Slow down light question | FerrariChat

Slow down light question

Discussion in '308/328' started by jonesdds, Mar 10, 2008.

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  1. jonesdds

    jonesdds Formula 3

    Aug 31, 2006
    2,163
    SB,CA & Park City UT
    Full Name:
    Jeff
    1989 328 GTS. Did some spirited driving yesterday and met up with a friend so he could see the car and drive it-little nervous but he's a good driver. The car was fully warmed up and was running really well, no stumbles at all. Part way into his ride the cops show up on his tail so we drove a few miles, probably 5 at very low speeds and low revs. He did tend to shift up a bit too much so we were cruising at probably 40 mph in 4th so low rpms. Once the cop was gone he really pushed it for the last 1/2 mile or so to about 6K and the slow down light blipped on and off. Last time I had this happen it was running good as well but shortly after I started having problems and ended up having to replace a bad rotor. Plugs are new and the extenders and wires checked out good when the rotors were replaced.

    My question is would a slight blip in the slow down light be caused by the too low rev driving I described and then pushing the revs right after that-my thinking is unburned fuel due to too low of revs then into the cat, momentary heat up then OK. When we got out and check the cat it did not seem overly hot. I'm wondering if it just the driver's doing or if I've got something failing again.

    Jeff
     
  2. Bullfighter

    Bullfighter Two Time F1 World Champ
    Lifetime Rossa Owner

    Jan 26, 2005
    22,618
    Gates Mills, Ohio
    Full Name:
    Jon
    I'm not a tech, so I'll defer to any of our gurus who want to jump in here. That said, I've never had my Slow Down lights come on from the driving conditions you describe. My understanding of FI is that only the amount of fuel 'specified' by the throttle enters the cylinders. I think you'd need one or more cylinders missing to have fuel dumped into the exhaust.

    Do the caps look OK? :confused:
     
  3. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    Jon's reasoning is solid... pretty much gotta have unburned fuel dumping into the headers, then burning somewhere downstream, to elevate the exhaust temps to critical levels.

    Simply running lean or rich for short periods generally won't raise temps enough to trip the overheat sensors. More often than not, when the lights come on, the problem is a faulty crank sensor (or its wiring) --- causing sustained missing or late firing into the exhaust ports.

    Also, remember not to blindly trust sensors --- they go bad too, and can give false indicators. The cat overheat circuit is fairly complex ---- (2) thermocouples in the cats, a logic comparator circuit, power relays, reference ballasts, and the warning lamps themselves, plus all the wiring and connections --- lots of places for gremlins to hide...

    Did both lights come on or just one bank? Also, where were your water and oil temps?

    Also, the "little red box" that houses the comparator and relays is mounted behind the bulkhead in the passenger foot well. If a passenger kicks the footwell enough (happened to me once when I had a "scared" female companion along for a spirited ride), a connector can come loose / short --- which can trigger the lamps intermittently.
     
  4. jonesdds

    jonesdds Formula 3

    Aug 31, 2006
    2,163
    SB,CA & Park City UT
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    Jeff
    Mine's a 328-US model-so only one cat/one light. Came on for half a second only. Drove it again fairly spirited, high revs at times 15 minutes back home, no light. I'll check under the footwell for connection problems, come to think of it I might have been putting some pressure in the footwell-I'm not a good passenger. Last time I had it come on(when the rotor went bad awhile ago) it was running well and it was the same quick blip only. When it got worse and really was running bad, no light came on and when I shut it down the cat was glowing cherry. Why didn't it come on then? I'm thinking I've got a bad thermocouple or the ecu connection to it is bad possibly because of that past incidence.

    I've heard talk of using a laser temp guage to check the temp. At what temp is the cat too hot? Probably a good idea to really see what's going on.

    BTW, all temps at normal operating range.

    Thanks for the replys so far. Jon-what do you mean by caps-do you mean extenders?.

    Jeff
     
  5. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    Oh sorry -- didn't realize it was a 328...same applies, but of course you only have the single circuit...instead of two.

