308 No spark at plugs for cylinders 4 and 8

1984 308 QV - I had an overheated CAT and began to look for reasons...I'm getting no spark at the plugs on cylinders 4 and 8. Working on the rear bank, I've interchanged caps, rotors, and wires multiple times with known working parts and still no spark at the plug on cylinder #4 only. I connected a plug directly to the coil output and got steady and even spark.
I had recently installed new rpm sensors...
Greatly appreciate further diagnostic direction.

 

I do have the extenders but have already eliminated them as a variable in this test. (thought they do test out ok individually). I've been using the same spark plug directly into each of the cable ends (putting that little nub back on the spark plug tip that is removed when you plug it into the spring connector in the extender).

Then the spark plug is ground with a battery cable directly to the engine ground strap. I've been turning the engine over for a few seconds testing each plug wire. (the fuel pump relay is also disconnected for this testing)

As I mentioned I've switched plug wires, caps, and rotors, to eliminate them as a variable and its still the number 4 cylinder connection in the distributer that seems to not carry the spark forward to the plug.

This testing has all been on the rear bank and I do have one more distributer I could try that is currently on the front bank. But on the front bank cylinder 8 is not firing, which is the same contact position as 4 in the rear distributer cap I believe.

 

I appreciate your earlier response Steve M.
I also just tested both coil resistances, they are essentially the same at each coil:. both primary are at .4, and secondary at about 3,700.
From what I'm reading .4 primary and 3,700 secondary is low, or is that ok? Would that be the cause or non firing or erratic behavior? I believe these are the original coils.
Thank you.

 

Thank you all for responses.

To answer when it last ran well - I had the engine out this past winter for a partial resto. There were no issues prior, but I had recently purchased the car and it needed a lot of cosmetic care and most of the vac and fuel and lines were still original. I had already put in new Magnacore plug wires a few months earlier which were working fine. When the engine was out, the tranny was detached but not opened and worked on, as that was working fine. The cams were removed and I re torqued the heads but did not touch the valves except to re gap. I redid the timing belts and tensioners. I replaced the fuel pump and clutch. All vacuum hoses, o-rings, gaskets, gas lines (except the metal ones which were replated), and seals were replaced.

Once all back together and reinstalled, the oil pressure came up fine. The metering plate was a bit higher than it should be, which I attributed to the new fuel pump’s higher output pressure (this may have been an incorrect assumption). I adjusted the metering plate down to where it should be, and adjusted the initial fuel mixture according to Flowtech’s instructions to dial the mixture screw to where you start to see fuel coming up out of the FD, and then backing off a half turn. Coolant was where it should be too in terms of bleeding and making sure the entire system was filled.

I got it to idle fine with a combination of the throttle body stop screw and bypass screw. It idled ok too once it reached temperature for the fans to come on.

Then I got daring enough to drive it up and down the block, and after a few laps, that’s when the CAT got red hot and the engine started failing and I was glad I didn’t venture too far.

Some PO had installed a Hyperflow type CAT which I removed to inspect that nothing had melted inside. Nothing did, it looked ok.

My first thought was it was running too rich or too lean and I checked the plugs and they looked ok – greyish with some black but not terrible.

I thought next I’d check for spark and that’s when I noticed generally weak sparks and then none at all on 4 and 8.

This was prior to getting new caps, and I did notice wear on the contacts so I purchased new caps, not knowing how old the ones on the car were, and rotors too. Still no spark on 4. (I’m still diagnosing just the rear bank at this point.) I swapped out and tried various plug wires with the caps to eliminate that as a variable and still not spark on 4. As I mentioned earlier, I’ve also removed the extenders for these tests.

I wondered if maybe the rotor was not spinning evenly to hit number 4 contact correctly, as I had replaced the o-ring on the cam couplers (the coupler on the end of the cam where the rotor attaches) and so I made some videos so I can see if the coupler is rotating in a stable and fixed position (not wobbly), and that is indeed stable.

Thank you very much if you’ve read this far. Frankly I’m at a loss as what to look for next. This sparking issue may not even be the reason for the overheating CAT. Maybe I have a vac leak somewhere too, and even if I do, I still have to get the electronics working …

What should I diagnose next?

 

HI,

I do have some positive diagnostic results I believe. Still on the rear bank only right now.

Based on everyones comments (thanks guys) I was going under the assumption that everything pre dist cap was working fine, and also I knew that all cables were connected correctly and getting proper ohm readings, i then got suspicious that maybe when I changed distributer o-rings on the rotor coupler, perhaps I somehow bent the coupler shaft when banging that back in, and the shaft was rotating off center and hitting (shorting) the contact for #4. I took the cap off and put some white-out markings on all the contacts and put it back on. Sure enough the the rotor was hitting the contact. I sanded the contact down and retested with the white-out - I did this a few times - until there were no more markings on the white out. Then i rewired everything back together and STILL, 4 was not working. HOWEVER!!!! this time, when i switched to the Front coil. IT WORKED.

So on the rear coil, 1,2,3 but not 4 fired, and on the front coil, 4 fired and so did 3, but 1 did not.

So I think I have a combination of issues.... One was the rotor hitting the contact, and the other is failing coils (they are originals)..

I've ordered now coils and will test upon receipt and report back.

Regarding caps - yes I had ordered them from SP, and indeed you still have to drill out the wells a bit so they are deep enough for the pins to be able to grab the plug wire....

thank you all.

 

Appreciate any further direction you have John. I thought I'd just replace the coils since swapping them worked. Do you know what the diagnostic exercise I would do on the digiplexes directly to test them also?

 

@yelcab: "bang" is the wrong word. apologies. I should have said "tap". As in I had to tap out the roll pins that hold in the coupler, and tap new ones back in. The cams were out of the car and properly supported when I did this task. And the pins were easily removed and tapped back in. But its just a guess as at this point as I don't know what is causing this issue.

 

Good Afternoon,

I’ve done some further testing…and I hope someone can take a minute or two on this again to think about what might be going on. Remember my issue was not getting sparks at the end of some of the plug wires even though I was getting a good spark off the coils… So the assumption a week ago was that somewhere in the distributer cap or plug wires from the caps was problematic. Under that assumption I carefully re attached all the plug wires and confirmed via ohm meter that I had solid connections from the caps to the plug wires.

But I discovered a few other things today. I attached a chart of testing the spark at the end of each plug wire in various scenarios.

The first section (Normal Set Up) of the chart shows I still get a solid spark off both coils but no spark off 1 and 4.

If I swap the digiplex units by swapping the large connector on the digiplex boxes, I still no spark at 1 and 4. In other words, I assume, the issue does not follow the digiplex boxes.

Then below that, I had the idea to swap the wires directly at the coils (instead of the large connector at the digiplexes) This seemed to prove that both caps and all plug wires work, as I am able to get a spark at every cable in at least one wiring scenario.

I also checked to make sure the RPM sensors, which are new, were connected to their proper ports and they are.

So, looking at the REAR BANK 1-4 section on the chart, why would the rear coil, which seems to fire properly directly off the coil, not fire 1 and 4 at the cylinders? And more interesting, if I switch wires at the coil, then it is not firing on 2 and 3. You can see this clearly on the chart.

I really appreciate any insight anyone may have. I’m at a loss here as you can see…

Thank you.
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