Not Sure What's Going On

Discussion in 'The Mixing shop.' started by RedSkyV8, Jul 8, 2016.

  1. RedSkyV8

    RedSkyV8 Well-Known Member

    Engine starts fine and runs good on long runs. But, when I stop the car and shut off the motor when it's hot, it will start up and then shut off, right away. It happened today, after not doing this for awhile. I drove the car about 30 minutes, and stopped for gas (engine was off for about 5 minutes). I came out and the car started fine, but when I put the car in gear, it shut off. It took me about 10 minutes, off and on, to get the car running again, but when it warmed back up, I was able to drive the car home (about 15 minute drive with no issues).

    I am not sure where to begin in diagnosing whether it's a fuel problem, or a vacuum problem, or what?

    What do you guys think?

    (1970 Buick Skylark, 350V8, 4 BBL, AT)
  2. DaWildcat

    DaWildcat Platinum Level Contributor

    Sounds like a fuel issue, hot soak. During the longer runs the constant flow of gasoline helps.

    You don't still have winter fuel in the tank, do you? If so, fill 'er up now, that might be all it needs.

  3. RedSkyV8

    RedSkyV8 Well-Known Member

    I don't think I still have winter fuel in the tank. I have purchased fuel several times this spring and summer and made several long runs to car shows. I seldom, if ever, drive the car around town, and that is where this issue is occurring.

    Can you elaborate on hot soak?

    Thanks, Marshall
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    You may have a few things going on. An engine that runs very hot will heat up the carburetor more than a cooler running engine. After engine shut down, the coolant is no longer circulating, so it continues to absorb heat, (heat soak) raising the engine temperature even more. The intake manifold has an exhaust cross over passage that also heats the bottom of the intake manifold (this helps with fuel vaporization in cold weather). All this combined with fuel that is more volatile today can cause fuel to literally boil out of the small fuel bowl of the Quadrajet carburetor. In addition, the Quadrajet has 2 plugs at the bottom of the fuel well that can leak fuel. Any good rebuild addresses these leaky plugs by sealing them with epoxy.

    So what happens is, when you shut the engine down, the fuel bowl is emptying from evaporation/leaking into the intake manifold. This results in an overly rich mixture, so the engine doesn't want to start, and the fuel bowl is empty. Commonly referred to as vapor lock, it can cause the symptoms you describe. Cars with A/C commonly were equipped with a fuel pump with a vapor return line to help keep the fuel cooler and return vapor to the fuel tank.

    If you don't know how hot your engine is running, you may want to install some gauges to find out. If your carburetor has not been rebuilt in a while, you may want to look into that as well.

    When was the last time you tuned up the engine? If you have a points ignition, have you checked point condition and dwell? Engine tune especially ignition system condition can also make some difference.
  5. RedSkyV8

    RedSkyV8 Well-Known Member

    I'm in agreement and I certainly understand how this is occurring? The carb was rebuilt about 24 months ago and I have less than 1K miles on the car since the rebuild.

    Would swapping out the thermostat, to a 160 degree thermostat, help to lower the overall temp of the motor, thus stopping or helping stop the heat soak/vapor lock issue? Would swapping the ignition over to a Petronix system help with these issues? What about wrapping the fuel supply line with a reflective or heat insulating tape to prevent the fuel from getting so hot?

    Thanks for the info!
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    All of those things could help. A 160* thermostat may lower the temperature as long as the rest of the system is good. If you have an old clogged radiator, it doesn't matter what thermostat you use, it will eventually run too hot. You should get some gauges so you know where you are at. The Pertronix could help if your points are worn and out of adjustment. Does your fuel pump have a vapor return line?
  7. RedSkyV8

    RedSkyV8 Well-Known Member

    To the best of my knowledge, the radiator is original to the car, but seems to be performing well. I am not certain about a vapor return line from the fuel pump. It looks like the fuel line comes in from the back of the car (rubber line?) and then goes out towards the carb (metal line) across the top of the motor.

    I have had some concerns about the water pump. It seems to be making more noise than it used to (kind of a rattle or clatter) when he engine is running), and I think that it (water pump) is original to the car as well.

    The car has the rally gauges instrument cluster, but the temperature gauge is not working or the sending unit on the top of the block is inoperative. However there is a Mr. Gasket brand aftermarket radiator cap on the radiator with a built-in temp gauge that I have been watching closely. When I shut off the motor, the coolant temp will rise from the high "green" area (about 160 degrees) to the "yellow" area, about 180 degrees plus, in a matter of a couple of minutes. I'm sure these temp readings are just approximate and not exact.

  8. RedSkyV8

    RedSkyV8 Well-Known Member

    This is the radiator cap that I was referring to:

    Attached Files:

  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Original radiator? It's probably done, but you really need to know where the engine is running. You need real gauges. The radiator cap gauge is not going to cut it. Sounds like you have the standard fuel pump without a vapor return. That will make the engine more prone to vapor lock. This is the standard pump,

    This is the one with the vapor return,

    You would have to run a line as well as change the sender unit in the tank.

    Grab a hold of the fan and try to wiggle the water pump pulley. The water pump bearing goes first, the pump makes noise, and then the seal goes. The water pump replacement would be an easy job if the small bolts didn't break, but they almost always do.
  10. RedSkyV8

    RedSkyV8 Well-Known Member

    As it turns out, my problem was a faulty ignition switch (which was intermittently shutting off the motor, hot and cold) and not a fuel line problem or fuel pump or fuel filter or carb problem. There was a new GM replacement part available locally, so the fix was relatively quick and easy - this issue should never come up for me again.

    Oh, and the water pump was bad too, so it got replaced as well. So, in my case, I had multiples issues interacting simultaneously, which made for a difficult diagnosis.

    Thanks to all who pushed me to think and rethink all the potential causes.

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