Slow Down 5/8...?

Either your catalytic converter is getting too hot, or the Cat ECU has failed and is sending erroneous messages (and shutting down the 5/8 bank of cylinders)

If your cats are glowing red hot, then indeed there's a more serious problem. Cats could be blocked or something upstream could be messed up. Did this occur in the middle of a hard drive, or when the car was stone cold?

Likely, the cat ECU has failed. What year is your car?

142581: http://www.ricambiamerica.com/product_info.php?products_id=261104

 

Your cats will heat up (eventually catching fire if the situation is never remedied) quite hot if raw fuel reaches them in quantity...this can happen with a failed spark plug, failed spark plug wire, timing problem, etc.

Blocked cats (Ferrari cats eventually implode/erode over time) will also heat up quite hot.

It's generally accepted to swap the left side and ride side exhaust ecus on 348's...if the Slow Down light switches banks, then the problem is typically a failed exhaust ecu (water evntually seeps in and destroys them).

However, if the Slow Down light remains on the same side, then you quite likely have a real fuel/cat problem or a grounding issue with the thermo sensor on that side.

 

Then I'd treat it as a real fuel/air problem.

However, you can first measure with an ohmeter the resistance between the two wires that connect to the thermo sensor.

A faulty thermo sensor or shorted wires would probably yield 10 ohms or less when the cats are cold.

The Ferrari system is normally expecting 350 ohms between those two wires up to 800 ohms.

When the resistance falls below 350 ohms, the exhaust ecu reports an overheated cat, but something less than 10 ohms would probably be a fault/short instead.

So after measuring the resistance when cold to rule out having a thermo/wiring fault/short, I'd start looking at fuel/air problems (a wideband exhaust sniffer would help here).

Do any of your spark plugs appear fouled? Notice any misses? Backfires?

I'd suggest switching coils from left side to right side to see if the problem switches sides with a bad coil, if not, I'd try swapping the left bank of 4 spark plug wires with the right bank of 4 wires to see if the problem switches sides...which would identify a bad spark plug wire.

Measure the resistance under the MAF cable on the male MAF #1 and #6 pins (extreme ends of the cable connection, but on the MAF itself) with the engine off. You should see identical resistance values on left and right sides (for reference, the factory pre-set value is 383 ohms).

You can then swap MAFs from left side to right to see if the problem switches sides in order to identify a bad MAF.

If none of the above helps, then the cheap thing to do would be to swap cats from left to right side to identify a failed/clogged cat (if the slow down light swaps sides then, of course).

 

You mean the same problem on the same bank of cylinders, right?

 

Angelo, please don't take offense at this, but I'm wondering if it's certain that you know what the exhaust ECU's are, since it doesn't spell it out in this thread. We're not talking about the Motronic ECU's behind the seats, we're talking about the exhaust ECU's that are mounted near the tops of the rear shocks. These are so notorious for failures that they are ordinarily the first place to look. If you definately swapped the exhaust ECU's and the warning light still appears on the same side, then you need to find out what is causing the cat to overheat.

 

Wow! Good catch, Miltonian.

 

Using my famous "instructional arrow", here is a picture showing the exhaust ECU location for the left bank. The ECU for the right bank is in the same position, on the right side. The Ferrari part number is 142581, the same for each side. They are very easy to swap from side to side. If the exhaust ECU is causing your problem, once you swap them from side to side you will now get a warning light for "slow down 1/4".
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And this is what they look like. This is a bad one that I replaced several years ago, when my 348 was showing the same symptoms that yours is showing now. There was nothing wrong with my engine or my catalysts, just the ECU's. The problem has not returned since I replaced the ECU's.

Edit: Oops, I forgot that Daniel that already posted a link to show what the ECU looks like.
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The famous pool stick has returned.

 

Is there any evidence that the cats are overheating? Glowing red? Bad smell?

If not, disconnect the wires from both exhuast ECU's and take it for another drive. The warning lights shouldn't come on any more, and if the ECU's were signalling the Motronics to shut down a bank, that shouldn't happen any longer.

It's entirely possible that both of your exhaust ECU's are malfunctioning and sending false signals to the Motronics. But be careful to keep an eye on your cats, they will get really, really hot if there is an actual problem, and bypassing the exhaust ECU's won't solve that particular problem!

 

Exercise caution.

Use an ohmeter to measure the resistance coming from the thermo sensor. This is easy to do when the engine and cats are cold.

Disconnect the thermo sensor wires from each exhaust ecu and tape them where you can safely measure resistance again when the cats are hot. This will trigger CEL warnings, but you need to know if your cats are overheating or if faulty warnings are being reported.

NOTE: if your cats are glowing a dim red, stop this and probably all testing.

 

Well since you got the light again on the same side, try switching the thermocouples from side to side. Those are the wires that are coming out of the catalytic converters, not the O2 sensors, but the thermoucouples that are screwed directly into the cat. When you unplugged the cat ecu's, follow the wire that you unplugged back to the thermocouple. If the exhaust ecu's are working fine then it could be a bad thermocouple.

 

Hey Ernie, Miltonian...would it make sense to unplug the thermo sensor wires, while otherwise leaving the exhaust ECU's plugged in...such that a faulty exhaust ecu would still report a slow down light?

I mean, would that tell you that you've got a bad exhaust ECU if it lights up the SLOW DOWN light without the thermo sensor being connected?

 

You mean pull the thermocouple and put a cap nut on the bung? If the ECU's suck, they suck.

Although I don't recommend anyone follow my back-country advice, I run Hyper Flow cats in my car with no ECUs and the bung capped off. I figure if things fail so miserably that a HyperFlow cat catches on fire, it's time for a new car anyway.

 

Well you better let it burn to the ground then, and make sure your full coverage insurance is paid up to date.

 

Mon Dieu! Mais bien sur.