308 Backfire/Missing Help

I'm not sure what you mean by the tack bouncing all over the place?
Do you mean that tack AND the engine speed are changing dramatically,
Or that the engine speed remains constant, and just the tack needle is bouncing around?
Just a guess, but if just the tack needle is bouncing around and the engine speed seems to remain constant, it's probably a poor electrical connection to the ignition. Could be something as simple as a loose or oxidized connection at a fuse, or a loose wire in the ignition circuit.
I’m sure someone more knowledgeable will chime in with a more specific probable cause to get you sorted out.

 

This is an indication that the problem is not in the rotor-cap-wires-extender-spark plug part of the system. The tach signal (when the tach gauge itself is not broken) is a derived signal coming from one of the Digiplex ECUs (i.e., having the tach signal go bonkers and the engine backfiring/stumbling at the same time indicates that the firing events for that bank are not occuring at the right times).

IMO, the area you should be investigating is the flywheel sensors-Digiplex ECUs-coils part of your ignition system (see pages 70-71 in your OM). One thing you might try is just switching Digiplex ECUs:

if the problem then becomes "the tach works OK, but the bad running still occurs" that would be a bad sign for one of the Digiplex ECU

if the problem remains the same, I'd be more suspicious of the flywheel sensors or coils (and switching the coils might be a next step if this is the case).

Good hunting!

 

The tach runs solely off the front bank ECU (cyl 5-8, under rear window) thus an iratic tach is a sign of problems on that bank.

Your problem most likely lies with a Ground Failure in the luggage compartment (left side) where the ECU are mounted and the their Grounds are attached (to a bolt that holds the ECU mounting plate down, lift the carpet up and you will see a metal plate with 4 bolts...the ECU's are mounted underneath to that plate).
Run a ground wire (10 or 12 GA) from where the coils are mounted back into the luggage compartment and connect it to the grounds for the ECU's. Follow the vaccum advance tubing.
Even if you find the ground bolt loose do the upgrade as the ground will fail eventually in every 308 as they age.

I have now had to this in 5 308's that have developed floating grounds in the lugage compartment.
I would recomend that anyone with a 308/328 do this upgrade.

Paul

 

I had the same problem, ended up being the Alternator bracket being worn out and going slack, check the belt and all connections, hope this helps, good luck

 

Did you do the ground update?????

 

It will.
If you own a 308/328 do the ground update as explained above.

 

Hi Paul: I have an 82 Mondial. The tach was erratic, but now the only symptom is power loss. What does the ECU look like?
Thanks
Sam

 

I’ve been battling what seem to be ignition misfires for the last few months, should have looked harder at these grounds when I’d checked them earlier. Even if they aren’t the source of my problem right now they were on borrowed time.

I’d previously checked/cleaned these grounds a few months ago and thought they were good. There didn’t seem to be a lot of corrosion externally, so I just cleaned them up and cleaned the mating surfaces and bolted them back in the stock location. Since then I’ve cleaned then replaced the crankshaft position sensors, reinspected the new spark plugs and extenders, replaced broken distributor rotors and damaged distributor caps (thought for sure they’d fix it), replaced the ignition wires (twice ), ordered new ignition coils, and in the meantime decided to add auxiliary Digiplex module grounds as Paul recommended.

I clipped the existing eyelets off and stripped the tip of the wire again, found corrosion all they way around just under the insulation. The inside wires were fine, but the outside wires that were in contact with the crimped eyelets were corroded. I then proceeded to add auxiliary 10 gauge grounds from the coil mounting bracket directly to the ECU cases.

I’ll have to drive it for a while to know if it’s actually fixed (it’s fooled me several times before), but I’ve got confidence now that they WON’T be giving me any problems from now on.








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This is my vote. I'm not sure if your Model Year has a protection relay as well for the ECU.(s)... but that could be your culprit as well. in my car the relay was lose and would wiggle out over a bump or vibation. I just used a zip tie to ensure its seated.