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Slow Down Light From New Fuel

Discussion in '308/328' started by hanknum, Dec 10, 2009.

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  1. hanknum

    hanknum Formula 3

    Nov 1, 2003
    2,050
    Santa Barbara
    Full Name:
    Henry
    #1 hanknum, Dec 10, 2009
    Yesterday I drove down to LA from Ventura (50 miles) to go to the LA Autoshow. Everything was fine on the way down. When I left to come home, I went to a gas station (Unocal) and filled up with 91 (I almost always go with 89). On the way home one of the "slow down" lights would flicker (sometimes stay on for 3-4 seconds, other times just flash). I know that this means that one of the cats is overheating. But, it just seemed random, almost like a wire was making contact. Can a bad batch of gas cause this? One thing though, I haven't driven the 308 at night in quite some time. Maybe the light has been flashing during the day and I haven't noticed. What do you guys think? I'm going to pull the cat this weekend and take a look.
     
  2. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    #2 Steve Magnusson, Dec 10, 2009
    Last edited: Dec 10, 2009
    Can also just be a flaky warning light ECU (and "random" fits that picture). Try swapping the two ECUs first (which also gives you a chance to reseat the connections) -- if the flickering goes to the other light = bad ecu; if the flickering stays on the same light = reason to dig deeper into other things on that side. Also, watch your self-tests carefully -- if it isn't working perfectly every time, that's not a good sign for the corresponding ECU either (i.e., no light at all during the self-test doesn't look bad, but it is not a "pass").
     
  3. FasterIsBetter

    FasterIsBetter F1 Veteran

    Jul 22, 2004
    5,855
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    Steve W.
    #3 FasterIsBetter, Dec 10, 2009
    Hank,

    When I got my '78 GTS, I had a similar problem with the slow down light. It flickered and sometime just stayed on steady. I thought it was the CATS overheating, but it turned out to be a broken wire to the sensor that was shorting out. When you poke around under there, you'll see that the wires are fairly exposed. You might want to start there and see if that shows any possible problems. Sometimes it really is just something as simple as that.

    Steve
     
  4. hanknum

    hanknum Formula 3

    Nov 1, 2003
    2,050
    Santa Barbara
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    Henry
    #4 hanknum, Dec 10, 2009
    Thanks Steve(s).

    Great idea about the ECU...you're talking about the ones under the passenger's foot panel, right?

    Yeah, I thought about the wiring...I just had a new exhaust system made up about 2 months ago and maybe they guy did something under there. I'll check it out.

    Thanks again guys.

    Henry
     
  5. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,786
    30°30'40" N 97°35'41" W (Texas)
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    #5 Steve Magnusson, Dec 10, 2009
    Yes
     
  6. hanknum

    hanknum Formula 3

    Nov 1, 2003
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    Henry
    #6 hanknum, Dec 10, 2009
    What do you mean by "self test"? How is this performed?
     
  7. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    26,786
    30°30'40" N 97°35'41" W (Texas)
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    Steve Magnusson
    #7 Steve Magnusson, Dec 10, 2009
    Last edited: Dec 10, 2009
    See the descriptions given on page 25 for items 84 & 85 in your 150/78 OM -- every time you turn the ignition key from "off" to "on", the warning lights in the dash should come "on" for a second or two and then go out. A warning light that wrongly stays "on" after this self-test easily gets noticed, but if the warning light never comes "on" that is also a self-test failure.
     
  8. hanknum

    hanknum Formula 3

    Nov 1, 2003
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    #8 hanknum, Dec 11, 2009
    Thanks Steve, the self test does operate correctly. I'm going to take a look under the car and also switch the ECUs. If I don't get any answers from that then, I'm going to fill up from my regular place ans go from there.
     
  9. Doug n Terri

    Doug n Terri Rookie

    Mar 12, 2004
    21
    Thousand Oaks, CA
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    Doug Anderson
    #9 Doug n Terri, Dec 13, 2009
    OK - um, slow down light - why is it there in the first place? It's a real rudimentary cat saver. Huh? Yeah. What kills a cat faster than anything? An engine miss. A perfect air/fuel mixture touches off in the cat - hot hot hot. Now I am not saying your car has an ignition miss – but that’s what those little thermal gizmos are there for.

    OBD II – 1996 and newer – has a VERY sophisticated engine miss system that can actually sense an inconsistent crankshaft frequency (in-between spark firings!!) and sets the check engine light (or MIL – malfunction indicator light) BLINKING. The worst of the worst. Stop now! It is trying to say. Yer gonna melt the cat substrate like a blowtorch to honeycomb.

    Anyway – our 78’s system is REAL prehistoric. BUT it works – baring opens, shorts or grounds. Ahem. So the previous advice is very good – check this wire system out and determine if there is a short - - - or do you have an engine miss caused by gunk in a carb?

    Just wondering. Keep us posted. What did you find with your cats this weekend?
     
  10. hanknum

    hanknum Formula 3

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    Henry
    #10 hanknum, Dec 18, 2009
    OK, so I took a look at the exhaust system and the thermocouple wires and all seemed good. I decided to put the belt on the air pump back on and see what happens (I thought it might actually run hotter due to more O2, but I just wanted to see). Started up and ran for about 2 miles then light came on and stayed on, oh crap. I pulled over and pulled the belt off. The light stayed on even after sitting for about half hour. When I got home I removed the cats and inspected them both. No melted substrate, they both seem fine. Reinstalled cats and turned on the ign. The slow down light now just stays on all the time (even after the self test AND after sitting overnight). I think there is a system failure now. I have an Electromotive ign and don't think I have a misfire.

    Could overheating the cat cause it to fail? I don't think so, I think this was just a coincidence.

    Cylinders 5-8 are the front cylinders, right?
     
  11. hanknum

    hanknum Formula 3

    Nov 1, 2003
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    #11 hanknum, Dec 21, 2009
    Thanks Doug. Are you still in Moorpark?

    As I mentioned in my previous post, I've got an Electromotive and don't think I have a miss at all. Both cats looked fine after I inspected them. Don't know if the cat was actually overheating or if I got some bad fuel or if it did overheat and killed the probe/sensor (doubt it).

    I'll swap ecu's to see if that is the prob. Cyl 5-8 is the front bank right?
     
  12. hanknum

    hanknum Formula 3

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    Henry
    #12 hanknum, Dec 23, 2009
    OK, I figured it out. It appears the probe/thermocouple died. I checked the resistance of both and the good one read abouth 8 ohms cold and the other was open. I'm going to try and trace the whole thing to see if it got cut/damaged somewhere.

    Anyone know where to get one of these probes?

    Oh, and 5-8 cyl. are the front bank.
     
  13. hanknum

    hanknum Formula 3

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    #13 hanknum, Dec 29, 2009
    Last edited by a moderator: Sep 7, 2017

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