3.9 EFI Fuel Pressure

Wizardof · Post by **Wizardof** » Fri Jul 27, 2007 10:57 am

Hi Guys,

I've had a problem with fuel delivery for a bit now, which first looked like dodgy electrics, but thats ok now and I was advised that the problem could lie with low fuel pressure / damaged fuel pump / fuel regulator... After having checked all the bits and pieces it looks like the problem is with the fuel regulator..
So I have a new reg now, which came with a pressure guage, so it can be set up... Unfortunately, no where in the instructions does it say with the pressure should be.. so can anyone advise? Its probably going to be set right already, but there is no harm in checking is there?..

Cheers
Kevin

badger · Post by **badger** » Fri Jul 27, 2007 7:30 pm

Standard pressure on a 3.9 hotwire setup is 36psi with ambient air pressure in the manifold. Bypass the pump relay to keep the fuel pump running as you check/set it up.

Wizardof · Post by **Wizardof** » Sat Jul 28, 2007 12:03 pm

Hi Thanks for that...

Have done a few tests now and have some mixed results..
On the fuel pressure:
the 3 second fuel pump prime - 30psi but drops off very quickly
engine running - around 20-25psi
blip the throttle and this will try and rise but not always

The engine will start and if the throttle is used gently the revs will rise.
If the throttle is used a bit harder, the revs will flat spot and fall off.

Could this be a weak fuel pump? not delivering enough fuel?

Cheers!

badger · Post by **badger** » Sat Jul 28, 2007 5:00 pm

The regulator controls pressure relative to manifold depression, what you are seeing (a reduction when running and a change with load) is correct. That's why I said to test the pressure with the engine switched off.

What isn't correct is the static drop from 30psi. Remove the complete rail and injectors and repeat the static test - if the injectors leak or flow fuel, they are shot and need replacing. If they don't leak, and there are no external leaks, then it's the regulator bleeding back when it shouldn't (remove the return line, plug it and see if anything comes out of the reg. when the pump runs), or the pump itself is duff.

Badger.