Fuel pump activation....

[ducman82]

how do i get the MS to just turn the pump on with Ignition on and stay on? seems to help with my hot start issues if i cycle the key a few times to get the fuel flowing before starting.... id like to see if i can get the pump stay on 12 switched. i could "re wire" but I'm not sure if i un hook the FP control from the MS box, if that will mess anything else up.

[billr]

You don't need the pump to stay on, just increase the priming or cranking pulse. If your pump/fuel system is good, pressure will come up from zero to full pressure in a second or less. The benefit you are getting from that "key dance" is repeated priming pulses. As always, an MSQ would help...

[ducman82]

Hmmmmmmmmmmm

[Yves]

Not sure what your hot starting problem is, but some aftermarket FPR's loose pressure real quick. Within MS there is no setting that allows you to have the pump running with ignition on. You'll have to rewire and use a programmable output to do this.

[Raymond_B]

That's really dangerous, key on engine off fuel pump on? Recipe for disaster. What you indicate happening by cycling the key is that you are unable to hold fuel pressure which is usually a check valve in the fuel pump or a fuel pressure regulator.

[ducman82]

just key on pump on. like an old car. fuel pressure seems to be holding steady, (but i will run some more tests). cold start is fine. only have an issue after she gets hot from sitting. runs lean and as she drives and the IAT drops she riches up. i was playing with the MAT Density table, but super finicky. i was thinking that was the issue, but i have not been able to narrow it down.

added at DL on how she swings up and down in RPM's after it does start. until she drives and gets in her groove. makes me think heat/lean issue.


thanks guys!

[billr]

Look at fuel pressure with a sensor or gauge, I think you will find that pressure is well-up almost instantly, regardless of whether the FPR or pump check hold. I concede, though, that I don't know when the "priming pulse" occurs. If it is at the same instant that the pump turns on, then the pump may not even have a chance, since priming pulses are typically in the 10 msec range; and a mechanical FP relay may not even make that fast. I have priming turned off and use a hefty cranking pulse, so really can't help much further.

[DaveEFI]

Would feeding the pump relay coil from the starter circuit help? In addition to from the normal MS pump output? So the pump is forced to run when cranking, before the tach signal is seen by MS and turns on the pump? If you try this, use a couple of diodes to prevent any interaction between the two circuits.

[grom_e30]

what about adding a longer 'priming pulse delay' it will run the pump longer before firing the priming pw to allow the pressure to build


edit: a bad start log may shed some more light on the issue

[Yves]

Look at fuel pressure with a sensor or gauge, I think you will find that pressure is well-up almost instantly, regardless of whether the FPR or pump check hold. I concede, though, that I don't know when the "priming pulse" occurs. If it is at the same instant that the pump turns on, then the pump may not even have a chance, since priming pulses are typically in the 10 msec range; and a mechanical FP relay may not even make that fast. I have priming turned off and use a hefty cranking pulse, so really can't help much further.

Most aftermarket FPR's loose pressure real quick. What happens then is the fuel pressure in the lines drops really fast to 0. Since the engine is hot, you'll get vapor lock in the fuel lines. Once you restart the engine the fuel pump will first have to remove the air from the lines and the injectors have to clear them as well. This takes a couple of tries. MS does not allow you to lengthen the priming pulse enough to do that. On my setup it's similar. I need to switch on my ignition 2 or 3 times and then she fires right away.

[billr]

Have you looked at FP with a gauge or sensor? After sitting for a long time, and no priming pulse, my fuel pressure goes from zero to 3 bar in 700 msec after cranking is detected. Surely no FPR leaks down to worse than zero? And I have said before, my pump isn't "super-duper", I got it out of a '76 Caddy many years ago because it was getting noisy; there was worry that it was failing.

[billr]

Yves, every time you mention running the pump to "get the air out" I'm puzzled; where is the air getting in? Then it dawned on me: is your return line from the FPR open to atmosphere? Is the tank end dumping above fuel level? I believe you have a C3 Corvette and am wondering if you re-purposed a vapor return line as an FPR return. If that FPR return enters the tank below fuel level, then no air gets in and the pump can pressurize everything near-instantly, as I have been claiming.