6.5 TD having a weird starting issue...?

Discussion in '1982-Present GM Diesel' started by shady, Oct 13, 2020.

  1. shady

    shady 1 ton status Premium Member

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    Only diesel I've ever owned before this was a 6.2 that never needed anything but oil changes..... So I'm lacking in diesel knowledge lol.
    It's a 95 C2500 suburban, 6.5 TD
    What I got going is a random starting issue that goes like this....

    Key on, wait for GP light to go out, turn key and hold it..... No turny turny, but I get a sound like a normal electric fuel pump running (only while the key is being held in start position).

    If I hold the key that way for a while, some times it'll randomly start turning over after a few seconds. Most of the time all I get is that pump running sound.

    BUT!.... The odd part is that probably half the time I turn the key it starts just fine exactly as it should :dunno:.

    My only theories so far are,
    1. I'm losing fuel pressure somewhere while it sits (happens more often after sitting, but not always), and that pump sound is building it back up.....
    Would that prevent it from cranking though?
    2. There's something wrong with the key/switch that's not cranking it.
    3. Completely random thing that I have yet to understand because I'm dizzle dumb.
     
  2. shady

    shady 1 ton status Premium Member

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    Nobody...? Lol. I figured 2 or 3 guys may have had the same issue at one point or another.
     
  3. TreeFarm

    TreeFarm Internet fabricator. Premium Member

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    well i'm no 6.5 guy but i know engines and diesels. I highly doubt low fuel pressure will keep you from cranking the engine. that fuel pump is just a lift pump to get fuel to the injection pump. Now if you had hard starting issues, like cranking and cranking before it fires, then I would look at fuel. it could possibly be your ignition switch.

    First thing I would do is verify if its the starter or not. You can do this two ways. 1. jump 12v positive to the trigger on the solenoid and see if the engine turns over. if it does then you are not getting voltage there, problem in the wiring. Another way to do this is to check for 12v at that same connector when someone turns the key. voltage present means bad starter, no voltage means something in the truck.

    Another thing you can check is the neutral safety switch. try starting it in neutral rather than park. OR hold the key to start and move the sifter up and down to see if it makes it crank.


    I just replaced a SBC starter doing this same thing. and then a few weeks later had the neutral safety switch mess up and I had to by pass it.




    Start with checking for voltage at the starter/giving voltage to the starter. Work backwards from there to see where the voltage stops
     
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  4. shady

    shady 1 ton status Premium Member

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    Only thing that kind of goes against that is that if I'm driving it and shut it off to run in a gas station it starts right back up every time... Has never failed in that situation. But if it sits a half hour or more, it's a 50/50 shot.
    That's why I thought pressure.... Like maybe it has a pressure switch and won't allow it to crank without so much pressure so it doesn't kill the injection pump or something.

    I'll have to find the pump that IS running too. Because I have an aftermarket lift pump on the frame and I know that pump is not the one I hear running.
    Unless there's still a factory one after that one :dunno:.
     
  5. KirsL

    KirsL Diesel maniac Premium Member

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    The lift pump in the 6.5s is turned on via the oil pressure switch. Now a previous owner could of changed that. And it wouldn't stop the engine from cranking, just starve it of fuel under any kind of load.

    So it sounds more like an electrical issue then an engine issue. I'd start by checking all the usual connections between the batteries and starter. It could be the ignition switch. My old 91 ford the ignition switch would become hard to turn when cool/cold.. so it's worth checking that too.
     
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  6. diesel4me

    diesel4me 1 ton status Premium Member

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    I'd do as suggested already--try jumping the solenoid at the starter,if it cranks OK then,then its not getting enough amps & volts to kick in the solenoid from the ignition,neutral safety,wiring,etc..

    If it still doesn't crank when jumped,or struggles to,it may be the Solenoid itself ..

    Inside the starter solenoid is a copper disc that does the transfer of power from the battery cable stud to the starter motor windings..
    On diesels especially,that disc gets a beating,the high amp current arcs and blackens the copper disc and makes for a poor contact..or none..with two batteries and a cold diesel,long cranking periods,the disc can heat up and lose conductivity..
    After the engine has been run and warm the starter needs less amps to kick in and crank,so it works till it gets cool enough not to again..

    I've had this happen on a few starters and I took off the plastic cap on the solenoid and put the disc on the wire brush on my bench grinder and shined it up good,and spun it so the two spots that were "burnt" were no longer aligned with the stud and other contact,to give it a "new spot"..didn't act up again as long as I had the trucks..
     
  7. Wes Harden

    Wes Harden 1/2 ton status Premium Member

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    Check positive cable at starter, it may be loose. This will cause a no crank cold, and crank just fine after warm up, once the terminal gets hot and makes contact.
    It sounds like your ign switch may need adjustment, or replacement.
     
  8. Truckman4life

    Truckman4life 3/4 Ton Status Premium Member

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    Yeah I've had weird starting issues due to crappy/loose connections at the batteries also. Could be ignition switch or the trans range switch might be my guess.
    upload_2020-10-16_7-41-46.png
    Dizzle sub starting 1.png
     
  9. shady

    shady 1 ton status Premium Member

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    Thanks for the diagrams. I was cooking for those.

    I'm actually surprised that it doesn't have a start relay.

    Terminals at the batteries are all clean and tight.

    I have tried to wiggle the switch and the shift lever both while holding it in the start position, but it's never caught and tried while I was doing it. :dunno:.

    I've been dealing with all the crap on my dually and haven't put much effort into this one yet lol.
    I'll see if I can find and bypass the trans position switch first.
    See what that does.
     
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  10. yellowK20

    yellowK20 Well Lubricated Premium Member GMOTM Winner

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    Sounds like the ignition switch might be going Tango Uniform . ( not the tumbler , the actual switch) or the connector to it.

    it could also be a neutral safety switch , that should be in the trans on a 4L80e ?
     
  11. shady

    shady 1 ton status Premium Member

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    Not sure. In the trans is probably right. :dunno:

    Hopefully I can get to checking it tonight yet.
     
  12. Wes Harden

    Wes Harden 1/2 ton status Premium Member

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    should be on the steering column just a few inches from firewall
     

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