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Thread: Is this problem a bad fuel pump?

  1. #1
    Join Date
    Apr 2002
    Location
    Norcross, GA
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    3,326

    Is this problem a bad fuel pump?

    The problem showed up as an occasional stumble that cleared up after a couple of seconds. Then it started doing it regularly but would clear up everytime. Now it's doing it all the time. It cranks right up but runs really rough and sputters out. It won't respond to the throttle at all but tries to idle. When I pressed the shrader valve the gas sprayed out but was a thin spray for only a few seconds. I put a pressure gauge on it, jumpered the pump on and the pressure came up very slowly struggling past 20 psi and just barely made it to 30psi after running for 30 secs. It sounded a bit rough while it ran, too.
    This is on a friends 91 SC. I've got a spare pump to put in it, but if that's not the problem I'd rather not go to the trouble for naught. I'm going to replace the fuel filter too to eliminate that. I don't think it's ignition. I swapped out some questional plug wires, tried another DIS to no avail and it starts up too easily to be a crank sensor.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  2. #2
    Join Date
    Oct 2006
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    48125,michigan
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    Quote Originally Posted by Mike Puckett View Post
    The problem showed up as an occasional stumble that cleared up after a couple of seconds. Then it started doing it regularly but would clear up everytime. Now it's doing it all the time. It cranks right up but runs really rough and sputters out. It won't respond to the throttle at all but tries to idle. When I pressed the shrader valve the gas sprayed out but was a thin spray for only a few seconds. I put a pressure gauge on it, jumpered the pump on and the pressure came up very slowly struggling past 20 psi and just barely made it to 30psi after running for 30 secs. It sounded a bit rough while it ran, too.
    This is on a friends 91 SC. I've got a spare pump to put in it, but if that's not the problem I'd rather not go to the trouble for naught. I'm going to replace the fuel filter too to eliminate that. I don't think it's ignition. I swapped out some questional plug wires, tried another DIS to no avail and it starts up too easily to be a crank sensor.
    dont we need more then 30psi to run?

    most cars need around 50psi sounds like filter or pump to me..

  3. #3
    Join Date
    May 2011
    Posts
    825
    Mike, you may try changing the fuel filter first.

  4. #4
    Join Date
    Apr 2010
    Location
    SLO County
    Posts
    358

    722 Agree

    The filter is definitely the easiest to change and cannot hurt. Change it. Then Check fuel pressure again. It should be between 30 and 40 PSI at idle. There are a bunch of procedures to differentiate the low pressure from fuel pump and the fuel pressure reg. I can Email or fax them to you if needed. PM me if so.

  5. #5
    Join Date
    Apr 2002
    Location
    Norcross, GA
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    3,326
    That's the plan. I'll change out the filter first thing in the morning and try it again. I'm hoping that's it but not counting on it. I've got a 190 lph to replace the existing pump if I need to.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  6. #6
    Join Date
    Jul 2002
    Location
    Manchester, NH
    Posts
    2,064
    It's the pump.

  7. #7
    Join Date
    Nov 2012
    Location
    McKees Rocks, PA
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    79
    I have a similar issue with my 90 SC. When the weather was a tad bit warmer (above 50 degrees) when I would drive the car in heavy traffic, stop n go traffic, etc. and the car would get up to operating temp. I would stop somewhere, home, work, store... maybe for 15-30 minutes. I'd come out and my car would be hard to start, then it would finally start but run rough for like 20 seconds and once I started driving it the problem would clear up. I was going to start with the fuel filter. However now that it is colder out everyday I haven't had this issue in a month.

  8. #8
    Join Date
    Oct 2004
    Location
    Metro Denver Area
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    2,514
    Mike, I'm curious to see if the filter or pump was your issue. Please post your results when you can.

    I have a stumble but only at idle, I think I have a bad wire but I suppose it could be fuel delivery....

  9. #9
    Join Date
    Apr 2002
    Location
    Norcross, GA
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    3,326
    I replaced the fuel filter but it's still not right. Tries to start right up but stumbles and won't run but a couple of seconds or so. The pump does run and I jumpered the pump on for about 30 secs or so before cranking to build up pressure in the fuel rail but it's the same as before. Although the pump runs it doesn't sound right and it changes tone as it runs. On my SC the pump has a higher pitched sound and it doesn't vary in tone. I've got it jacked up in the back, ready to drop the tank to swap the pump. I want to pump out as much gas as possible before I open it up. I'll be working on it this afternoon and hopefully finish it up today.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  10. #10
    Join Date
    Jul 2002
    Location
    Manchester, NH
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    2,064
    Quote Originally Posted by Mike Puckett View Post
    I replaced the fuel filter but it's still not right. Tries to start right up but stumbles and won't run but a couple of seconds or so. The pump does run and I jumpered the pump on for about 30 secs or so before cranking to build up pressure in the fuel rail but it's the same as before. Although the pump runs it doesn't sound right and it changes tone as it runs. On my SC the pump has a higher pitched sound and it doesn't vary in tone. I've got it jacked up in the back, ready to drop the tank to swap the pump. I want to pump out as much gas as possible before I open it up. I'll be working on it this afternoon and hopefully finish it up today.
    Good luck man. I am certain she'll be squared away once you swap out the pump.

  11. #11
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    Jul 2002
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    Manchester, NH
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    Just out of curiosity. Why not use an Aeromotive Stealth 340?

  12. #12
    Join Date
    Apr 2002
    Location
    Norcross, GA
    Posts
    3,326
    Well, I got it jacked up and ready to drop the tank and thought, I need to pump the gas out before I start. So I disconnected the fuel line at the rail, stuck the end into my gas can and jumpered the pump on. Instead of fuel flowing out I got a noise, bum-de-la, bum-de-la, bum-de-la, (as best as I can imitate it) coming out of the line. So, I checked the gas gauge and it showed 1/8 tank, well off of the red mark. Hmm, dropped the rear of the car and tried it again, and still got bum-de-la, bum-de-la noise out of the end of the line. I double checked the gas gauge by switching the ignition off and on several times and it was still showing 1/8 tank. Ed told me he thought there was plenty of gas in it, apparently not. Off to the gas station for a few gallons, dumped it in the tank and after a couple of false starts, it starts up and idles just fine and moved it back and forth no problem. I guess you could say it was the pump in a way. It looks like his gas gauge is off, either the sending unit on the pump or the slosh module could be the culprit. Thankfully it looks like I won't need to drop the tank. Whew.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

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