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Over-reving TBI when cold (UPDATE)!!

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by smack, Jan 15, 2003.

  1. smack

    smack 1/2 ton status

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    Sorry for being redundant but I couldn't find the right thread.
    My TBI seems to have what seems like a sticking throttle when the engine's very cold. However, the throttle itself is not sticking (manually forcing it back doesn't help). No codes. Have sprayed WD-40 and carb cleaner into butterfly. No vacuum leakes. Replaced IAC.
    I'm considering replacing the whole TB, but first I have two questions.
    1 -- with a 160 thermostat the engine won't warm up beyond 120. This might be the cause, right? Not sure which switch to replace (I'm guessing the one that controls the fan on the intake).
    2 -- could a faulty TPS cause a condition like this??
    TIA,
    smack
     
  2. TopOff

    TopOff 1/2 ton status

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    Re: Over-reving TBI when cold

    A faulty TPS - could cause this, although I am not sure how the computer would react.

    So could a IAC valve (I think it's your IAC valve - but I have not had this problem before). I have had to reset the idle on my TBI though, using some weird procedure when jumping the ALDL.

    It could alsobe a bad (or leaking) MAF or MAP.

    Have you let it run with the ALDL jumpered? This will keep the engine in open loop, and shut down some sensors, although I forget which ones. This can help --- in someway.

    Good luck.
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Over-reving TBI when cold

    Go back to a 195 for sure, there is no real reason to be running a 160. The power "gained" if even real, isn't enough to think about. If its not getting to 160+, it won't go into closed loop.

    Engine takes a long time to get up to 195* even, so I doubt thats the cause. My truck takes a good 5 minutes to get up to operating temperature even if I jump in, let it idle for 15-30 seconds, then drive off.

    Replacing parts gets costly, it's better to make sure all the parts are working correctly. How high are we talking about? Haven't dealt with TBI before, but injected vehicles typically have at least a short period of higher idle on startup. Maybe with extreme cold, the high idle on startup is increased somewhat. I doubt it, but again, no direct experience with the TBI.
     
  4. Sidepipes

    Sidepipes 1/2 ton status

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    Re: Over-reving TBI when cold

    With the TbI there should be a short period of high idle(around 1000rpm by my tach) and quick ( within 5-10 sec max) down to the normal idle of 800rpm. The period of high idle is while the sensors are adjusting mixture, timing, etc for choke operation

    At least thats how the 2 TBI 350's I have had operated.

    It must be nice to have your truck able to come up to operating temp. since my rad change I can't get the engine over 150 deg with the 180deg stat I have(I think its leaking around the stat)
     
  5. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    Well, I'm running a 160 because of the stage 2 chip, but according to my gauge, the engine's not getting anywhere near that hence the thought that maybe my fan is always on because of a possibly bad temp. switch.
    Before I swap back in the orig thermostat I'd like to get the temp in the right zone.
     
  6. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    In actuality, my truck idles fine and not overly high when cold, and correctly low when warm. The problem is that when I hit the accelerator it takes forever to come back down and actually shoots up when I'm shifting = scary.
     
  7. TopOff

    TopOff 1/2 ton status

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    Re: Over-reving TBI when cold

    FYI: My TBI goes into closed loop at ~90 degrees.

    It sounds like you were eluding to the use of a custom aftermarket type chip...

    Is this the case?

    If so, change back to the original chip and try it out...

    Good luck.
     
  8. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    Yes, I do have an aftermarket chip but it did have this behavior before I put it in. Again, this only happens when it's very cold so my first thought was that the butterfly is sticking open but spraying cleaner in there doesn't help. If I lift the filter and watch them move, rev it up and pull them back, it stills over revs even when I'm seeing down into the bowl that the flys are closed. It's like some air/injection/idle path that's overworking or something.
    smack
     
  9. TopOff

    TopOff 1/2 ton status

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    Re: Over-reving TBI when cold

    Yep, your IAC allows a kind of vacuum leak to control idle and keep it stable under load.

    On my TBI setup I have this in the front towards the front of the vehicle.

    I think on the factory TBI this is on the rear. There should be four wires going to this little black looking thing on the TBI body.

    I know these get stuck over time, maybe yours is stuck open. It should be vibrating when the truck is idleing. So if you put your hand on it, or a screwdriver on it and to your ear (use with caution) you should here it tapping and clicking like a loose valve... /forums/images/graemlins/laugh.gif

    IF not, that would be your culprit.

    If this is not clicking, you can TRY taking it off the TBI, and spraying some carb cleaner down it to loosen up the oil deposits and get it working again. But most likely you will need to get a new one.. /forums/images/graemlins/frown.gif

    I would hate to try and shotgun approach like this though, because I am pretty sure your computer should through a code that the IDLE circuit is not responding...

    Good luck.
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Over-reving TBI when cold

    Theres some variance in temps between the systems, so I might be able to get an exact number.

    122* in one camaro application for instance, so it may be more variable than I thought.
     
  11. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    Well, I did replace it with a new one and that didn't help.
     
  12. TopOff

    TopOff 1/2 ton status

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    Re: Over-reving TBI when cold

    Bummer...

    How high of an idle does it run at?
     
  13. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    The idle is fine.
    The problem is when you hit the pedal and release the rpm will stay there for a bit and slowly come back down. When driving it's a real PIA 'cos if I'm not careful, as soon as that clutch hits the floor my rpms go sky-high. Makes no differnece if I stick my foot under the gas pedal and pull it back.
     
  14. TopOff

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    Re: Over-reving TBI when cold

    Hmm maybe it's an EGR or vacuum routing problem...

    weird. Still, I would think the computer would have some codes... So you get a code 12 when you jumper the ALDL then?
     
  15. Thunder

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    Re: Over-reving TBI when cold

    Have you checked the base gasket on the TB. It may be leaking. Might want to double check all the lines.
    Sounds like a vacume leak to me.
    Could be possibly be EGR related. EGR should be closed when the engine is cold.
     
  16. R72K5

    R72K5 Banned

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    Re: Over-reving TBI when cold

    personally id just plug off thr EGR valve vacuum line, and check for vacuum leaks as previously said, and stil sounds like it is IAC or TPS really
     
  17. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    Yeah, code 12 is there, vacuum routing has not changed.
     
  18. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    I was considering EGR, too, but the valve is (fairly) new. With that I mean about two years old. It being stuck open would make the most sense but how could I bypass it if it's open?? I seem to remember it being pretty hard to move the diaphram by hand.
    I do have a TB spacer but the condition existed prior to it's installation (I think). Maybe I'll put it this weekend and see what's up then.
    Plus, I'm wondering if I fu..'d up something on the TB 'cos after I put on the spacer I left my cleaner lid flipped and noticed there was pretty little clearance for the hood. I set it down easy but who knows what could have happended.
    smack
     
  19. smack

    smack 1/2 ton status

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    Re: Over-reving TBI when cold

    Do you know specifically what symptoms a bad TPS would be?
    smack
     
  20. TopOff

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    Re: Over-reving TBI when cold

    If you flipped the lid, then maybe the MAT sensor is not getting a true reading of the incoming air. I don't what or how this affects the engine/computer. I am not familiar with where the MAT is installed on stock engines - sorry.
     

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