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?

 

Brian,
You could also have a look at and inside the front distributor.

- Ensure the screw piercing the coil wire is actually tight, holding the coil wire with good electrical contact.
- check the centre electrode and rotor in the distributor.

I had a high rpm misfire, which was a bit better after changing the coils but was still present. When I checked inside the front distributor, I found that the centre electrode was chewed up and the rotor had carbon transfer instead of a polished spot like in the rear distributor. I think a weak spark contributed to the carbon transfer. After replacing the rotor and centre electrode, the misfire was gone.

Here's a photo of the new centre electrode next to the chewed up one, with the bad rotor in the background:
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Absolutely - if the trouble moves to the other bank = not a good sign for DigiPlex ECU that the problem follows; if things stay the same = good sign for the two DigiPlexes, but not much help to figure out what is the problem is other than knowing it's not the DigiPlexes; if the problem goes away completely = thank your lucky stars .

The next diagnostic level is to measure the AC voltage (or o-scope it) of the TDC sensor for the troubled bank when the problem occurs (i.e. with the engine running) and the RPM sensor.

 

How about the plug extenders?

 

^

Sometimes you really have to look close to see holes in the extenders..

Why not throw a new plug in too?

 

Do you think bad plugs and extenders would show up with a timing light clipped on to the wire from the coil to the distributor?

 

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.

 

Did you wash the car recently? A few years back, I had similar problems. Went through just about everything you did. I did need new wires, extenders and plugs, so not wasted effort. But it did not fix the misfire. Only after I unplugged the white connector on the driver side near the oil cooler to my digiplex did I find the culprit. The rubber boot around this plug had failed. Had water in the plug. Odd thing was that I had washed the car almost a week prior to the misfire showing up. I would have thought it dried out in the Texas heat. I was wrong. Dried and cleaned the connector and my issue went away completely! Just a thought.

 

There is one more simple thing to check before you do that.

Check the vacuum hose that supplies the ECU boxes --- it runs as a single line from the intake plenum to the boot (trunk), then splits (with a t-fitting) into 2 lines supplying each box separately. Check carefully the condition of the one that is supplying the ECU which is running the front bank.

These are rubber hoses that are now 30+ years old. They get dry, brittle, and will develop cracks and splits --- causing leaks. Without good and accurate vacuum supply, the ECU will not advance the ignition correctly.... and the entire bank will stumble and misfire as it revs.

 

Yup. Check the Digiplex boxes in the trunk. When I bought my 83 it ran well sometimes. It would change when closing the engine cover. No kidding! People thought I was nuts. But it came down to a loose plug on the Digiplex. Good luck!

 

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.

 

Plug extender. Has to be.

 

One or more of the plug extenors is sketchy. Extendars? Extenders? Extendurs? Xtendoors?

Sorry. Can't spell.

 

After reading again, Man, those Digiplex supercomputers gotta be good.

Your symptoms sound exactly like mine.

Do your best on those. Just time. No cash.

AND! Make sure they are ground fully to the chassis. Employ dielectric grease like a fool!

Go ahead, scratch that paint a lil to get a metal to metal.

 

I'm sure you just didn't list it but did you actually replace the spark plugs? It's rare but plugs have gone intermittent ....,


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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.

 

Looks can be deceiving.

 

Okay. Bought some.

 

Be great if it's that simple. Good luck, fingers crossed.

 

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.

 

Glad you got it sorted. Have fun.


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Brian A - did you change the seals on inside your distributors? Mine both leak oil and I'm into that next.
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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.

 

This is the first time I've read this thread and I did not see it but the answer to that question is "yes".

At any given point there is only one single current path - from the coil to the cap, through the rotor to the terminal and then through the plug wire and across the plug gap of the cylinder that is firing at that moment. An open anywhere along that circuit path will result in no current and the timing light will not fire. The next logical step for troubleshooting would have been to go to each individual plug wire and see which one was misfiring. Only if all of them showed misfiring could you conclude that the problem was "upstream"

I've only had my 308 for about three weeks now and it had some rough running issues when I first picked it up which seem to have gone away now that I've put about 500 miles on it and driven like it was stolen but my advise to anyone who is inclined to DIY things is to go through all of this stuff because we're talking about a 33 year old car here and from what I've seen and read, past documentation of supposedly "professional" maintenance may not always be what you were expecting it to be. I mean the bills are there, but was the care given to really do everything right?