#6 cylinder not firing

[BadDog]

'73, stock distributor, Pertronix Igniter, MSD 6A.

Symptoms started one day last week after winding out 2nd gear. My tach jumped from around 4K up to around 6K, then back down to around 4K. After that, the engine was running roughly, not revving easily, and backfiring under anything more than light load. I thought maybe a plug was fouled because it definitely felt like it was down a cylinder.

Here's what I've done:

1) Changed the plugs

2) Checked to make sure the distributor didn't move.

3) checked the cam timing, thinking it was timing chain slip or the tensioner broke. It was dead on.

4) Checked valve clearance, since it'd been about 2 years. No difference.

5) At this point, it occurred to me to pull the plug wires one by one. This is when I found that #6 was not firing.

6) I swapped the wires for positions 5 and 6. The problem stayed on 6.

7) Swapped in a new rotor. No difference.

8) Swapped in the last distributor cap I had. No difference.

9) swapped in another new plug on #6 just in case I had a bad new plug.

10) Propped the #6 wire and plug up on top of the value cover so I could start the car and watch for a spark. No sparks.

At this point I'm thinking that something is wrong with my distributor, but I don't know what to do next. Help?

[Walter Moore]

Check the compression on the #6 cylinder. Except for item 10 of your list, everything else seems to eliminate ignition. The valve cover isn't necessarily a good ground, so you might not see a visible spark with the plug just touching the cover.

[BadDog]

Ground? I guess I never really thought that the plug, when installed, was grounded to the block. Maybe the plug isn't grounding for some reason? Because every other plug, I could hear the spark jumping from the wire to the plug if I wasn't quick enough hooking the wire back up...

I guess I'll go pickup a compression tester, may come in handy later with my aging engine...

[westpak]

Check the compression on the #6 cylinder. Except for item 10 of your list, everything else seems to eliminate ignition. The valve cover isn't necessarily a good ground, so you might not see a visible spark with the plug just touching the cover.

yeap I chased mine the same way until I found no compression on the faulty cylinder

[FastWoman]

Run your engine in the dark, and look for stray sparks. Your current might be shorting to ground prior to the plug.

Check the connection to your #6 injector to make certain it's firing.

Obviously check compression.

Your problem is probably in there somewhere.

FAIW, a weird problem one forum member had (argneist) was that the EGR port on his intake had eroded through to the manifold, bypassing the EGR valve. The leak was dumping lots of exhaust exhaust into his #4-#6 cylinders, causing them not to fire. There was possibly some blockage that prevented the exhaust gas from reaching #'s 1-3. I've not heard of this problem in any other car, but I thought I'd mention it.

[240260280z]

you can pull the end of the wire from the plug and feel the sparks jump to the plug top as you pull it away a few mm. This works fine with bare hands and ngk and stock plug wires... bad wires or other brands may spark to your hand.

[BadDog]

Thanks for the replies. For some reason, I'm not getting e-mails about responses. FastWoman: I have carbs, so no injectors to test; I'll have to try the "dark" trick soon. Blue, that's exactly what I was talking about, I could feel the spark jumping on all cylinders except 6.

I just got done running the compression test. It seems to look surprisingly good and consistent:

1: 155

2: 155

3: 160

4: 155

5: 155

6: 160

This was after a short warm-up, around 4 or 5 minutes.

Edited May 17, 2011 by BadDog

[FastWoman]

In trekkie alien voice:

What IS... "carbs?"

But seriously...

:bulb: Do you also have points? I'm just guessing here, but if the cam (correct term?) in your distributor is worn a bit more on the #6 than the other positions, OR if your distributor shaft is wobbling just right, AND if your points are adjusted so that they barely break as the distributor cam rotates, then you might be (barely) breaking on #'s 1-5 and not on #6.

[BadDog]

yes, carbs. Points? No points. See the first line of my post: Pertronix Ignitor, MSD 6A. I've have the Pertronix for at least 5 years I think, and its been rock-solid. I've had the MSD box since last summer. I'm assuming they're OK, because from what I've read, if they had issues, I wouldn't have any spark at all.

At this point I think I'm going to order a remanufactured distributor and install it. I seem to remember my manic Z mechanic mentioning something about a little bit of play in my distributor's shaft a year or two ago. I'm not sure how to check that, but there is a tiny bit of play if I try to wobble it around. I can also easily rotate it a little back and forth, which may be normal, I'm not sure.

[FastWoman]

D'oh! I can't read.

Might still be in your distributor. You can try swapping BOTH your #5 and #6 wires AND plugs to see if the problem migrates to #5. If it doesn't, well, that's the entire ignition system after the distributor. I would think that would mean the problem is in the distributor.

[ezzzzzzz]

Put a set of points back into the distributor and see what happens. It's quick and simple with the petronix setup. Just one cylinder not firing points to a bad plug, wire or cap externally. I'm thinking a magnet has moved in the petronix setup. They're only held in place with that green tape.

Edited May 17, 2011 by ezzzzzzz

[ezzzzzzz]

f'ing slow internet made me think I hadn't gotten the last post out...nevermind.

Edited May 17, 2011 by ezzzzzzz