Fuel System Leakdown Test: FAIL and FIX

[HOrnbrod]

Lately my truck has been cranking a bit longer before it fires up, so I decided to do a leakdown test at the fuel rail test port. The FSM states that on startup, the fuel pressure should jump immediately to vacuum regulated pressure (40 psi in my case), then should hold a minimum 20psi of pressure for at least 30 minutes after shutting down the engine. My pressure dropped rapidly to zero in less than a minute - not good. Fuel pressure internal leakback is normally caused by a bad check valve at the fuel pump, or a leaky fuel pressure regulator and/or fuel injector(s). Since the pressure drop was so quick, I suspected it was the pump check valve. To verify, I pinched off the return line at the rubber soft-to-hard line transition near the rear axle, and the fast pressure drop was unaffected, so I eliminated the FPR and injectors as the cause.

I picked up the above 5/16" inline check valve HERE, then using fuel injection hose clamps, plumbed it in the fuel filter 5/16" output line. I selected this check valve because it has Vitron seals that are supposedly impervious to fuel.

 

When I repeated the leakdown test the pressure after shutting off the engine dropped slowly to ~20 psi over an hour or so, then held for three hours more until I got tired of timing it. That's more than satisfactory according to the FSM. The next morning on a cold start after ~14 hours, the engine started immediately instead of the normal 5-10 seconds of cranking. It worked! So this check valve will remain inline as long as the current Bosch fuel pump lives. And I suspect it will live a bit longer now than before as it won't be working as hard.

I don't know if this will help the Renix system trucks as they are computer-dumb and have to crank longer than the HOs to relearn their settings each time before starting, but hopefully this little mod might help them start faster too. 

[fiatslug87]

Nice simple fix. 

[cody4359]

I suspect I have leaky injector orings. when the truck sits for a few hrs to a day, it'll crank a couple seconds before firing. my remedy, and I'm curiouse if you tried, is to turn the ignition on til the fuel pump shuts off, turn off, then fire it up. seems to work til I get around to fixing it. good idea on the check valve

Sent from my SM-G900V using Tapatalk

[MeanLemons]

Highly interdasting