4x4CPOSEADOG

Hey guys, I'm about to do a compression test on my 3.0 slow. How do I disable the fuel pump, injectors and ignition? Any help is greatly appreciated. Thanks.

sphealy

iTrader: (1)

Easiest way is to pull the fuse for the fuel pump if your set on disabling it. It really isn't that big of a deal to leave it alone because it will only run as long as you are turning over the engine which should only be for a few seconds. Just pull the coil wire from the dizzy and it won't start and test away.

scope103

Which is where ?

There is no "fuel pump" fuse. But pulling the IGN fuse will disable the EFI main relay, which powers the COR and fuel pump. Also disables the injectors and ignition.

But even easier: https://www.yotatech.com/forums/f116...switch-287859/

MudHippy

Pull the ignition coil wire, and unplug the VAFM(fuel pump no get power). Easy peasy.

EDIT: You're supposed to disconnect the CSI too. Though I never have understood why. Since if you've done as the FSM says and removed the air cleaner hose, that's basically the same as unplugging the VAFM(exc. no DTCs to clear afterwards). Vane stays shut = No power to fuel pump = No fuel pressure(other than residual) to injectors/CSI.

scope103

Isn't the COR, which powers the fuel pump, ALSO closed by the STA signal (when the key is turned to start)? Once you let go of the key, if the engine is sucking air the VAF takes over and keeps the COR closed, keeping the fuel pump running.

So if you're checking compression by turning the key, the fuel pump starts immediately, pressurizing the rail (and spraying fuel through the injectors, including the CSI, because the EFI relay will close with key-on).

MudHippy

The EFI main relay sends power to the COR. The FC switch in the VAFM controls whether or not the fuel pump receives that power(via opening/closing of the fuel pump power circuit in the COR).

So no. I'm 100% correct. Don't believe me? Try starting/running your engine with the VAFM unplugged. There's a reason it won't happen.

EDIT: Let me simplify that, and address this more to your point. It takes BOTH the ignition switch ON(not just at START, which is also ON) AND the vane in the VAFM to be OPEN for the fuel pump to be running. Or by bypassing ALL of that and jumping the FP and +B check connectors.

scope103

100%? Wow.



The COR has two windings, so that it can be closed initially by the STA signal (via the Starter Relay, and with A/T the PN switch). Then once the engine is pulling air the VAF FC switch closes, grounding the second coil in the COR, holding the COR closed once you let go of the key. So the FC switch can keep the pump running, but it can't prevent the pump from running with the key to start.

You can test this if you want by unplugging the VAF and listening for the COR to close when you turn the key. Or if you're really daring, loosen a fuel line connector. The truck may not start with the VAF disconnected, but it's not for lack of fuel pressure.