1983 308 GTS QV: Trouble-shooting an ignition stumble on the front bank.

I need some troubleshooting suggestions.

My 1983 Ferrari 308 GTS Quattrovalvole has developed an audible and noticeable ignition stumble. It is significant at idle, is discernible up to about 2,500 rpm and above that is not noticeable. I have stopped driving the car, except to troubleshoot.

Using a timing light on the ignition wires between the coils and distributors, it appears that the stumble is on the front bank and upstream of the front distributor. Eyeballing it from the flashing of the timing light and the sound of the engine, it appears to me that the front bank is intermittently (about once per second) dropping out completely for a single rotation then kicking back on again. It is erratic (non-rhythmic) so does not appear to occur predictably at a single point.

I have done the following trouble-shooting:
- Have replaced the coil ignition wires (also all the other ignition wires to the spark plugs).
- Have replaced the front (lower) TDC sensor.
- Have cleaned the TDC sensor harness connectors within the engine bay.
- Have swapped the coils and renovated the coil ground wires.

Failing further trouble-shooting suggestions (I feel stumped), I feel like my next step is to swap ignition boxes and see if the stumble moves to the rear bank.

What is confusing me most is that the timing light on the rear bank ignition wire between the coil and distributor shows that there is a very slight intermittent miss there too. Alone, it would be too minor to notice. The two banks should be independent of each other.

Is there something else I should be looking at to fix the problem?

 

Everything downstream of the coils have been checked: rotors and caps look good, plug wires and extenders are fresh, plugs look good.

Sure seems like the problem is upstream of the distributor though.

 

Some excellent suggestions have come up. Thank you for the ideas.

I am now going to do more now than just simply swap the digiplexes. (I usually like to make only one change at a time to see what the problem is, but with the digiplexes buried under the trunk carpet I will probably will do all of the below at the same time.)

Reflecting back, I manhandled my dixiplex boxes when I renovated the trunk carpet/liner a few months ago. Maybe I bumped something loose.

... or cracked a petrified vacuum hose. The erratic nature of the stumble points me elsewhere, but I will certainly check my digiplex vacuum hoses.

Actually, I have never washed my car. Instead I have just used a sissy spritzer bottle of that "speed shine" stuff and microfiber towels. But that doesn't mean I won't check that connector again!

 

I have put a total 80 miles on my car over several trips this weekend. I seem to have eliminated the major “stumble” on front cylinder bank (as measured with a timing light on the ignition cable between the coil and distributor) that I was reporting, but after about 30 blissful misfire-free miles, an occasional erratic misfire in both cylinder banks seems to have taken its place. I am disappointed that the system is not running flawlessly now, but it is running much better.

I will drive it now like this for a while to watch the misfire symptom (it still shows as missed flashes of the timing light on the coil cable) and will continue trouble-shooting if it gets worse.

I don’t like making a bunch of changes all at once without necessarily testing to see their individual impact but it was most prudent to do so from a time perspective. I did these things:

- Replaced front cylinder bank Top Dead Center sensor. Engine stumble did not change. (Decided to not change the rear bank or tachometer sensor they didn’t seem related to the problem.)

- Swapped ignition coils. Engine stumble did not move banks suggesting coils were not the fault.

- Refurbished ground terminals (2) on ignition coil brackets. Used star washers to improve chance of a good ground.

- Replaced all ignition wires, both distributor caps and both rotors (plug extenders and plugs are fine). I used NOS Magneti Marelli caps and rotors, Kingsborne 7mm Mag wire and Kingsborne terminals. Reused sheathing and boots. This is really routine maintenance but these changes were past due on my car. The cap terminals were looking burnt, but I don’t know how bad they can get before they affect performance or how many miles a distributor cap or rotor can last.

- Swapped Digiplex ignition modules, inspected digiplex vacuum hoses and cleaned Digiplex connectors. Vacuum hoses were fine. One of the Digiplex connectors on the wiring harness may have been repaired with liquid metal or something. This is a smoking gun. I am pleased (and surprised) to see that the Digiplex connectors are available as replacement part number 115598. I may replace both.

I have now gone through the system from TDC sensor to Digiplex to coil to distributor to spark plug. After all these changes, most of the problem has gone away but not all of it. I can’t think of much else to test or try. It is running satisfactorily but it is not perfect, and perfect is what I am working toward.

 

Hummm. Maybe. I changed the extenders to the black ones from Kingsborne 2,000 miles ago.

I have not. You make me wonder. I inspected and gapped them 2,000 miles ago when I changed the extenders. They looked nice. Currently I am running NGK Iridium BPR7EIX.

 

Okay. Bought some.

 

Just to close out this thread, my engine idle problems seem to be now completely solved. Since completing the work, I've taken the car out about 10 times and probably have put over 100 miles on it.

It purrs like a kitten now on idle; probably better than when I got the car.

Now that it's all done, I am thinking I was dealing with two problems with the ignition system:

1. The major stumble on the front bank may have been the worn out distributor cap/rotor. It looked bad. Alternatively, it may have been bad grounds or connectors. When I swapped my coils and digiplexes, I cleaned and revamped all connectors including grounds.

2. Once that was resolved, I saw minor misfires on both banks. I suspect it was poorly installed plug extenders or, less likely, tired plugs. When I pulled the plug extenders to replace my spark plugs, several pulled off with minimal effort. The springs in the Kingsborne extenders are much stiffer (and smaller diameter) than the stock ones, so they are trickier to put fully on. I may have installed them poorly when I changed the extenders a while back. Now they are all on properly and quadruple checked.

Thanks to all for the help troubleshooting the problems.

 

Neither of mine were leaking oil so I did not change.

What was interesting when I removed the front distributor was that there was no cork gasket installed. There was a light haze of rust on the rotor shaft. Since the 4 distributor terminals were so badly pitted, I wondered if someone just removed the gasket to move the cap inward to move the contact point for the rotor to fresher metal. If so, kinda sneaky.