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Thread: 1988 K5 Blazer 5.7l TBI will not run - need help!

  1. #1
    Carb and Points!
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    1988 K5 Blazer 5.7l TBI will not run - need help!

    Been working on this for a long time. Here is list of new stuff:

    1. spark plugs and wires.
    2. Ignition control module.
    3. Fuel pump assembly. then just in case another new fuel pump
    4. 2 new fuel filters
    5. Fuel pump relay
    6. battery
    7. Starter

    So I did the following - FYI they all result in NO START:

    1. Tested voltage at relay. With key on I have 12v at power IN connection

    2. When turning the key I have 12v for about 2 sec at the output side of the relay.

    3. I tested the other end of the output from the relay at the fuel pump assembly and have 12v for 2 sec.

    To me the means that the existing wire is OK!

    4. When turning key and holding the relay in hand can feel the click and 2 sec later another click.

    5. Connected 12v to red wire (relay bypass) at relay. I have 12v at the fuel pump assembly.

    6. Connected new 14 gauge wire (temporary splice) from "relay out" to the fuel pump assembly. I have 12v at the fuel pump assembly.

    7. I have tested the ground from fuel pump assembly to frame using continuity. Result is .000 on the meter.


    Following resulted in fuel flow and start.

    a. Connected power directly from battery to fuel pump assembly.


    So if I am getting 12v to the fuel pump assembly via the existing wire and the temporary new wire why doesn't it run the fuel pump and start ?

    Been reading and wondering if the cause is a lack of a engine start signal or an engine running signal? I have been unable to find what creates these signals.

    Am I correct that while I have the initial 12v at the pump, that voltage stops after 2 sec and without the appropriate start or running signal voltage to the pump is cut off ?

  2. #2
    EFI GearHead ! EagleMark's Avatar
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    On the ECM pin B2, see if it gets power during the 2 second prime when key is on? If not fix it, look at diagram. If it does? Replace ECM...

    If you have a known good working 1227747 ECM just plug it in with your chip and try

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  3. #3
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    The original circuit included an oil pressure switch that would also close if oil pressure existed at the engine. This switch may have been hiding another problem and may have recently failed. Use the wiring diagram and test for power and ground at various points to determine what's going wrong.
    Last edited by 1project2many; 06-26-2013 at 12:21 PM.

  4. #4
    Carb and Points!
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    Still not starting

    New ECM - no start - fuel pump not running.

    To reiterate new parts:


    Spark Plugs / Wires
    Oil Pressure Switch
    Fuel Pump Relay
    ECM
    Fuel Pump - Entire Assembly
    Battery

    I have power for 2 sec while cranking at the following places:

    ECM port B2, Fuel Pump in , Out side of fuel Pump Relay.

    Following copied from another post so people don't have to scroll back - all result in NO START:

    1. Tested voltage at relay. With key on I have 12v at power IN connection i.e. "A".

    2. When turning the key I have 12v for about 2 sec at the output side of the relay. "E"

    3. I tested the other end of "E" at the fuel pump assembly and have 12v for 2 sec.

    To me the means that the existing wire is OK!

    4. When turning key and holding the relay in hand can feel the click and 2 sec later another click.

    5. Connected 12v to red wire at relay. I have 12v at the fuel pump assembly.

    6. Connected new 14 gauge wire (temporary splice) from "E" to the fuel pump assembly. I have 12v at the fuel pump assembly.

    7. I have tested the ground from fuel pump assembly to frame using continuity. Result is .000 on the meter.


    I replaced the oil pressure switch a few days ago and today replaced the ECM. Still not starting, fuel pump does not run. I wish I could understand the wiring.

    Does anyone know if it works like I suspect, as follows:


    1. Cranking activates the fuel pump relay for 2 seconds to prime the fuel lines.

    2. Cranking causes oil pressure to build. When it reaches 4 lbs the oil pressure switch closes allowing the fuel pump to continue to run.


    Also, Since the engine runs when I apply 12v directly to the fuel pump does that CONFIRM that my Coil, Pickup coil, Spark plugs and wires, distributor cap, rotor, TCM are all ok ?

  5. #5
    Fuel Injected! jim_in_dorris's Avatar
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    I have to suspect something else is wrong. Try testing your fuel pressure at the throttle body. If the pump primes for 2 seconds like it should, you should have pressure at the throttle body. If it immediately bleeds back, then you wouldn't have fuel to get a start condition. With the pump running constantly, the bleed down problem would be masked. Won't guarantee that is the problem, but something to think about.
    Square body stepsides forever!!!

  6. #6
    EFI GearHead ! EagleMark's Avatar
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    On B2 there should be a 2 second on with key on, as soon as you turn to crank it should again have power. Yours does not? Mine does I just checked!

    Then if there is a crank signal to C9 when key is cranked the fuel pump trigger at A1 turns relay back on and looks for the signal again at B2.

    Do you have power at Orange wire B1 and C16 Batt Power? Key off, key on, key Crank?

    Do you have power at Pink wire A6 Ignition, should be hot key on and key crank?

    I'm wondering if your ignition swicth at bottom of column is faulty?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  7. #7
    Fuel Injected!
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    black ground wires on the engine block,or battery ground to body?

  8. #8
    Electronic Ignition!
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    have you checked the ignition module? more than once i have found a bad ignition module to be the cause of a no start on a chevy tbi ( sometimes will even have spark but no injector pulse) a set of noid lights to test injector pulse might help too................just my thoughts................

  9. #9
    Fuel Injected!
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    if you have had your ecm out try disconnecting it an reconnecting, some times the little pins don't make a good contact

  10. #10
    Carb and Points!
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    update

    I put the new ecm in today. Grounds are good.

