355 355 5.2 slow down and check engine light when reaching certain RPM

Lots of experts on here so await more detailed and intelligent responses than mine But FWIW and being a 355 2.7 owner who also experienced both "real" slow-down/check engine lights and "limp mode" and also "ghost" slow-down/check engine lights what you are experiencing is most likely due to a cat heat issue triggering the ECU protection sensors that warn you and/or shut down the bank of the engine to avoid damage. These can be finnicky and various things can trigger them. If you have original cats I would check to make sure the ceramic linings are not loose/collapsing - that can cause the cats to get too hot and result in what you are describing. Since you have already replaced the spark plugs that would be the next area to look at and diagnose. These conditions usually throw OBD codes/errors as well so either obtaining an OBD diag tool or having your shop/dealer do it will usually help with the diagnosis.

Again - Not an expert by any means - and there a lot of experts on here - so see what else comes back but this is at least a start.

Cheers and welcome to 355 ownership

 

I believe it should but could be some variance amongst OBD devices - some may have phantom power and some may not (unsure so don't take this for gospel) - I had a thought - there is a lengthy and very comprehensive thread in the 355 forum entitled "355 Stuttering" - the OP was also experiencing somewhat of the same things you are describing and the thread goes DEEP into OBD diag tools/connections/potential causes and solutions etc. - I weighed in a couple of times but it was obvious the OP was very comfortable from a DIY standpoint and the other FChat members who chimed in were much more knowledgeable and helpful than I was. You might take a read through that thread and perhaps pick up some things - not suggesting collapsing threads because your symptoms are a bit different but potentially the same cause or potentially can derive some additional answers to the diagnosis steps and OBD tools - just a suggestion.

Cheers

 

You will still need power for the OBD scanner ... and an earth. That is what the red and black leads (with crocodile clips) are for.

I used a cigarette lighter adaptor to power mine. Something like this:

https://www.jaycar.com.au/cigarette-lighter-cable-with-bare-ends/p/PP1995

Just make sure you don't short out the two wires.


My lighter only works when the ignition is on, but you need the ignition on anyway (and, importantly, the immobiliser disarmed) for the Motronic ECU to power up properly.

 

Because you have a SLOW DOWN light, the problem is related to cat temperature (whether real temperatures or improperly sensed temperatures).

If you suspect moisture is, say, causing your Thermocouple ECUs to malfunction and cause your SLOW DOWN issue, you can fabricate a Thermocouple ECU "bypass" with a resistor and a zener diode... or even a 1.5 battery). But first check to see if your OBD scanner pinpoints which bank the problem is on.

Here are instructions for fabricating a Thermocouple ECU bypass

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

This is only for faultfinding in the short term, however. Your cat/s may be overheating for real.

The engine doesn't need to be running to get fault codes.

 

See @bennzwel - Told you there were some real experts on here

 

This is assuming the cat itself is causing the overheat. You could have, for example, have an intermittent injection/ignition problem causing a real cat overheat .

Irrespective, I should have specified that what I said was more of a general statement on SLOW DOWN lights.

 

Thanks for the update, but I can't see how a faulty battery charger would cause a problem with an ECU which would disappear 40 minutes after disconnecting the battery on a 5.2 car.

The ECU doesn't like overvoltage/undervoltage, but I don't think the ECU would care if the ignition was not currently turned on.

Did you get your OBD2 port to operate? Maybe that would offer some further clues.

Humidity may be be an issue.

 

Understood, but the ECUs probably won't be doing anything at this time other than preserving volatile memory (fault and learned parameters, depending on the Motronic version).

Were there no errors because you disconnected the battery?

 

If you think it’s in the wiring and has something to do with weather and it’s not show signs at this time you can try spraying a mixture of water and ammonia in small areas around the engine compartment then drive the car and see if something happens but doing small areas you can then narrow down the area




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If your indie understands the sdl function it will be an easy fix likely.
It is working as designed and your bypass is not functioning correctly
Many many causes but all easy to fix if they understand

 

You could start by telling us about any mods to the engine
If there are none then likely your cats on bypass need replacing or the thermo system or components on bypass needs replacing