Slow down light question

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?

 

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.

 

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"...

 

Distributor cap - breakage, damage, etc. But I'm not sure an intermittent miss would cause a 'Slow Down' condition.

 

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.

 

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.