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  1. #1
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    car fuel pressure question

    I have a 97 Mercedes e320 that has had a few warm crank/no start issues that I will be replacing the CPS this week. I also did a quick fuel pressure check to see how my fuel pump is doing and had some funny results. When i connect the fuel pressure gauge and the ignition "ON" it stays at "0" but as soon as I crank it jumps to 48 PSI and stays there as it idles. When I shut off the engine the pressure holds around 45. Should I be concerned about the lack of fuel pressure before cranking ? Is this a sign the fuel pump is not holding pressure or one way valve is toast on it ?

  2. #2
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    Could be engine temperature sensor. When the engine is cold, it richens the mix, if it doesn't lean out the mix when hot it can flood the engine and kill it, and also cause a hard/no start condition. You should be able to look up a spec and debug with a volt meter.

  3. #3
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    Just a thought, is it possible that in a Mercedes turning the key to on does not activate the fuel pump prior to cranking? 48 lbs holding at 45 sounds more than adequate.

  4. #4
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    Have you checked the fuel filter(s)?

  5. #5
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    Quote Originally Posted by Storm Hood View Post
    Have you checked the fuel filter(s)?
    I have not but the 48 psi at crank and idle seems to tell me it's not clogged.

  6. #6
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    Quote Originally Posted by lucknau View Post
    Could be engine temperature sensor. When the engine is cold, it richens the mix, if it doesn't lean out the mix when hot it can flood the engine and kill it, and also cause a hard/no start condition. You should be able to look up a spec and debug with a volt meter.
    Car never dies and mileage has been the same.

  7. #7
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    Do the test again see if you have different results... Keep the pressure tester kit with you and use next time you have the warm crank issue. You can also run the tester hose up to the windshield and pin it there with a wiper blade so you can look at your numbers while driving just route the hose away from moving parts so you don't spring a leak and burn your car down
    I just replaced the pump on my ML and it did some things I'd never seen a fuel pump do like it would run at 25 psi (not very well) had almost no pressure with key on and after removing and replacing the fuse pressure went up... My experience with other cars electric pumps either work or they don't. Not so much with Benz I guess.

    Start shopping for a replacement pump (autohauz) shouldn't be too expensive or difficult to replace. I believe you have access under the rear seat but you should verify.

    Good luck

    Sent from my I3123 using Tapatalk

  8. #8
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    Quote Originally Posted by fatnslow View Post
    Car never dies and mileage has been the same.
    Not sure what you're saying here. Did you do the voltage test? Mileage in reference to what? Don't rule anything out until you've verified it with 100% certainty.

    Quote Originally Posted by fatnslow View Post
    I have not but the 48 psi at crank and idle seems to tell me it's not clogged.
    Have you tested it while hot or only while cold? Keep thinking. Don't rule anything out until you've verified it with 100% certainty.

  9. #9
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    A grand seems a little steep... This site is known for good oem parts amongst the Benz forums

    Sent from my I3123 using Tapatalk

  10. #10
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    Quote Originally Posted by lucknau View Post
    Not sure what you're saying here. Did you do the voltage test? Mileage in reference to what? Don't rule anything out until you've verified it with 100% certainty.



    Have you tested it while hot or only while cold? Keep thinking. Don't rule anything out until you've verified it with 100% certainty.
    In reference to the coolant temp sensor I have not noticed ANY drivability issues or lower than usual MPG usually seen in a failing temp sensor. One thing I didn't mention was when I did have the no start issue I wiggled the CPS connector and it fired right up and hasn't been a problem since so I ordered one and also wanted to check the fuel pump pressure. Just rechecked the fuel pressure on a warm engine and the reading is the same at 48 PSI and when I shut the engine off it holds at 46 PSI so no problem with the backflow valve on the fuel pump or leaking injectors. KInd of on the fence about ordering fuel pump/filter($100 total) and replacing it preemptively at 22 years and 145K miles. I usually don't just throw parts at a problem but this is my kids car that uses at school about a 100 miles away so I need to get in dialed in this week so I can go swap cars.

  11. #11
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    Quote Originally Posted by SirVicSmasher View Post
    A grand seems a little steep... This site is known for good oem parts amongst the Benz forums

    Sent from my I3123 using Tapatalk
    Sorry that was typo, I can get a beck arnley one for about $70 from Amazon.

  12. #12
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    Quote Originally Posted by SirVicSmasher View Post
    A grand seems a little steep... This site is known for good oem parts amongst the Benz forums

    Sent from my I3123 using Tapatalk
    Sorry, that was a typo, about $100 for beck arnley pump and filter.

  13. #13
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    car fuel pressure question

    Could also be the fuel pressure regulator. That's the little valve with a vacuum hose attached to it. If you pull the vacuum line and there's gas on the vacuum side, it needs to be replaced. There's a little diaphragm that wears out. I think they're cheap to replace.

  14. #14
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    Quote Originally Posted by lucknau View Post
    Could also be the fuel pressure regulator. That's the little valve with a vacuum hose attached to it. If you pull the vacuum line and there's gas on the vacuum side, it needs to be replaced. There's a little diaphragm that wears out. I think they're cheap to replace.
    Checked it, when I unplug it the fuel pressure rises and no leaking gas. I think the fuel pump is fine but thinking about replacing it to ease my mind. Not worth the $100 savings if it breaks down on my kid.

  15. #15
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    Quote Originally Posted by fatnslow View Post
    Checked it, when I unplug it the fuel pressure rises and no leaking gas. I think the fuel pump is fine but thinking about replacing it to ease my mind. Not worth the $100 savings if it breaks down on my kid.
    right on. I read your previous post. Looks like you figured it out. I'm assuming you checked already, but could it have just been the wire or the connector, as opposed to the CPS itself?

  16. #16
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    Quote Originally Posted by lucknau View Post
    right on. I read your previous post. Looks like you figured it out. I'm assuming you checked already, but could it have just been the wire or the connector, as opposed to the CPS itself?
    Connector was tight without any visible damage. I replaced this damn thing a few years ago due to long cranks but for the life of me can't remember if it was a oem/Bosch. I'll be replacing it and fuel pump/filter this week. Hopefully I won't take too much of a gas shower on the ground. Gotta go find my hose clamps now. I swore I would teach my kids how to fix shit but all 3 are utterly useless but I can't really complain. Never in trouble, great people and kicking ass in life.

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