Cylinder not firing

I am still fighting this issue. I have confirmed that upon revving the car, voltage does go up to 13.7, which is where the spark seems to come in and the car starts running ok. My problem is only at idle.

I replaced the distributor cap and rotor and it seems to make no difference. My AAV valve is open when the car is off/cold. I am not sure when it is supposed to close. I have thought about clamping off the vac hose to see if it makes a difference but have not gotten to the garage yet.

I also see a lot of posts about the Warm up regulator but not sure how to test it. (Or where it even is)

I also saw a post about a relay in the back right corner of the trunk for a some valve that is supposed to be vibrating? Can anyone tell me about that?

 

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.

 

I did check the cables and they seemed ok to me. The problem has also changed a little since I first started the the thread. I though it was originally a spark problem on one cylinder but I am seeing spark across all cylinders, but I can still feel a really rough idle. I have weak spark at idle, and I have not yet figured out If that is normal or not. (See video posted previously)

Many people have had similar symptoms with other components, so I am expanding my search.

I am also going to replace all of the plugs just to see if anything changes

 

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.

 

Thanks guys. Interesting. Does anyone happen to have a wiring diagram for the coils?

 

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.

 

Lol. You can read that wiring diagram! Your eyes are better than mine!

Below are a few pics. I will test again tonight to confirm it is both banks. Last night I tried to hook up an ignition scope to it, but for some reason my scope would not read anything. I am not sure if I had a lead go bad or what.

For the alternator voltage, it looks like it maxes out at 13.7. If iI replace it with a 14.7 regulator will that hurt anything? I would think it would be ok but to be honest I am a little scared of this car. View attachment 3342568 View attachment 3342569 View attachment 3342570

 

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?

 

Interesting. I will try to post the images again to see if it makes a difference. Also, although not a good video, this kind of shows what I am dealing with for the rough and surging idle.

https://drive.google.com/file/d/1PIx4qf35LZZe7fBK0MzwRk7fsCR_3H81/view?usp=drivesdk

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… also, it’s a single v belt

 

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.

 

I don’t see exactly what you are referring to, but I think this picture should show that area. I do have the documents from when the vehicle was imported that showed that it met all requirements.

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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).

 

Cool. One more possibility checked off the list.

 

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.

 

That’s a good thought. I may try that this weekend. If one of the crank position sensors is acting up, what would the behavior be? Could it cause similar issues?

 

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.

 

Ok, here is my update for today.

1. I checked every plug extender. They were all .5-.7 ohms.
2. While I had the extenders out, I went ahead and replaced all of the plugs. Gapped at .025
2. I ran a new power wire to the coils temporarily to ensure they were getting enough current
3. I installed a 2 ga braided ground strap from the battery to the chassis

None of this made any difference at all.

I noticed that the top of the throttle linkage was all the way against the plenum, so I adjusted the top idle screw just a bit, and I was able to get the idle back up in normal range. It is running better than it has in the past week, however the miss is still there. There is also a fuel smell but but not sure if that is just because I am running it in the garage. You can feel the car shake at idle. Over about 1800-2000 RPMs it is not noticeable, but could still be there.

Tomorrow I will check the rotor resistance as suggested above. I really wish I could replace the plug wires, but I can’t allow myself to spend that much on something I am not sure is the problem. They are crazy expensive. I did check resistance on them and they seemed to be in the 500-700 ohm range. I think that sounds reasonable, but don’t know the expected range on them.

I am also going to try to get my scope working on it so I can see what is going on.

Thanks for your input guys, I really appreciate it.

 

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

 

I did hook up the second battery, no change

 

Can anyone point me to the proper procedure for setting idle speed on a QV? I am not finding it anywhere.

 

Thanks Mike. I actually have 4 of those so I can look at a full bank at the same time.