Ferrari Mondial 1984 Misfire

i have a problem which is driving me crazy lately,
the car idle excellent
runs excellent
but between time to time it start backfiring for 15 seconds and then back to normal.
did anyone had this issue before and can help

 

Does it start well, both cold start and hot start?

If so, and the car is generally running well most of the time, it would seem to be ignition related. Some intermittent spark issue is causing unburned fuel to be dumped and causes the backfire.

First I would check to make sure all the wiring connections to the coils, and fuse box are all snug. Same with the spark plug wires connections. Need to make sure nothing ignition related has jiggled loose.

Some items that cause spark issues...Are the spark plugs new and/or gapped correctly? The spark plug extenders sometimes burn through, you can usually tell by inspection of each one for a black arcing trace. The distributor caps and rotors are also not changed as frequently on these cars as on normal cars as they are expensive, so that can cause some intermittent spark misfires.

In general one would want to rule out various ignition issues.

There are also fueling issues that can present symptoms that mimic spark problems, but if your car starts and runs well most of the time, fueling issues usually present under more operating conditions.

 

If it does this during idling, does the engine idle RPM drop significantly? I'm with moysiuan -- seems like a bank of ignition going and coming back is the most likely suspect to rule in, or out, first.

 

I was reading some of the other threads, and it seems that i missed checking the Frequency Valve Relay...

 

Are you saying that you confirmed the frequency valve was not vibrating/buzzing when the problem occurred, but was vibrating/buzzing when it ran OK?

 

FV is working perfectly,
now checking on the Sensors, there are 3 sensors.
- when disconnecting 1-4 sensor bank the car shut down
- when disconnecting 5 - 8 sensor car works on one bank

we removed the sensors and they where dirty we cleaned them and checked them by magnet they are both working.. tomorrow the car is going to an electrician to check all the wiring in the engine bay and fuse box

 

Sort of glad that that is the case as "backfiring" has never been a reported symptom of the protection relay (and, therefore, the frequency valve) not working. Good hunting with your search -- you'll be lucky if it's just a loose/flaky connection, and unlucky if it's one of the Digiplex ECUs getting flaky. One thing that might help you determine if it is one of the Digiplex ECUs is to watch the behavior of the tach (the tach signal comes from just one of the Digiplex ECU):

1. If the tach dies completely when the problem occurs = try swapping the connectors to the Digiplex ECUs. If the tach then doesn't die completely when the problem occurs = bad sign for one of the Digiplex ECUs.

2. If the tach doesn't die completely when the problem occurs (maybe just reads lower and gets a little erratic) = try swapping the connectors to the Digiplex. If the tach then does die completely when the problem occurs = bad sign for one of the Digiplex ECUs.

3. If the tach works the same before and after swapping the Digiplex ECUs when the problem occurs = problem probably elsewhere.

 

but did it ever occurred if crank sensor is faulty , it works sometimes and it disconnect for a while and work again?

 

Yes, that's also a possibility (and can give the same symptom as a bad Digiplex ECU). The strategy there is to connect an AC voltmeter to the two pins of the TDC sensor where it connects to its corresponding Digiplex ECU -- when the engine is running, there should be a small AC voltage present there (not 0 V AC). There's a decent schematic of the ignition system in Section 3 of the Mondial QV OM to determine which pins (if you don't have access to the wiring diagram).

 

i will check them tomorrow , and check all the wiring between the ECU and the crank sensor

 

just an update,, we found out that both Rotar Arms where leaking electricity when they were hot and that was the cause of the misfire sometimes.
Changed both of them and the car is Healthy now
thank you all for your support

 

Glad it was something simple!

There is no reason these cars can't run perfectly well, especially in cars that have been driven regularly.

 

Hi Guys, .. am suffering from the constant failure from the rotars arms,, i change to new the car behave amazing for a week and then have the same problem. i change the rotar arm and the car runs amazingly again.
do you have any idea what makes it fail...

 

Could it be possible that the "new" dist rotors you are getting are actual the older version of the rotor that had the known problem of the arcing from the carbon button contact plate to the top of the distributor shaft? Have you looked at pictures of the newer rotor on some of the F part supplier websites to compare (my foggy recollection is that they changed the plastic material and it may be a different color)?

This thread has some prior discussion (and has a link to other threads with photos of other failed rotors for comparison to your failure):

https://www.ferrarichat.com/forum/threads/ignition-problem-on-308-qv.642903/#post-147929536

 

do you have by any chance the part# for the modified rotar arm

 

I'm not sure that there was a Ferrari part number change as I'm having a recollection that Rifledriver (Brian Crall) mentioned that Marelli changed the rotor's plastic material to address the problem, but never informed Ferrari that there even was a problem. I'd be 99% sure that this black material 124288 one from Maranello Concessionaries would be an updated material (as the "bad" ones were a whitish or clearish material):

https://www.ferrariparts.co.uk/part/ferrari/124288

Are you seeing arcing between the top of the dist shaft and the metal piece of the dist rotor where the carbon button touches?

 

Yes am seeing that, and brownish colour and the rotar shaft,, next time i will take a picture and post it