Correlation Fuel Level - Cat Temperature??? | FerrariChat
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Correlation Fuel Level - Cat Temperature???

Discussion in '308/328' started by wernerg, May 21, 2007.

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  1. wernerg

    wernerg Karting

    Jun 29, 2006
    50
    Danville CA
    Full Name:
    Werner Goertz
    #1 wernerg, May 21, 2007
    my '86 328 has shown three instances over the last 6 months where the "Slow Down" light came flickering on. This after very moderate (sub <4k RPM) driving in 80-degree weather. Visual inspection of the cat showed no redness or any other signs of excessive heat.

    I was thinking this is the classic electronics/sensor problem, rather than real overheat.

    Ironically, though, all three instances occurred after filling the gas tank more than half full. This might be a stupid question, but is there any reason why a full tank could lead to the Slow Down light coming on? E.g. engine running too fat and unburned excess fuel burning in the cat?
     
  2. TestShoot

    TestShoot F1 World Champ
    Silver Subscribed

    Sep 1, 2003
    12,288
    Beverly Hills
    #2 TestShoot, May 25, 2007
    weird and worthy of a bump. I can't figure out how the two would correlate unless it was a bizarre electrical short between guage and the sensor :/
     
  3. wernerg

    wernerg Karting

    Jun 29, 2006
    50
    Danville CA
    Full Name:
    Werner Goertz
    #3 wernerg, Jun 11, 2007
    OK - I must give it another try, and maybe be more specific: the problem occurs after pumping gas and driving, at very moderate levels (this side of 4k rpm) for about 3-4 miles.

    It happened again today - I bought 5 gallons of gas which brought the level to about 1/2 tank, and after a little bit of driving, the Slow Down light started flashing intermittently.

    Is it possible that unburnt fuel gets into the cat during the process of filling up, and combusts there to (temporarily) cause high temperatures? Oil and collant temps were showing low normal, btw.

    All hoses were replaced about 4 weeks ago by an independent in my area, whose reputation is beyond suspicion, so I am ruling out leaks in that area.
     
  4. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,934
    Outside Detroit
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    Don the 16th
    #4 don_xvi, Jun 12, 2007
    Sorry, I'm sure I'm in the same boat with a lot of readers here, I don't have any real good ideas.

    Here are two thought starters I'll throw out: (I don't believe either one!)
    1) A siphon is created in the fuel injection system when there's enough head pressure in the tank. This draws fuel up through the fuel distributor, through the injector(s) and into the cylinder(s). This could turn really bad by hydrolocking the engine!
    2) There's a ground hang-up between the fuel gauge and the TC circuits. Put in your 1/2 tank of gas and then disconnect the fuel level sender on the top of the tank (3 wires, note which one goes on which terminal and stick some tape on them to prevent shorts--easy enough to do at the gas station)
     
  5. FasterIsBetter

    FasterIsBetter F1 Veteran

    Jul 22, 2004
    5,855
    NoNJ/Jupiter FL
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    Steve W.
    #5 FasterIsBetter, Jun 12, 2007
    One other thought -- you may have an intermittent short in the wires of the cat sensor. When you fill the car up, you lower the rear suspension, possibly causing the wiring to rub on something or make contact that is doesn't make when the rear end is a bit lighter. Get the rear end up in the air and trace the wire for the sensors, make sure it's not being pinched somewhere back there, that it's not worn or broken.

    Pick yourself up an infra-red temperature sensor (they are relatively cheap now). Then run the car and see if the light comes on. If it does, immediately check the temperature of the two cats. If they are both the same, that's a pretty good indication that it's a problem with the sensor. If the that is flashing is significantly hotter, then you need to look at what might be causing that. But if I had to guess, I'd check the sensor and wiring first. Eliminate the easy explanations before you look to the complex ones.
     
  6. Brian Harper

    Brian Harper F1 Rookie
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    Feb 17, 2006
    4,078
    San Jose area
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    Brian Harper
    #6 Brian Harper, Jun 12, 2007
    I have a suggestion of where to look, but admit it is kind of unlikely. The gas tanks are vented to a charcoal cannister and the charcoal cannister is vented to the engine's air intake. Could you have an issue here where you are pumping excess fuel into the intake, enough to not get burned in the engine and turn on the cat light? I don't think it's likely, but at least it is easy to check - just disconnect the char cannister during fill up and look for fuel. Leave it off when you drive (perhaps pinching off the tube would be a good idea!) and see if the symptoms are still there.
     
  7. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,934
    Outside Detroit
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    Don the 16th
    #7 don_xvi, Jun 13, 2007
    Ooh! What if the charcoal cannister is missing (or bypassed) or doesn't have charcoal in it! Then its buffering action wouldn't be happening. Does it only happen after the fillup, or will it continue happening until the fuel level drops below 1/2 tank?
     

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