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Surging and Misbehaving

Discussion in 'Truck & Equipment Repair' started by OBXFishinFever, Sep 26, 2007.

  1. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    1988 Chevy R-30, 454 7.4 TBI with 95,000 miles, and a 4-speed manual trans.
    For several years now I have had a surging problem while accerlating, it skips and misfires while idling, it started off a small problem, and all I had to do to correct the problem is let the truck warm up to normal operating temp before driving it. Now that does not work, I have changed every sensor that I know to change and have been told to change, I have rebuilt the TBI unit and put a new IAC and TPS on it, I have totally rebuilt the distributor, and the fuel system except for the fuel pumps in the tanks. I even changed the computer last year before our biannual fishing trip. The truck ran very well while at full throttle until lately, now when I give it full throttle, it stumbles and hesitates until it is at full throttle, then as I back off the pedal, it picks up power and rpms. When I try to set the timing, I pull off the wire for the computer and set the timing to 4 degree BTDC, the engine will barely idle, then I plug the wire back to the computer and it sends the timing mark way off the scale, but the engine runs better and increases idle to around 1100rpms.
    I took it to a local shop a couple of years ago and after $1200 all they could tell me is the O2 sensor was not hooked up to the computer.
    I am leaving Sat. the 29th of Sept. going to the beach pulling my 9500lb camper, it will still pull it OK, but not as good as it should or used to.
    Please Help, I am lost and almost out of money...

    Thanks for any and all help,
    Bobby
    OBXFishinFever@aol.com
     
  2. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Sounds like a fuel delivery problem Bobby....

    1) When you rebuilt the TBI unit did you replace the injectors? Its pretty common for the
    injectors to get weak on the 454's. The rough running after a cold start is a good indication that the injectors are weak and/or failing.

    2) Did you at least clean/replace the screens on the injectors when you did the throttle body?

    3) The fact that it has more power as you ease up on the throttle is also an indication of weak/restricted injectors or low fuel pressure (which is VERY common on the TBI trucks).

    4) Has anyone done a fuel pressure test on the truck while under a load?
     
  3. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    1) When you rebuilt the TBI unit did you replace the injectors? Its pretty common for the injectors to get weak on the 454's. The rough running after a cold start is a good indication that the injectors are weak and/or failing.
    YES, I replaced both injectors, man them things ain't cheap.

    2) Did you at least clean/replace the screens on the injectors when you did the throttle body?
    YES, I replaced the filter screens, I replaced everything replacable in the throttle body unit, I even replaced the springs and the diaphragm for the fuel pressure regulator.

    3) The fact that it has more power as you ease up on the throttle is also an indication of weak/restricted injectors or low fuel pressure (which is VERY common on the TBI trucks).
    I have not checked the fuel pressure, I have not thought about it and no one has suggested it until now. I do remember that when all this first started that the truck ran better off of the right tank than it did the left tank, I will check in to that. I did find this morning that the TPS was not adjusted right, I put my OHM meter on on the center terminal and the front terminal and the OHM's were really high, around 14.75, as I opened the throttle, the OHM's went down to 2.20 then started back up, when I adjusted the arm for the TPS, I made the OHM's bottom out at 2.20, now it goes up to 6.31 OHMS at full throttle.I adjusted the TPS this morning and now the truck does Not pick-up as I let off of the throttle, it acclerates good now. It still surges real bad and hesitates real bad on bottom end.

    4) Has anyone done a fuel pressure test on the truck while under a load?
    No pressure test at all, would you know what the pressure should be? I will look it up if you don't know right off...

    Thank you for your help, I will check the fuel pressure sometime tomorrow and I will post my findings...

    Thank You Very Much,:)
    Bobby
    OBXFishinFever
     
  4. streetsurfin'

    streetsurfin' Senior Member
    Messages: 770

    You say you went throught the fuel system. Did you change out the tank switching valve? What you say about it running worse on one tank makes me think that the valve may be the source of the problem.
     
