69 psi key on, engine off fuel pressure?!?


New Member
Dec 8, 2003
San Diego, CA
So I'm finally getting around to properly adjusting the fuel pressure on my suposedly '89 5.0 (turned out to be an 86 - flat top pistons, E6 heads with 89 computer and wiring, 19 lb injectors and unknown sensors). It's a very mismatched engine...

So anyway, I connected a tester under the hood and set it to about 38 psi but it ran crappy (had a hesitation) and my air fuel ratio guage read at the very bottom of LEAN. So I cranked it up til, at idle and warm, it sat at on the very bottom 2 stoich LED's which ended up being around 69 psi (when tested engine off, key on)! I know this is way high for a stock engine, but what can I do? Can't run it lean, but shouldn't run such high pressure. Anyone got any tips for me? Thanks!
  • Sponsors (?)

I gotcha. It's just a digital Autometer.

I actually rechecked it and it was more like 60 psi.

But anyway, I took it out on the highway, and after about 5 minutes of 65 mph, it was up at 2-3 (of about 8) into RICH. So I dropped it down to about 58 and I'm gonna test that out and get it to a shop when I have a chance. But just out of curiousity, is there something that would cause my engine to need a higher pressure to run properly (maybe because of the mismathced parts and sensors)?

Also, how does a wideband gauge work?
The stock lambdas dont actually measure the A/F ratio. They toggle back and forth between telling the puter to richen the mixture to telling it to lean the mixture up (Kind of like if you wanted a bowling ball to roll straight down a bowling lane, but you rolled it crooked initially. You might tap it from the each side as it rolls to get it to hit in the middle of the bowling lane). The range on a stock O2 is very limited. Widebands are another story.

The problem with the adjustments you are making is that the computer takes time to adapt, so it is not like you can change the FP setting and see results in real time initially.

I would have cleared the KAM and started out at stock pressure and went from there.
good luck.
Booksix said:

And what is stock FP?
KAM = Keep alive memory. it is the memory in the computer which saves adaptive strategies and modifications (like if someone plays with the FP and deviates from the stock setting, the computer will adapt how long the injector stays open so the same amount of fuel is squirted is the same).

It remembers these settings, and if you change the FP (for example) it will take the computer some time to adapt.

Stock FP (warm motor at idle) is:

with the FPR vac line off and plugged: ~39 psi

with the FPR vac line on the FPR (like you have it normally in day to day operation), the pressure drops down to ~32 PSI.

good luck.