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tb injector not firing

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 89Jimmy, Oct 28, 2003.

  1. 89Jimmy

    89Jimmy 1/2 ton status

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    Hello,
    This isn't really a K5 question, problem is occurring on a friend's 94 pickup. 350 tbi, 700r4, stock stuff. The truck will not start. Pour gas into the tbi and it will run until that gas burns off. Gas is present to the tbi unit while cranking. Injectors are not "buzzing" when cranking. Test light shows voltage at injectors during cranking. Sorry if this has been covered, but I didn't find anything in my forum search. Any ideas? Thanks.
    John
     
  2. NITRO

    NITRO 1/2 ton status

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    fuel pump bro...

    NITRO
     
  3. rampage

    rampage 1/2 ton status

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    Did this happen suddenly or was the problem getting worse and then finally wouldn't start?
    I'd start with the cheap and easy things first, like the fuel filter. When the key is turned to the accessory position do you hear the fuel pump pressurizing the system? If not then it's the pump. Could also be clogged injectors.
     
  4. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    [ QUOTE ]
    fuel pump bro...

    NITRO

    [/ QUOTE ]

    Agreed. I've had them just DIE without warning in the past.

    -Dan
     
  5. Thunder

    Thunder 3/4 ton status

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    Check or replace the distributor Module before you go messing with the fuel pump.
    If the module is bad the ECM sees no spark singal and shuts off the fuel so the engine does not flood.
     
  6. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Good point there too Thunder...

    -Dan
     
  7. BurbinOR

    BurbinOR 3/4 ton status

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    Make sure the fuel pump relay or fuse is good before you think pump........... /forums/images/graemlins/cool.gif
     
  8. 89Jimmy

    89Jimmy 1/2 ton status

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    Still not starting. The problem started suddenly. As stated, gas is present at the tbi during cranking. That should eliminate pump,relay,filter, and other fuel delivery components from the problem. Gasoline preasure and volume are in spec. The distributor pickup coil has been replaced and the ignition module was tested good at the parts store, but we had a known good module available, so we swapped that in anyway. Still no start, but very good thinking, Thunder, thanks very much. As stated in the first post, the injectors are not "buzzing" while cranking. We borrowed a noid test light and it indicates that there is no electrical impulse to the injectors while cranking. We have swapped in 2 junkyard ecms with no luck. It's possable that those ecms were bad, but kinda' unlikly both would be bad in the same failure mode. We don't know if the ecm directly triggers the injectors or is there another part involved? Is there another system, other than lack of distributor reference signal that would direct the ecm not to fire the injectors? Still lookin' for input. Thanks to everybody for pitchin' in.
    John
     
  9. Silverado_K5

    Silverado_K5 1/2 ton status

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    There should be (2) ECM fuses in the fuse block and the crank fuse too. Check all your fuses in the fuse block to make sure you power going through them.

    Doug
     
  10. POWERMAD

    POWERMAD 1/2 ton status

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    It sounds to me like the MAP sensor has gone south.
    From what you posted it sounds like that has gone out.
     
  11. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    The injectors are wired directly from the ECM. The ECM grounds the 12v provided by the fuse mentioned above.

    See this wiring diagram for pinout info:

    INJECTOR/MODULE WIRING DIAGRAM

    Another thing to check is SPARK. Do you have spark? Check to see if the rotor on the distributor is turning while you (or a buddy) crank the motor over. I've heard folks blowing the cotter pin out of the gear on the distributor... its worth a shot.

    Otherwise, be sure the connections at the distributor are good, and that those wires aren't broken anywhere...

    Good luck,
    -Dan
     
  12. 89Jimmy

    89Jimmy 1/2 ton status

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    Yes, We have spark. The motor runs if gas is poured into the tbi. All the fuses check ok. The noid light says no impulse. But a test light shows voltage steady on one wire and pulsing on the other when grounded to the block. Will check the MAP sensor. Thanks for the info.
    John
     
  13. KRAZIE87K5

    KRAZIE87K5 1/2 ton status

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    Ron-

    I've never heard that before... a dead MAP can cause the injectors to quit firing? Just curious... never heard of that before... and I installed my last EFI system from scratch... so I'd like to think I have SOME idea what I am talking about... Sorry if this comes across wrong... I'm just real curious???

    I'd also check for a SOLID ground to the ECM itself... even if you just doa quick OHM test from the case to the batt NEG. terminal...

    -Dan
     
  14. POWERMAD

    POWERMAD 1/2 ton status

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    From everything he described it all points to the MAP sensor. But I'm not there to look at it so it could be any number of things
    A quick test is to measure the signal wire voltage, it is typicaly 4.6 - 4.8 volts 0 vac. apply a vaccume to 5in-Hg and it should drop to about 3.75. pull it to 20 and it should drop to around 1 volt.
    The MAP sensor sends engine load data to the ECM which uses that info to adjust spark advance and fuel delivery.
    Think of it as the electronic equevilant of vaccume advance and a power valve in a carb.
    I have seen them tell the injectors to be full on at an idle and flood the crap out of the engine.
    Or fail in the other direction. Basicly telling the ECM that the key is on but the engine is not cranking.
    That's why there is fuel pressure, spark and so on and will only run with raw fuel poured down the pipe.
     
  15. 89Jimmy

    89Jimmy 1/2 ton status

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    Thanks for the map sensor diagnostic data. We'll check out the reference signal. Will also check the ecm ground. thanks to all. I'll keep you posted.
    John
     
  16. R72K5

    R72K5 Banned

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    i found out the hard way that if coil primary wire is not supplying power to coil then the TBI injectors will not inject

    had this problem with 92 4.3L TBI not too long ago, it would sometimes not fire up during crank out of the blue for no reason at all, would fire on starting fluid however, so we replaced fuel pump and that didnt fix it, one day i was messing with the pink 12v feed wire to coil while cranking engine over and it fired up and ran good, so i jiggled the wire some more and it killed engine, the wire was broken inside insulation real close to plug at coil end of the wire, it runs from there to distributor,

    check that because i have a feeling its happening to alot of TBI chevy engines

    cant explain how it will fire on fluid or gas, but it loses and gets connection at the brake in wire randomly
     

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