What should fuel pressure be at after sitting overnight?

T

truthmatters

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Jan 21, 2019
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Location
VA
Hey everyone, quick question (I hope).

I have a 99 Wrangler tj 4.o and I can't seem to find the answer of, how much pressure should remain in the fuel rail after it sits overnight? I know it runs at about 49psi and will always leak down a little when I shut it off, but what pressure should it stop at so as to allow an easy start in the morning without priming it? Thanks
 
It won't stop at any particular pressure if there's a leak in the fuel pump's check valve. If it's dropping off overnight enough to cause a starting issue, the only solution is to replace the fuel pump. Bosch still sells the actual fuel pump for your '99, it just no longer sells the entire assembly.

Whatever you do, don't decide to go with a store brand, Spectra, Airtex, etc. fuel pump assemblies which don't last long enough to warrant the considerable effort it takes to replace them. I made the mistake of installing a Spectra years ago and it failed after 13 months... one month past its warranty expiration.
 
John Scoggins said:
Or a leaking fuel injector
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Possibly but that is definitely much less likely than the fuel pump's check valve which very commonly eventually develops a leak. In nearly 23 years of TJ ownership I've had four fuel pumps with leaky check valves I ended up replacing but never a leaky or bad fuel injector.
 
Thanks guys. I checked for the leaking fuel injector but all was good; no leaks on any of them. I ask about the fuel rail pressure because I have seen some people say that it's normal to be "0" in the morning, that doesn't seem right. I've put on the pressure gauge and it stays at a solid 49psi when on, but when I shut it off, it doesn't go down quickly, but it will reach "0" when i go to start it in the morning. I push in the schrader valve in the morning and no air or gas comes out.
 
it will start a little rough, but only for about 3 seconds and then run fine all day....as if no problems at all.
 
truthmatters said:
Thanks guys. I checked for the leaking fuel injector but all was good; no leaks on any of them. I ask about the fuel rail pressure because I have seen some people say that it's normal to be "0" in the morning, that doesn't seem right. I've put on the pressure gauge and it stays at a solid 49psi when on, but when I shut it off, it doesn't go down quickly, but it will reach "0" when i go to start it in the morning. I push in the schrader valve in the morning and no air or gas comes out.
Click to expand...
0 pressure after an extended period of time sitting like overnight is normal according to the Mopar factory service manual. But if there's insufficient fuel in the line from it having drained out & back down to the gas tank it's going to cause a slow start caused by the extra time it takes for the fuel pump to get gas back up to the fuel rail.
 
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