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#51751
Hi everyone, I've built a Lotus 7 replica using a Miata engine, redoing all the electronics from scratch (including coil-on-plugs, slightly beefier injectors, inline fuel pump: it's not turbo-charged, but wanted to keep that as an option). Anyways, soldered together a 0.4.4b, did a lot of tinkering, and to my great surprise, I'm able to start the engine and it idles indefinitely.

Before digging into the tuning, I noticed something un expected (to me): I have it set so the fuel pump just runs continuously. It's rated at 225LPH, and I have a regulator set to ~35 psi. The pump needs to prime to get the engine to start, but after that, by flipping off the pump (cut-off switch) the idle gets much smoother, and doesn't seem to ever get fuel-starved (I've let it run for ~10 minutes with no problems). In fact, flipping it back on will usually kill the engine.

So, my theory (bear in mind I'm new to this) is that the setup is far too rich (dwell time too long etc) so the engine is flooding, but this also makes it possible for piston suction/gravity to provide sufficient fuel when there's no actual load (I can rev the engine just fine etc, but maybe if it was actually moving the car it would stall out due to fuel starvation?).

OK, with that background, my questions are basically 1) does any of this sound wrong/more of a problem than I'm seeing and 2) what best practices are for fuel pump control here? Is it OK to just run the pump all the time (other than some unnecessary electrical load)? And a more specific question, I've heard people talk about how their Miata's fuel pressure goes from ~30 psi "up to X", where "X" might be 60 psi etc...how does this happen? There's just a single pressure regulator, so are these people setting that to the highest value and having a combination of sensor/pulse/ECU dial in the pressure on-the-fly? I hope not, because for the moment I'd like to just set the pressure, turn on the pump at full blast, and drive it a bit...

Thanks for any help/advice, I'm genuinely amazed at how approachable Speeduino made this whole project, this is the first time I've had to even ask something I couldn't quite grasp from the documentation!
By dazq
#51752
Fuel pump pressure is usually set just simply at idle to the required pressure.
The pressure regulator then keeps the fuel rail at that pressure +- the manifold pressure.
So if it goes into vacuum then the rail pressure would be seen to drop , or if in boost ( turbo/ supercharger application) then it will raise the pressure.
But it always stays at that set pressure with respect to the manifold
Unless you are running a returnless fuel rail then your pump would usually just run constant once then engine is running.
In a returnless setup then the pump speed would be modulated in some way to keep the rail pressure constant in the same manner as the pressure regulator would do in a normal return line setup .
#51753
Ah perfect: that's what I hoped, so I can just experiment on the software side and not have to add more circuitry. I'll just try dialing in the pressure regulator and injector settings: it is indeed a return system. Thanks!
User avatar
By PSIG
#51756
Set your fuel pressure with KOEO (key on, engine off), pump running, so MAP is at atmospheric pressure. Set for the pressure spec your fuel injector flow was rated-at.

The injector calculations are based on tested or published flow at that pressure specification, e.g., 500cc/min at 3-bar (43.5 psig) or whatever it is. You can alter the pressure later if injectors are found to be oversize (non-linear low-flow) or other concern; but for now you use spec's for a baseline so the calc's are reasonably close. Tune and adjust from there. Have fun!
#51760
Hi Tom,

We need a photo of your Lotus 7 replica !
Here's mine. Scratch build, 2 litre Duratec, early MX5 box + IRS, DIY CAN module emulator to keep Focus donor instruments happy.
Running OEM ECU for now (required to pass emissions test for registration), aiming to use Speeduino + ITBs in future.

Crazy fun.

Cheers - Gavin

Image
#51769
Thanks! I'll give this a try.
PSIG wrote:
Mon Jul 12, 2021 6:22 pm
Set your fuel pressure with KOEO (key on, engine off), pump running, so MAP is at atmospheric pressure. Set for the pressure spec your fuel injector flow was rated-at.

The injector calculations are based on tested or published flow at that pressure specification, e.g., 500cc/min at 3-bar (43.5 psig) or whatever it is. You can alter the pressure later if injectors are found to be oversize (non-linear low-flow) or other concern; but for now you use spec's for a baseline so the calc's are reasonably close. Tune and adjust from there. Have fun!
By neovin
#51791
I don't think the first part was answered.
To clarify, you are removing power to the fuel pump and the car continues to run? How have you wired the system? What pin and extra circuitry is set up ecu side? And does the fuel pressure change with your shutoff switch and how is that wired?

If my fuel pump cuts it will die in seconds
#51872
Your fuel pump does not shut off if you think it does. There is no way for an electric injector valve to run on the engines vacuum. For that you need a carburetor...
The usual fuel pressure range is 2-3.5 bar, with the regulator port open to atmosphere. Set it once, then do not change it any more, as the whole fuel tune depends on it. When the engine is running, this port has to be connected to manifold pressure, so the differential pressure is kept constant, independent of trottle position/ engine load.
I prefer the fuel pressure to be on the high side, as fuel atomization is a little better, but you have to have an eye on injector opening times at idle, so you do not get too rich.
#51981
So, that was my expectation! But I am *definitely* removing power from the pump completely (there's a cut-off switch, the pump is very audible, and the regulator does not hold pressure. Also, when the fuel pump is turned off, the engine starts idling with very small, fast explosions. Taken together, I'm wondering if maybe something's happening to keep the injectors permanently open, such that the vacuum pulls just enough fuel to keep going...

...this might also relate to another issue that's come up: at idle the exhaust is almost clear, but revving it up to a couple thousand+ RPMs it turns black (I think: could be dark blue). And occasionally sounds like a small backfire. I'm hoping this is just related to the fuel weirdness and not oil getting into the cylinders...
neovin wrote:
Tue Jul 13, 2021 10:40 pm
I don't think the first part was answered.
To clarify, you are removing power to the fuel pump and the car continues to run? How have you wired the system? What pin and extra circuitry is set up ecu side? And does the fuel pressure change with your shutoff switch and how is that wired?

If my fuel pump cuts it will die in seconds

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