    This engine starts right up and runs if I bypass everything and put 12v directly from Battery to the fuel pump.

    Just bought a new key assembly and ignition switch.

    Will install them on Monday and see what happens.

    Thanks for all the input.

  11. #11
    EFI GearHead ! EagleMark's Avatar
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    Don't bother, it's not the key switch, it may be the ignition switch on bottom of column that is activated by a rod from key cylinder...

    Your trying hard and this is not an easy find, I've never seen it before. But go through this before you go and just replace another part, I'm trying to figure out what is wrong?

    On B2 there should be a 2 second on with key on even if you don't go to crank, as soon as you turn to crank it should again have power. Mine does I just checked! Yours does not? Why?

    Then if there is a crank signal to C9 when key is cranked the fuel pump trigger at A1 turns relay back on and looks for the signal again at B2.

    Do you have power at Orange wire B1 and C16 Batt Power? Key off, key on, key Crank? Should be hot always.

    Do you have power at Pink wire A6 Ignition, should be hot key on and key crank? Should be hot key on and key crank.

    I'm wondering if your ignition switch at bottom of column is faulty? This will prove ECM has all Batt power and Ign power needed for operation. Something is not triggering fuel pump relay in Crank! But it does Key On?


    SO, is it ignition switch faulty and loosing power to ECM? Or id ECM not getting a crank signal during crank at C9?

    I brought up the C9 Crank signal in your other thread at CK5 but never heard back? It's a wire on starter solinoid to ECM C9... since it runs with power to fuel pump? And not if relying on ECM to trigger fuel pump relay, my bet is on no Crank signal from C9?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  12. #12
    Carb and Points!
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    thanks for replying to both threads Eaglemark. I posted to both in case anyone else has to go through this. As I said taking the day off will replace ign switch tomorrow and then if it does not start I will run these new tests.

    Thanks for everything.

  13. #13
    Fuel Injected! phonedawgz's Avatar
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    Your problem is the +12v that is present on pin E of the relay isn't making it to the fuel pump. If that voltage made it to the pump, your pump would run and your problem would be fixed.

    A few things. Test with a 12 v trouble light. Using a meter puts no load on the circuit and can give you misleading information. Test with everything plugged together. If you had the 2 sec +12 on E with the fuel pump unplugged, reconnect it and test again.

    Your problem is not the ignition switch.

    If indeed you have +12v on E of the relay for two seconds but nothing at the pump you will either have to find why it's not connecting, find the break in your wire, or add an additional wire between the two. Most likely your problem is a connector that is corroded and not making contact. Is the point of your tester piercing the corrosion but the terminals are not making contact? Use the point of your tester to poke directly to each sides terminal and verify the voltage is making it across. Also look to make sure the wire itself is not corroded off of the metal terminal. Again this is where the test light comes in play. You might have a high resistance connection that with no load allows the +12 to cross, but with load the voltage drops too low. The test light puts some load on the circuit and can show the high resistance joint. A meter is designed to put practically zero load on the circuit.

    Many of the relays used in 88 were of poor design and if left dangling up side down by their wires filled up with water and corroded the relay socket terminals badly.

  14. #14
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    I agree with using a real test light when checking for problems such as this. And I agree that the test for power at the pump connection should be check with connector plugged in due to difficulty in nailing down this problem.

    You asked how the fuel pump circuit operates. In original form the pump circuit contains redundant paths. The oil pressure switch closes at about 4psi oil pressure and provides power to the pump. The pump will run as long as battery power is present at the switch and oil pressure has closed the switch. The relay can be removed completely in a vehicle with this circuit and the engine will start and run. The only complaint may be long crank times if it takes a while for oil pressure to build. The relay is controlled by the ecm. In most vehicles you will hear the relay close for about two seconds on key on. The relay will also close when the engine is cranking and will remain closed for about two seconds after cranking stops. The oil pressure switch can be completely disconnected on vehicles with this circuit and the pump will operate just fine.

    So, you're troubleshooting a no start. How do you know the fuel pump is not working? Are you basing the diagnosis "pump doesn't operate" on the fact that the vehicle starts and runs when a wire is attached from battery to pump? You may be complicating the problem by making this decision. After all, what if the pump is running slowly with insufficient voltage? Subtle distinction, yes, but it turns what seems like an impossible situation into something more workable. Can you hear the pump run with key on, engine off? Can you hear the pump run when the relay is jumpered? Can you hear the pump run when power is applied from battery to pump?

    Some what-ifs to test for since I might not be able to check back for a bit. If voltage is dropping below minimum acceptable levels on one or more circuits during cranking, and you're testing by yourself, you'll have to come up with a few dynamic tests. It appears that you have proper voltage during Key On Engine Off which is good. What happens if you connect the jumper from battery to pump, start truck, then disconnect long jumper? If truck stalls, suspect insufficient voltage at pump. If truck continues to run, suspect insufficient voltage during cranking. Try running jumper from battery to factory pump relay bypass and see if truck starts. If yes, move up the line and run jumper from battery to BATT side of pump relay. If no, diagnose problem in pump wire between sender and factory relay bypass. Continue to test the circuit moving up the line closer and closer to the battery until you locate a problem.

  15. #15
    Carb and Points!
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    ok besides truck not working my printer stopped but I had an old one lying around and was able to print all this. It's raining, don't have a garage. As soon as I can I will go through all these tests and report back. Rain forcast for NJ for next few days but will try to get it done asap.

    Thanks for the detailed explanations.

    Will report back in detail after next round of tests.

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