355 loss of power and slow down light

Sounds electrical.

 

Do you have a 2.7 or 5.2 (i.e. two Slow Down lights or one)

 

I'm assuming the light is coming on because of a false overtemperature signal being sent to your ECU. I don't know if a bad connection triggered by vibration can cause the thermocouple or Thermocouple ECU to generate a light, but the internal electronics have been known to break down in the Thermocouple ECUs.

What colour is the potting compound on your Thermocouple ECUs? Green or black. Your car should have 3 TCUs. Two on the right and one on the left.

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Left TCU with green potting compound.
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You may have to invest in a cheap OBD scanner to tell you which one is faulty. Otherwise, you'd have to either buy a new one and swap it out with the old ones (one by one) or fabricate a dummy TCU from a resistor and zener diode. The dummy ECU bypasses both the thermocouple and Thermocouple ECU.

Note.... Aftermarket, cheaper than OEM TCUs are available from Technistrada (New Zealand)

Of course, I'm assuming your cats are in good condition. Broken bits of catalyst may be rattling around, periodically causing real overtemperature.



By the way, is your Check Engine Light working? Does it come on at engine start?

 

More info on dummy Thermocouple ECUs..

https://www.ferrarichat.com/forum/threads/simple-2-7-slow-down-ecu-replacement-only-for-no-cats.538719/

A useful faultfinding tool for cars with CATs

 

Bad (loss of) connection of a Thermocouple to the TCU is interpreted as if the EGT is too high and will cause SDL and bank shut-down.

 

I had a similar problem earlier this year. I would get a momentary flicker of both the SDL and CEL accompanied by a violent, but momentary, loss of power. It happened so briefly that it didn't set the CEL.

It was a cracked connector to the crank sensor. Vibration at various RPM or bumps would cause a loss of signal. "Pending" P0336 trouble code gave it away immediately. Replaced sensor and no further problems.

 

It should be in the same general area under the secondary air pump. See the graphic I posted earlier (item G is shown twice). I don't know if your Capristo installation deletes the exhaust bypass valve Thermocouple ECU. I think you would need some kind of Motronic ECU software update to do that, though. A few folks have tried to put a dummy TCU in that position, but still get warning lights. The logic in the Motronic ECU processes the signals from exhaust bypass Thermocouple ECU differently from the CAT Thermocouple ECU signals.

Or did this warning light only appear after the installation of the Capristo?

It looks like your car has been driven in the rain.... and I don't like the way the potting compound is pulling away from the case.

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Your potting compound seems to be the better green colour, but these green ones still do fail. Anyway, clean the pins on the plugs with non-residue electrical spray (not WD40). Let dry and reinstall the plugs.

If you can't find that 3rd Thermocouple ECU, see what the single ECU is attached to... The exhaust bypass or the RH Cat.

I have seen a workshop manual graphic showing the bypass valve thermocouple wired directly to the Motronic ECU, but no one seems to have that configuration.

 

If you do find the 3rd Thermocouple ECU... and the wiring harnesses are long enough on the RH side, try swapping over both plugs on those RH ECUs (it may be slightly easier than swapping over the TCUs). If the RH Cat Thermocouple ECU is causing the issue, you may solve the bank shutdown issue, but still get warning light/s during a drive.

How easy is the problem to reproduce during a bumpy drive?

 

If the plug cleaning doesn't work, for faultfinding, unless you haven't got a soldering iron to string a diode and resistor together, I would be buying a dummy TCU from Technistrada or even one of their Thermocouple ECUs.

https://technistrada.com/

They are a different size to the OEM ECUs, and lighter, but unless you are stickler for originality, it won't make a difference.

 

Replace that TCU with the lifting epoxy.

 

Just tested this on my own car ('98 5.2). I pulled the two pin plug on the left TC ECU and started the engine. After about 5 seconds the engine tone changed (although I was surprised at how relatively smoothly the engine ran... just a little imbalance at idle). I turned it off a few seconds later in case I broke something. I forgot to check which lights were on. I reset the plug and all was ok.

EDIT: Unfortunately, I can't see how this can be used as a faultfinding tool where the car is only fitted with one SD light on the dash. I repeated the exercise with both sides pulled, but the engine still ran on one bank. I turned off the engine, reset the plugs and restarted the car, but the SD light was now flashing. I pulled and reset the plugs again but still no joy. I don't know if this was a glitch, but I used my scanner to clear it and the light disappeared. My previous single plug test didn't generate an SD light after reconnecting the plug and restarting the car. Maybe it was a case of two strikes and you get a permanent flashing light, even if you put everything back to normal?