Fuel Pressure Question

silver03svt

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So recently, I burned up a 30 amp fuse going to my KB Boost-A-Pump. Replaced the fuse, found out the BAP was wired wrong (at the relay), and rewired it correctly (power wires were in wrong location). Ever since then, when I let off of the gas pedal quickly (decelerating), the fuel pressure will spike up into the high 60's or low 70's. If I let off of the pedal slowly, it does not do it.

Also, When the car is turned off, the relay is staying warm, as if it is getting current when the car is not running, and it does drain the battery.

Any USEFUL comments or help would be appreciated.
 

01yellercobra

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You might need your tune adjusted for the BAP. There's a couple things that can be done to control the spikes.

Are you good with a DMM? Make sure the relay is opening when the car is turned off when the car is off. It may have gotten damaged when it was wired incorrectly.
 

silver03svt

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You might need your tune adjusted for the BAP. There's a couple things that can be done to control the spikes.

Are you good with a DMM? Make sure the relay is opening when the car is turned off when the car is off. It may have gotten damaged when it was wired incorrectly.

I replaced the original relay with a new one. You can grab it when the car is off and it is definitely warm to the touch. And no on the DMM. Relay is relatively cheap, so may try getting another one and try it.
 

MG0h3

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If the relay is powered(closed) the pumps should be running. I've never experienced a relay that is getting some current to get warm but not enough to close.

Sounds like you just corrected a wiring error but Malcolm's thread on wiring the pump for factory fuel pump priming has a good diagram
 

TRBO VNM

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Sounds like there may still be a wiring issue or relay may be bad. Actually, what amperage is the relay? It should be 30 amp minimum. A lot are 20 amp, so check that. Should not be getting warm obviously.
 

silver03svt

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Replaced the relay with the exact same one that came out of it, and it had been powering the BAP since 2011 and 20,000 miles. I just only discovered that the relay was wired wrong when installed. Looked at SEVERAL different wiring diagrams (most from members on here), and found that 2 wires were reversed at the relay.

With everything off, the relay still gets warm to the touch. Not sure if it is from the lead wire from the power distribution block or what, but it's aggravating as hell. I even did away with the wire harness at the relay, and added terminals to the wires for a direct plug in.

Will purchase another relay and see if it still does it.
 

01yellercobra

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The other concern would be if it's been on there for 20k miles, what suddenly caused the fuse to burn up? I can't remember which pins they are, but two of them can be reversed. They're just the pass through for the heavy current. I want to say they're 85 and 87, but don't quote me on that.
 

TRBO VNM

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Make sure it is a 30+ amp rated relay. My other question would be to feel the 8 gauge wire and see if that is hot as well. I had a guy that was return style and for some reason his 8 gauge was getting pretty hot. So we removed it and put 4 gauge wire and it has been fine for 6 months now.
 

Bigggy

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Had the exact same problem when I replaced my pumps (for GT pumps) and not readjusting my BAP. I think that the PCM is sending minimum voltage to the pump but it's still to high with the BAP, that the pressure spikes. Try turning the knob down. Only happened off the throttle when engine was decelarating.
 

silver03svt

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FINALLY, I think I have gotten all of the wiring issues with my Boost-A-Pump worked out. Bought a new relay, and wired the BAP to be PRE-FPDM instead of POST-FPDM. Car sat for OVER 48 hours without the battery draining.

NOW, when I am driving and I let off of the gas pedal, I am seeing the fuel pressure go up to around 75psi. If I just rest my foot back on the gas pedal, the pressure drops to about 29psi for a fraction of a second and then levels at 40 psi. If I put the car in neutral to coast, it also drops back to the normal 40 psi. Again, it only spikes when I lift off the go pedal to decelerate.

Any ideas on what is causing the fuel pressure to jump so high while the car is decelerating?
 

ShadowDrake

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FINALLY, I think I have gotten all of the wiring issues with my Boost-A-Pump worked out. Bought a new relay, and wired the BAP to be PRE-FPDM instead of POST-FPDM. Car sat for OVER 48 hours without the battery draining.

NOW, when I am driving and I let off of the gas pedal, I am seeing the fuel pressure go up to around 75psi. If I just rest my foot back on the gas pedal, the pressure drops to about 29psi for a fraction of a second and then levels at 40 psi. If I put the car in neutral to coast, it also drops back to the normal 40 psi. Again, it only spikes when I lift off the go pedal to decelerate.

Any ideas on what is causing the fuel pressure to jump so high while the car is decelerating?

I know you didn't get any timely replies on this but:

Tune - 100% the tune. Start by adding some D for the fuel pump PID controller, it doesn't need much. If it still is missing target increase P a little too. You can help alleviate this as well by disabling DFSO or setting it to target AFR of 18-19 instead of 21 during DFSO so it'll still open the injectors a tiny bit to bleed out the pressure if it does overshoot during throttle closed (inevitable to some degree, fuel pump is mechanical and can not respond to changes as fast as you can move the throttle plate).

Additionally if your injector flow vs fuel pressure is set up right, your AFR will not drift even if the fuel pressure moves off its 40 psi target. The stock sensor can read from 0 to about 75-80 psi. If your pressure is actually going higher than this the injector vs flow compensation won't be effective, clearly.

If you're not tuning it yourself, get it to a tuner who won't feed you a line of BS that you need to buy X, Y, and Z or switch to return style. Your issues can be fixed in the tune alone.

Interesting about the FPDM wiring - mine has had a ghost drain for so long now I've just gotten used to it. I'll have to check my BAP installation and probably cut the notorious wire for the GEM and see if I can get it so the battery doesn't go dead in 2-3 days if it doesn't get driven.
 

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