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Stalling EFI truck

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blue85, Jan 26, 2001.

  1. Blue85

    Blue85 Troll Premium Member

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    Can you help out? This is not my truck, it's my parents and it's too far to go look at it. It is an '89 3/4 ton Suburban (read "long K-5) 4x4 with TBI 350:

    "We have a "dead 4X4" in the yard. It runs for exactly 20 seconds & shuts off completely, as if someone turned the switch off. First the alternator died (ground out the rear gears). He replaced that. Then the stalling began, when the truck was idling, but not at the "20-second" mark. Then it went to the 20-second shut-down. It starts right back up again... for 20 seconds. Why at exactly 20 seconds?"

    I suggested that it is an electrical problem with the circuit that runs the fuel pump while the engine is running (as opposed to the "priming" circtuit.) and went into some detail as to why it would do that and what to check. Are there any other suggestions or quick troubleshooting aids?

    <font color=blue>"As of tomorrow, employees need security cards for building access. Pictures will be taken Wednesday and cards received in 2 weeks."</font color=blue>
     
  2. Brian 89KBlazer

    Brian 89KBlazer 1/2 ton status

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    Blue

    Best thing I can think of is get a code scanner. Sounds like it takes the computer 20secs to register the problem & shut down. Maybe a faulty temp sensor or something like that that the computer would think is fatal to operation (overheating) and thus shuts down??
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Does the engine have oil pressure? There's a fuel pump relay on the firewall that provides power to the pump while cranking the engine. After it times out, there's a sensor down under the distributor that needs to have at least 4 psi of oil pressure present to keep sending power to the fuel pump. It's there to keep fuel from spraying out of a severed fuel hose in an accident. When the engine dies, the pump stops running.

    If there's no oil pressure, or the swtich is bad or the switch is unplugged, the engine will stop when the ECM stops sending its override signal to keep the pump powered.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <A target="_blank" HREF=http://ThunderTruck.ColoradoK5.com>http://ThunderTruck.ColoradoK5.com</A>
     
  4. pcorssmit

    pcorssmit 1/2 ton status

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    There should be a short lead comming off this relay. If I remember correctly, it is a fuel pump test connection lead. Try hooking it up to a 12v source and then starting the engine. The fuel pump should run all the time if hooked up this way, and would tell you if the problem lies in the oil pressure switch or wiring.

    Pete

    '83 K5, 350 TBI (ex 6.2), Dana 60/14 bolt, Detroits, 3" lift, 15-39.5x15 TSLs
    '97 Dodge 2500 4x4 CC LB Sport, Cummins 5 spd
     
  5. K5 NUTT

    K5 NUTT 1/2 ton status

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    If the cable was damaged a case of high resistance might of helped out with the demise of the fuel pump...what i used to do at the shop with GM's is put another guy under the hood with the aircleaner off and a full can of carb cleaner in one hand ...if it woudn't start we used to spray into the tbi while cranking...and they will start and run as long as you keep feeding them the juice...so when it starts to die spray in some carb cleaner...if it continues to run you need to r/r the intake pump...if not i will need the codes she most likely is throwing to help you out any farther...and if your parents happen to live in Az. i can do a house call for a dellow CK5 member! I own a scanner.

    Azblazer



    1979 K5
    1976 K5 Chalet
    1972 K10 Suburban
     
  6. FWP

    FWP CRS

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    Yep, sounds like not enough fuel pressure....

    AZ, Bout time you posted[​IMG][​IMG]. Where you been??? When's the next wheelin trip? BTW, got 2" snow right now, and it's still comin down[​IMG]-- Time to go play... hooo hooo.......

    [​IMG]<P ID="edit"><FONT SIZE=-1>Edited by FWP on 01/27/01 03:14 AM (server time).</FONT></P>
     
  7. Blue85

    Blue85 Troll Premium Member

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    The problem was a bad fuel pump relay. As it turns out, the ideas that I gave them initially were right on the money as I pointed to the "engine running" fuel pump circuit, including the fuel pump relay and oil pressure switch. Looks like you guys were right on top of it, too.

    Thanks for all of the help.

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