Slowdown Light (very specific)

Secondary cat delete meaning you have no cats other than the cats in the bypass? Or the cats in the bypass are deleted?

First thing I'd do, if you have the primary cats, is check the temp of them when the light comes on with a hand held high temp thermometer. If the bank that the light was coming on is significantly hotter than the other bank, you have an issue causing it to run rich. If they are the same temp or close to it (and not in the 900C+ range), then it's either the thermocouple or cat temp ecu, which you can troubleshoot by swapping the ecu's side to side and see if the problem moves or not.

Here is some more information on the operation of them:
https://www.the355.com/index.php/f355-buyer-s-guide/24-cat-ecu-s-explained

Wanted to add - if you don't have the primary cats, then you can unplug the ECU's and ignore the lights, they're not doing anything.

 

Unless, of course, you wanted to know if your car was running rich

So just the SD light, not the CEL?

Let's establish if your car has had any other modifications.
Would you know if you have a Capristo bypass valve solenoid and Capristo bypass valve? These looks different to the OEM solenoid. The OEM solenoid should normally look the same as your Secondary Air Solenoid Valve (behind the RH rear wheel arch). If you have an aftermarket Capristo valve and something breaks (no signal from the Motronic ECU or loss of vacuum), the valve opens. Normally Ferrari valves won't stay open unless they have been lockwired open (or someone has messed around with the vacuum tubing to keep a constant supply of vacuum to the valve). Look for lockwire. The vacuum tubing mods are probably a lot rarer than lockwire.

Does your car have an OBD2 port?

 

If no cats, the exhaust won't get hot enough to trigger a slow down, so they won't be doing anything. In a rich condition on na motor, the exhaust temps would be lower, not higher, pre cat.

 

Good point. I do vaguely recall reading somewhere that pre-cat and post-cat temperatures are opposite under rich conditions.

You can buy or make jumper plugs to keep the ECU happy (The ECU still seems to know you have disconnected the thermocouples).

https://technistrada.com/dummy-load-sdl-bypass---not-for-installation-into-vehicles-equipped-with-catalytic-converters

 

Let's have more description.

1. Do you have main cats (L and R)
2. What exactly is secondary cat delete? Is the Y pipe simply rodded out, or something else installed in its place?
3. What year is your car, is it OBD1 or OBD2?
4. Did you read the error codes? There surely is one or two.
5. What if any modification was done to the bypass valve system? Is it Ferrari, or Capristo, or something else?

 

Either you have a faulty solenoid valve or someone has rerouted the vacuum plumbing.

Here is the standard plumbing.

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In theory, you could join up 35 and 11 and bypass the bypass valve solenoid. This would create what you are experiencing. Note that 11 is the source of vacuum from your vacuum tank.

Well, there are four cats, but two of them are the small ones in the bypass exhaust path.... unless you have custom bypass plumbing.

If you disconnect your thermocouple ECUs, to stop warning lights appearing, just buy a simple dummy resistor kit from Technistrada

https://technistrada.com/dummy-load-sdl-bypass---not-for-installation-into-vehicles-equipped-with-catalytic-converters

They simulate a normal safe cat temperature. You can make these "dummy loads" yourself, but it is easier to buy them than buying the components and assembling them. It's probably cheaper to buy them, too, especially if you have to buy the tools required to make them.

 

What colour is the connector on your exhaust bypass solenoid valve?

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I don't think the plugs are swapped over, but it would be good to confirm that the blue plug is on the right.

 

Confirm that you have Ferrari bypass valve, normally closed.
The Capristo bypass valve is normally ... open.

 

If the main cats are deleted then the cat temp sensors aren't doing anything, so you either have a faulty thermocouple (not so common) or bad cat temp ecu (common) or bad contacts (also fairly common). You can swap thermocouples side to side to see if the problem moves, if it does it's a bad thermocouple or connector on the thermocouple, if it doesn't it's a bad cat temp ecu or connector on the temp ecu. If it's a bad thermocouple you can replace with a resistor to simulate good cat temps, replace the thermocouple, or just unplug the cat temp ecu's and ignore the dash lights. If the cat ecu is bad you can either replace it, try to fix it (which is usually doable but a bit of pain as it's a sealed unit that needs to be opened) or just unplug it and ignore the lights. You can also remove the lights from the dash and forget about it altogether.

I would go through the pretty easy process of finding out what's bad and labelling it/marking it down (or posting here) for future reference in case you want to put cats on or sell the car so the new owner knows what is necessary to fix the system or return it to stock if they want to. I ended up putting cats back on my car as I hated the cold idle smell, and it'd make my garage stink a bit, so I'm glad I never disabled the system. My thermocouples were still attached just sitting in the engine bay tied out of the way, so adding cats back in with a functioning cat temp monitoring system was easy.

I think if you're after performance, gutting the secondary cats and leaving the primary is the better method as when you're getting on the car hard it uses the bypass anyway, so making that the clear path will have minimal impact on emissions and still give the added performance (and give better sound as you'll get more exhaust through the bypass due to lower back pressure). A lot of the primary cats were deleted due to the factory units having a fragile ceramic element that would break up and potentially hurt the engine, the factory replacements are now metallic core and the aftermarket available units are also metallic core, so not really much reason to delete them anymore (imo).

 

You cannot ignore the SDL on a 355. It will eventually shut down the offending bank. You either have to fix it, or fake it.

 

The Thermocouples generate a small DC current (voltage) when heated up and this voltage is read by the Temp ECU, amplified 100 times, and sent to the engine ECU. The Thermocouple produces about 1 mV at 25C and about 37 mV at the exhaust temperature that calls for blinking SDL. A resistor cannot be used for any simulation here.

 

Shrug, I unplugged the cat temp ecus which made the lights come on but never shut anything down... Maybe it's different on 5.2 cars?

 

Same, never an issue on the F355 but on my F348 it would slow the Car Down, happened to me on the Track just when I was getting into the Grove

 

Those Technistrada modules simulate a good thermocouple and good thermocouple ECU. You can toss these Ferrari devices in the trash bin after you fit the Technistra modules. They are simply a diode and resistor in series. They will last the lifetime of your car.

No hidden ECU faults, no lights.

The diode and resistor combo does work, Miro, at least on the F355. I've made the dummy loads and used them temporarily until I fitted a new engine harness (I had bigger issues).

 

https://www.ferrarichat.com/forum/posts/145451778/

Original design by @wbt (and now sold by him)

 

From the WSM on a 95.

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Yes, it looks stock. The angle of the vacuum pipe on the capsule looks stock. I don't think the capsule is your issue.

Did you find the solenoid valve at the other end of the metal tube? This diagram from the parts manual:

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The Workshop Manual doesn't show the proper routing of the pipes, but I've marked the OEM Bypass Solenoid in this diagram:


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Hard to say what’s going in here. Double check that you have hooked up the thermocouples and ECUs to the correct wiring harnesses. Can you see the wire colours on the ECUs?

The Technistrada bypass plugs will only work for the main cats, not the exhaust bypass (in case you didn’t know).