Fuel swirl pot

I was trying to tune Idle on my 1JZ drift car and I had issues with RPM climbing uncontrollably. The only thing I could notice at that time was a change in pitch the fuel pump makes. Fuel pressure remained pretty steady. Pressure pump between the swirl pot and rail was pretty hot.

I tested each pump individually with and without the swirl pot. Now I don't have any issues. The pressure pump is cooler too. The one thing I did, was add another overflow line all the way at the top (number #4)

The setup is like this:

Aluminum tank - Bosch 044 #1 - swirl pot - Bosch 044 #2 - filter - fuel rail - tank

Swirl pot nipples top to bottom

Top AN8 newly added overflow to tank

Side upper AN8 overflow to tank

Side lower AN8 pump #1, feed from tank

Bottom AN8 pump #2, feed, pressure to rail

My question for you all is, is it possible that the first pump creates too much pressure in the swirl pot which affects the second pumps performance? Now, with two overflow return lines both are flowing full time and the issue is gone.

My best guess:

First, you may have too small a return line and/or regulator from the rail to the tank, or some restriction in it - this would mean the pressure would rise most at idle where the greatest volume would be getting returned to the tank.

Not 100% sure exactly how you had it, but I suspect the initial problem may have been three-fold with the first being the above. Because of the restriction, if it's there, the lift pump may have been pressurising the surge tank and the pressure pump - effectively doing all the work - and that resistance made the pump work harder and at the same time restricted the fuel through it that is supposed to cool the pump, this meant the pump ran much hotter and that was passed to the fuel that did go through. Did the swirl tank also get hot?

With the additional swirl tank' breather the lift pump was able to circulate more fuel via the breathers,which reduced the pressure and allowed more fuel to pass through it, and at the same time there was less fuel going through the pressure pump and return side from the rail was able to control that reduced volume, and the pressure pump is now taking over some of the load?

Matej at idle your engine fuel demand is very small, so once the swirl pot is full, you have 2 044 pumps at max output moving fuel with almost nowhere to go.

Improving return flow was a good move. It can be dangerous letting an 044 run full blast into a chamber without sufficient relief flow, so I'm glad you've made that change and things seem more under control.

In the future, I'd consider a lower output lift pump. All fuel not injected in the engine comes right back to the swirl pot anyway, so you only need to replenish what the engine actually ingests, and you're not driving against high head pressure when moving fuel into the swirl pot, so lift pump output needs are much smaller than post swirl pot pump needs.

Another option is PWM control using a pump controller, or ECU + solid state relay, PDM, or other method of duty cycle control of the pumps, vs. running them full blast all the time as you are now. For example if you ran the pumps at 33% duty cycle at idle, you would still deliver far more fuel than the engine consumes, but without creating anywhere near as much wasted flow and heat, then ramp up the second pump as needed for high engine fuel demand.

Hi! Typically these types of swirl pots have fuel rail return coming in at the upper side nipple.

I don't.

From inception I expected return fuel to be hot and wanted it to mix in the fuel tank and cool down. So it goes pressure pump - filter - rail - fuel tank.

I am already eyeing a Facet red top to replace the #1 044 as lift pump. From everything I read online, it should be adequate for feeding an 044.

If I wanted to control the pumps, I would in fact need solid state relays. Being my first ground up build, I went for simplicity and lower cost with standard relays. That restricts functionality unfortunately.

Mike's comment reminded me - if you have two identical model pumps, the one with the lower pressure head is always going to flow more than the one with the higher pressure head.

The lift pump having a greater flow capacity than the pressure is perfectly OK and normal, but it DOES need to be able to return the overflow to the tank.

My mistake was thinking the engine will gulp up enough, so that that won't matter. And it probably will. At WOT.

I think with 2 returns, unless something else happens, we can consider this a problem solved.

Matej thanks for the extra info. You're on the right track and it sounds like you've already got the system working well enough for now. Well done!

Matej thanks for the extra info. You're on the right track and it sounds like you've already got the system working well enough for now. Well done!