‘90 348- intermittent power loss. Like flipping an on/off switch.

I have the identical problem. I’ll be driving and then suddenly that engine note changes and there’s very little power. Then, as fast as it comes it goes. No lights.

 

I just realized that my 5/8 SDL light doesn’t illuminate at startup. Maybe bulb? My plan is to get those ECUs replaced and do an ignition overhaul. All stuff you’ve done. BTW, I have two brand new fuel pumps so that’s not it.

 

Dunno. Shop did it.

 

If I recall, manual says symptoms are as described then the 2 sides of the throttle linkage throws are not completely synched, it outlines the procedure in the manual.

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I keep saying it, and keep finding evidence in my own 348, RELAYS.
Cheap and easy to replace!!
I have had several "phantom" issues with mine which have gone away after replacing several relays relating to different functions.

Agreed that it may not be that but it sure is easy enough to replace the 30+ year old electro-mechanical devices which do not last forever and are often overlooked.

There are injector relays, fuel pump relays and so forth.....
I am actually just going to go and buy a pile of new relays and replace them all!!
I moved a few non critical ones to replace critical ones to find my problems went away......so new ones all around it is!!

 

I have a post on relays in case it's needed. Most of the original relays were not sealed and outside was metal and corrded from air exposure. God knows what was inside they are basically open to air.


https://www.ferrarichat.com/forum/threads/footpad-fuse-panel-relays.594744/

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Ok Just re-read the manual on this, the throttle synching affects throttle seizing or idle instability not mention of high rpm. I swear I read that somewhere but can't find it so disregard.

In that post link you can see the aluminum relays, 30 years is WAY past their service life. I'd start there, I did that first thing myself. Bit of a pain finding them that are not chinese knockoffs btw. Easy to find cheap ones, hard to find the good ones.

There are relays for fuel pumps and for injectors btw, (your post only mentions injector relays) as well as fuses. I replaced all the fuses too.

 

I pull these after the car has sat a few months to prime the oil system to build pressure before firing up. So I happen to have it color coded.


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Sounds like a signal thing not a mechanical thing. Its hard on these cars when there is no CEL's but you can also try the pin out box that fits between the ECU and plug to check the components voltage/resistance. Those are listed in the manual as well.

In front of the oil filter is a coupling that links both sides of the ECU together worth checking of its clean and not contaminated with oil etc, also cam position sensor is possible as is TPS.

Things like TPS and cam sensor are so cheap its costs less to replace it than it does to spend time to test it. You can also check TPS in place with an intercept harness like m.stojanovic showed me that I did. Used them to reset the TPS to idle condition when I reset the with throttle plates then tested and linkage throw and various % opening per the manual..

 

Maybe just disconnect the cat ecus one at a time to prove a point.

 

Did you try changing one/both throttle position sensors? You said you only cleaned the connections.

Does the 348 only use the TPS’s at idle and at wide throttle openings (like the F355)?

I was thinking perhaps there is a dead spot on one of the TPS’s and pushing further gets past the dead spot.

 

I forgot that the TPS on the Motronic 2.5 is actually switch, not a potentiometer. Could the switch be intermittently failing?

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They seem to be horribly expensive. At least there is only one of them on the 2.5.

 

On the 2.7 the manual has adjustment at idle10,30,80% opening where they have to be synched within a spec. Doubt it's just for idle.

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Is your car a 2.5 or 2.7? A 2.7 car has 2 TPS and they are pretty cheap if you search for the Bosch part number not the Ferrari part number, you have to set them to spec when replacing them its in the manual its pretty simple to do. Those pics above are me synching the TPS and throttle linkage. Thinking is not the TPS though. Still feels electrical like wiring issue or intermittent connection somewhere in the harness. I don't think its a component. Crank sensors, fuel pumps, fpr, igniters and coils, plug wires are the most likely suspects but you replaced those.

Personally I'd put your MAF's back to where they were and not 383 unless your positive someone messed with them previously.

These are handy for diagnosing wiring/component issues. Unit on the left is for the ECU's #95970072,the manual has the values and the pins for various components shown below a small section. You can do much of it without those boxes its just more of a pain counting pins etc. Diagrams below should help for that.

Really need the diagnostic sheet to be thorough, hard to find hem but gets one going regardless.

 

Diagnostic sheet
https://www.ferrarichat.com/forum/attachments/348motronicdiagnostic-pdf.1876343/

 

Not sure where your located, I own those diagnostic boxes.