  5. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    You say you went throught the fuel system. Did you change out the tank switching valve? What you say about it running worse on one tank makes me think that the valve may be the source of the problem.

    No, I have not changed or even looked at the switching valve, I have changed the filter which mounts beside the switching valve. I will check that valve out as soon as I can. I think that will be a good time to check my fuel pressure.

    Thank You for your help,
    Bobby
    OBXFishinFever:)
     
    Last edited: Sep 26, 2007
  6. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Since it does run better or worse depending on which tank/fuel pump your running on then I'd definitely say your fuel pressure could be a very likely culprit and you need to check the pressure. The factory spec for the pressure is 9-13 lbs but I Like to see no less than 12lbs. I doubt your switch over valve is causing your problem as they rarely cause a restriction problem but anythings possible...
     
  7. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    Since it does run better or worse depending on which tank/fuel pump your running on then I'd definitely say your fuel pressure could be a very likely culprit and you need to check the pressure. The factory spec for the pressure is 9-13 lbs but I Like to see no less than 12lbs. I doubt your switch over valve is causing your problem as they rarely cause a restriction problem but anythings possible...
    __________________
    87 GMC K1500 Meyer C-8.5


    Thanks for all the help and information, I will check the fuel pressure tomorrow and will look at the switching valve while I am under the truck, I need to change the fuel filter again anyway.
    I sure do appreciate all the help you all have given me, I will post my results as soon as I can.


    Thanks Again,
    Bobby
    OBXFishinFever:)
     
  8. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Glad to help bud, keep us posted on your findings...
     
  9. MickiRig1

    MickiRig1 PlowSite Veteran
    Messages: 3,617

    Why don't you get a tank switcher from the junk yard and try that?
    I chased a EFI problem for a year and did pretty much the same thing you have done.I even rigged my pressure gage with a longer hose so I could see the pressure while driving. ..... It ended up being the tank switcher. They charged me like $10 for it and no more problems. If it uses clips to hold on the lines use new ones, don't recycle them.
    If it uses the tank pumps pressure to switch the tanks it may be the same problem I had.
    Mines a Ford but a lot of parts are made by the same companies from brand to brand. Even the same design with a different brand printed on it.
     
    Last edited: Sep 27, 2007
  10. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    Checked the fuel pressure this morning...6-1/2 psi. I changed the fuel pumps...The truck now runs BETTER than the day I bought it.

    Thank You for ALL the Help
    Bobby
    OBXFishinFever:)
     
  11. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    That's great to here OBX, looks like you found out the TBI trucks will run with as low as 5 psi of fuel pressure, but they don't run well!
     
  12. OBXFishinFever

    OBXFishinFever Junior Member
    Messages: 6

    I would have NEVER thought the TBI would run with that little of pressure, I checked the pressure this morning and I found it had 6-1/2psi, after reading the posts last night, I knew that I needed at least 9psi to run it. I am more familiar with small blocks, 6-1/2 psi would be ok for most carburetored engines.
    I have owned the truck for a little over 16 years and for the last 4 years the truck has been almost undriveable. I bet I put 100 or more miles on it this afternoon, it is fun to drive again, the way it was when I bought it in 1991.
    I wished I had found this site a long time ago. It is hard to believe the kind of money and the sleepless nights I have spent over the last couple of years or so trying to figure out this problem.
    I Thank You Very Much for all the help, and I really Apprreciate all the help and advice from everyone here...

    Thank you ALL Very Much, and Thanks to the people that run this site...

    Bobby
    OBXFishinFever
     
    Last edited: Sep 27, 2007
  13. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Glad to help Bobby! That's why we're here, and happy motoring! :waving:
     
  14. MickiRig1

    MickiRig1 PlowSite Veteran
    Messages: 3,617

    Like my post, it drove me nuts for a year!
    It's great you found the problem!!!
    I am glad someone could be of help.