Stumblin Bumblin Z

[Paul Workman]

Originally Posted by scottfab

Ok, after pouring over this thread I picked up on something.
You say you have 42 lb of fuel pressure and it goes to 50 when you rev up. You should have 50lb or so without reving.
Could be that your secondary fuel pump is intermittent.
If you do some hard accelerations up to the stumble a few times see if a lean code gets set. Then its pretty certain.
I had something very similar but at 4500 rpm. It never set a code.
When I replaced the secondary fuel pump it went away.
My fuel pressure with both pumps whet to 44lb when I just turned the key on. Then it would leak down pretty fast.
When I replaced the pumps it went to 52 and came down very little over time.
You may need to connect a fuel pressure gauge and go for a drive up to the stumble.

I agree with you - far as a static test goes. Mine does that too. However, under dynamic load (pressure gauge taped to the windshield), with just the primary pump, pressure is in the 44-46 range (depeding on load). But, as soon as the throttle/rpm reaches the threshold where the secondary pump activates, pressure pops to about 52 ± a bit and holds steady.

For what its worth, when the secondary pump was not working, FP dropped to 38# at WOT, and the AFR went very lean!