Cylinder not firing

Other than the dist cap and rotor, none of the other parts you mentioned have anything to do with the spark plugs firing. So concentrate on the electrical side of things related to the ignition system.

Did you check resistance of the spark plug cables/spark plug wires between coils/distributers to determine if there is excessive resistance? This could cause the problem you are having. So could insulation breakdown/carbon tracking in the spark plug wires.

A resistance check will not find carbon tracking or insulation breakdown because they only show up when the engine is running and high voltage is passing through the secondary wires. For this, the "view it in the dark" test I mentioned earlier will usually make it visible.

I suggest both of the above tests before throwing any more parts at the problem.

 

Previous owner of my GT4 had issue of low voltage at the coils impacting spark so installed relays with power direct from battery. The power was run through the central tunnel with the coolant tubes which was not ideal. I have changed the 12v source to the terminal on the starter where the large gauge cable connects from the battery.


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"I have weak spark at idle, and I have not yet figured out If that is normal or not."

It is not normal to have a weak spark at idle. There should be no difference in the spark appearance at any RPM. A properly operating coil delivers whatever voltage it takes to jump the gap at any RPM/load. It might require, say 15kv to jump the plug gap at idle and 30kv or more at full load.

 

A euro 308QV doesn't have those components, BUT a 1984 euro 308QV brought into the US will have gotten some similar (but not identical) type modifications (if done legally ). However, these modifications have often been removed/disabled. Can you post some pics of your engine bay (especially around the fuel distributor fuel lines area)?

Your comment about the battery (alternator) voltage being low at idle is a concern and needs to be fixed regardless.

Does the "weak" spark (unless RPM increased) only occur only on one bank or on both banks?

The complete wiring diagram is in Section 5 of your OM, and there is a separate wiring diagram of the ignition system in Section 3.

 

I can't see the pics that you posted -- I get a "you don't have permission to view this page" error when I click on the picture links. Maybe a Moderator can educate me or get the pics to show directly in the thread?

I wouldn't resort to doing any alternator modification before just getting it bench tested and repaired, if necessary, as stock. The CIS-equipped 308 are all a bit marginal when it comes to the alternator idle performance. Do you have the single v-belt that drives both the alternator and water pump, or the later system with separate v-belts for each?

 

Can you post a picture of the connections on the top of the WUR (item B here):
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This photo shows the "US modification" as done on a 1985 euro Mondial QV -- note the two stacked banjo fittings on the single banjo bolt (and the thing with the yellow plastic end is the added frequency valve):

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If yours only has a single banjo fitting on each banjo bolt on the top of the WUR = the unmodified euro configuration.

Not the best for alternator performance at idle - but it should work better than you describe.

 

Yes, it does -- yours is unmodified, stock euro (so no frequency valve and no protection relay). Many, get the modifications removed later in life (if they were ever there).

 

I would just hook up a 12VDC battery in piggy back directly to the ignition system to rule out the low voltage issue. If you have VR sensors (not points) then an enlarged gap, bad sensor or bad wires could give a weak pulse at low rpm.

 

After ensuring that there is sufficient primary voltage at the coils, if the problem still exists, check the resistance between the distributer rotor tip and engine ground. It should show an open circuit. If there is any resistance shown, the rotor is faulty. Yes, I know you installed a new rotor...check it anyway. Modern rotors ain't what they used to be!

Also - in an earlier post you stated, "I did check the cables and they seemed ok to me." When you say "they seemed OK...," is that based on an actual resistance check of each wire? If so, what were the readings? If not, do that test!

Normally, ignition problems occur as engine load increases. IOW, an engine might run fine around town, for example, but misfire badly when accelerating at WOT, especially in higher gears. However, If there is insufficient primary coil voltage at low RPM/low load and/or there is excessive resistance in the secondary the coils may not generate sufficient voltage to consistently jump a spark plug gap until the rpm and associated primary system voltage rises.

 

Hmmm...Well, clearly the resistance in the cables is not a problem. Frankly, based on the tests/questions and answers, and parts you have replaced, I am pretty much out of suggestions for what might be causing the poor spark at idle but Ill throw one more suggestion out there...

With the engine running and your spark tester on the worst cylinder, move that spark plug wire away from its mounting points so that there is some distance between it and the other wires and as much distance as possible from the engine itself. See if the spark intensity changes. In doing this, it would be good to use something made of wood or plastic to handle the wire to avoid a possible shock if the insulation has broken down.

 

It may not help since you’ve done just about everything but for like $10 or so I suggest get one of these spark plug tester to let you visually see how each spark behave for each cylinder while it’s idling and at high RPM. Usually a yellow sparks indicate poor sparks and blue sparks indicates good or normal. Do you know what color spark you’re getting on the bad cylinder?

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I would still hook up a separate battery. If your system voltage is low, running a hot wire to the same system may not help. For $10 that tester is worth adding to the tool box, thanks Mike