    If your fluid temps were normal, and the car is running well, you probably don't have a serious miss / ignition fault. More likely just a glitch in the temp sensor system. Since it ran (likely too rich) for a while with the ignition fault earlier, you may have a contaminated t-couple. The t-couples are not too sensitive to gunk coating, but you may want to remove it and use a scotch-brite to clean the surface. While you're at it, do the same with your O2 sensor --- don't use an abrasive on it though --- nylon brush it with brake cleaner or acetone (obviously, make sure the fluid is completely evaporated before re-installing into the cat).

    As for normal operating cat temp, it varies, but 800-1400F is typical for a properly functioning, fully warmed up engine. At these temps, a red / orange glow isn't too unusual --- "cherry red" is surely too hot. Remember that it looks a lot hotter than it is in the dark, so make your observations in the daylight...

    Since your cat got so hot previously and the light didn't come on, you may have cooked the t-couple --- hard to do, but it can happen.

    If you can find the overheat system relay ---- on a 328, I can't recall whether it is with the control box or in the fuse / relay panel?? (use your wiring diagram from the owner's manual), you may try replacing it as all these old-style Bosch relays tend to live short lives.

    As for diagnosing the t-couple, all you really can do is an open check. It should have continuity with some resistance (the value is not important). If it is fully open (infinite resistance), it is bad. Checking it out completely requires a voltage output test, which is as they say, "beyond the scope of this course"...
     
  6. Bullfighter

    Bullfighter Two Time F1 World Champ
    Lifetime Rossa Owner

    Jan 26, 2005
    22,618
    Gates Mills, Ohio
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    Jon
    Distributor cap - breakage, damage, etc. But I'm not sure an intermittent miss would cause a 'Slow Down' condition.
     
  7. jonesdds

    jonesdds Formula 3

    Aug 31, 2006
    2,163
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    Jeff
    Gotcha-all new rotors and caps 4 months ago. Thanks for the responses.

    Jeff
     
  8. jonesdds

    jonesdds Formula 3

    Aug 31, 2006
    2,163
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    Jeff
    thanks for the responses. Changed to hyperflow about 9 months ago and thermocouple looked clean then. O2 sensor 4 months new. I will plan on removing the thermocouple and cleaning and checking. As far as checking resistance of it, would I just check the length of the thermocouple or what I'm really asking is check resistance from where to where(not electrically inclined!). I've got the meter to check just how exactly should it be checked? I'll go ahead and replace the relay as well but they were recently checked and all check out(if it's in the fuses block).

    thanks again,

    Jeff
     
  9. Darolls

    Darolls F1 Veteran
    BANNED

    Jul 2, 2003
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    Sparky
    Don't worry about it! If the light starts to come on regularly and you notice loss of power, then you can get your nerves in a tizzy.

    May have been some excess petrol dumping into tha cat due to guick slowdown.
     
  10. jonesdds

    jonesdds Formula 3

    Aug 31, 2006
    2,163
    SB,CA & Park City UT
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    Jeff

    That was my thought but the way the light reacted to previous ignition problems was identical so I'm a little concerned that a problem will surface very soon. Tends to affect how far from home I venture and I'm planning a 200 mile round trip in the near future.

    Jeff
     
  11. finnerty

    finnerty F1 World Champ

    May 18, 2004
    10,406
    This could be significant...

    Since I'm a believer in looking at the simple causes first, and there was recent R&R with the exhaust system; it makes sense to look there. Double-check that the thermocouple is correctly installed into the new HyperFlow cat. And, examine it closely from the cat to where it enters the body tub to make sure that the wire leads did not get damaged during the install --- although they run in a braided steel sheath, the wires are pretty fragile, and even a small kink or nick can affect their voltage signal. The leads are supposed to run continuously to the brain box, but sometimes people add in-line connectors in the engine compartment ---- if your leads have any connectors, check them for integrity.
     

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