check valve question

[uvbnbit]

I know fuel rail pressure is NOT maintained by PCM. The fuel regulator does at 55 lbs.,(key on), but how much pressure should be maintained in fuel rail when engine is off and for how long should pressure be maintained? I loose significant pressure (engine off, key off) in 12 hrs. I've ordered a fuel pressure test kit to find out just how much. 1Tony1 sez it's probably not an injector, more than likely a check valve. '98 RT/10, fuel pump in the tank.... Fuel pressure seems fine under WOT also.
comments please.
-Chip

 

[dave6666]

Short of having a session on pumping fluids and flow characteristics, which with nearly 30 years in chemical engineering I could have fun with, my guess is either the pump is losing capacity or like you've said, the check valve is not holding well. Quite honestly though, holding pressure for 12 hours isn't bad IMO. If it was perfect, why would they ever have that 1 sec pump run on key on? Just to bump the pressure back up?

Oh, and I'm pretty sure the PCM has nothing to do with the pump pressure. It's a factor of everything else that I said we wouldn't have a session on here.

 

[Vic]

Don't hold back, Dave!

 

[uvbnbit]

As usual, I stand corrected (misread service manual). Fuel pressure is maintained (regulated) by the regulator (appropiately named), diaphrams, and check valves NOT the PCM. Thanks for clarification Dave

 

[BOTTLEFED]

My fuel pressure is fine during driving and WOT, but my pressure in the lines has never stayed up for more than a couple hours after shut off. Lots of different cars lose pressure at different rates, so I just figured this one doesn't have a system to keep it from dropping quickly.

 

[ViperGeorge]

Don't think you have anything to worry about. 12 hours is fine IMO. If it bled down in like minutes then I would say for sure check valve in the pump has failed. Since pressure when running is fine you should be ok. I think The 3 second pump run as soon as the key is activated is to prime the system and get pressure to the level needed for the car to start. It is assumed that the pressure could have bled down while the car sat. If they didn't do this the injectors would not see enough pressure and the engine would simply crank until they did.

 

[uvbnbit]

Sounds good. I'll post my pressures when I get my kit.
On a side note, I cleaned my IAC last night with MAF, car fired right up, and idled better than it has in 2 years.
All of this started because I replaced a 4 yr old battery Sunday, and the car would only turn over. Wouldn't start until I kept "feathering" the throttle. I was afraid I had a fuel pressure problem.

 

[Tom F&L GoR]

All you need is for the check valve to keep the fuel line full, not pressurized. If it is full, upon crakning the fuel pump will pressurize it immediately; the problem is if the check valve allows the fuel line to drain the contents back into the gas tank. If that happens the fuel pump has to refill the fuel line before the engine gets anything, which is the long cranking problem.

 

[uvbnbit]

FOLLOW UP:

Got my fuel pressure gauge in yesterday and checked the pressure this morning. Car's been sitting for a few days:
no pressure at all. Key on, pressure goes immediately to 50, car starts right up and pressure is 55-56. Turn car off, and pressure holds for at least 15 mins. So, I assume it bleeds off very slowly, but at least pressure/fuel is there when I need it.
Thanks for all the replies/comments. See ya in SLC!

 

[RTTTTed]

Tom is correct (as usual) and there are a few threads on these fuel systems. If you have a fuel pressure on starting problem search the threads an you'll find the fix (I think Tom wrote that as well). Usually only an issue on the 96-97s (if I remember correctly Tom)?

Ted