure sensor on the line going to the PI system, but no sensor on the line going to the DI system. This makes it possible that both lines are low from a cold start, but I'm only seeing it on the PI line because I can actually measure it with a sensor. This also makes it possible for the car to not care, even if something is actually wrong, because MHD will throw a "low fuel pressure" code for the PI system but the DI system might be receiving sufficient flow to hide any errors. The car is cold and under low demand, so 40 PSI may be just fine, rendering the nature of the problem invisible.

Ahh I got you, I am also T’d. But my fuel sensor is at the T junction. I don’t have it specifically on that line. But the line continues on after that, although I would expect the pressure in both lines to be the same as where the T is. With PI injector not Firing there should be no reason to see pressure drop on the PI line versus the DI feed line.

 

[Thraxbert]

I would agree with all you've said above. Time for me to pull the EKP, give it some heat, and see what happens!

 

[Dubesa05]

I would agree with all you've said above. Time for me to pull the EKP, give it some heat, and see what happens!

If you have a heat gun watch the log while heating you’ll watch it slowly rise I think.

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