Matt Harwood

Diagnostic Challenge!

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OK, I've got a '50s GM car sitting in the shop and it's only running on 7 cylinders. My mechanic and I have been tearing our hair out trying to figure out why cylinder #2 (front left) isn't firing.

 

  • We have swapped the plug and plug wire with the plug next to it (#4) and nothing changed--cylinder #2 remained dead and cylinder #4 fired normally with #2's plug and wire.
     
  • We have pulled the valve cover and verified that the valves are moving properly. The cam does not have a flat lobe and the valves are not stuck open or closed.
     
  • We have done a compression test and all eight cylinders are between 155 and 160 psi. Cylinder #2 is about 158. Healthy.
     
  • We have removed, cleaned, and reset the points in the distributor. Most distributor problems would show at all 8 cylinders instead of just #2.
     
  • We have changed coils with a known good one from a similar car that runs great. No change.
     
  • We have pressurized cylinder #2 with air to about 80 PSI and it holds pressure for an extended period of time (like overnight). Seats are not burned or damaged, rings must be sealing well.
     
  • We verified that the intake passage between the carburetor and valve is open (air flows backwards from the spark plug hole to the carburetor), so the intake manifold gasket was installed correctly and isn't blocking the port--it is getting air and fuel.
     
  • Spark plug is not wet or fouled, and it is not soaked with gas or oil in the cylinder.
     
  • We have verified that the exhaust port is open and the system flows properly.
     
  • The oil is not diluted nor does it smell of gasoline, so it's not flooding cylinder #2.
     
  • We have a new distributor cap on order and will try that next.

 

What am I missing? The plug has a decent spark outside the hole, but in the hole it doesn't fire well enough to light the mixture. However, it is not the plug or the wire or the distributor, because the problem stays in cylinder#2 no matter what we move around. Is it possible there's a bad ground in that spark plug hole? It's clean, but could someone have rebuilt stripped threads with epoxy or something stupid like that? We're really grasping at straws at this point.


What else? We're about out of ideas.

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13 minutes ago, Matt Harwood said:
  • We have a new distributor cap on order and will try that next.

 

That was going to be my guess. You've obviously tested everything that would be common to all eight, and you've verified that the plug and wire aren't the problem. It's got to be ignition and the cap is the only thing left. I'm betting on some sort of crack or hidden carbon track.

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Hold the brass spark plug connecting end of wire #2 near something metal when the car is running to see if you get a spark to ground using your known good Spark plug wire.  If you get little or no spark I would be betting on a distributor cap problem.

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Stab in the dark.

Decades ago I had a miss i could not find. Turned out that ONE LOBE in the distributor was flat. The points opened for seven, but it was missing on one.

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1 hour ago, m-mman said:

Stab in the dark.

Decades ago I had a miss i could not find. Turned out that ONE LOBE in the distributor was flat. The points opened for seven, but it was missing on one.

 

Wow, that's not a failure mode I would have thought of.

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1 hour ago, TerryB said:

Hold the brass spark plug connecting end of wire #2 near something metal when the car is running to see if you get a spark to ground using your known good Spark plug wire.  If you get little or no spark I would be betting on a distributor cap problem.

 

Agreed, but Matt's already proven that the plug and wire on the bad cylinder are good and function properly in another cylinder. That kind of just leaves the cap (or the flat lobe on the distributor cam).

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Posted (edited)

Are there any vacuum ports on the intake runner that leads to the dead cylinder? Plug and try again if so.

 

I agree it is most likely the distributor cap, cracked or carbon tracked.

Edited by Bloo (see edit history)

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I thought of a cap issue first. Vacuum leak at the manifold. Check using propane to find leaks. Remote possibility......bad head gasket between the two holes. Remember, Compression, Spark, Fuel, and it should fire. A mixture that’s too lean or fat on any one hole will be an issue. Did you put it on a five gas machine? My money is a bad cap or a bad intake gasket.

 

 

 

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If you suspect a poor plug ground You could screw the plug in, if possible  connect  a dvom from the metal part of the plug to a good ground and see what the resistance is.  That would atleast eliminate that. 

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That's what I was thinking. I have seen freshly painted engines that plugs would not ground. Or a little rust where the plug seats keeping it from grounding. OHM meter. 

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I'm thinking look at the distributor cap under a really bright light.

 

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What kind of car is this? Does the miss continue up through all engine speeds?

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Yes, the miss persists at all speeds. Less noticeable as you go faster, of course, but it does have the smell of unburnt fuel so that dead cylinder is just pumping raw gas into the exhaust. I ordered a new distributor cap so we'll see what we get. Of course, this car is a low-production tri-power car (975 made) so the distributor cap is unique (AKA expen$ive). I'll do an autopsy on the current distributor cap this weekend and see what we find. That really has to be it. 

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I also have run into a flat lobe in the distributor. it was on an early 80s Subaru, so it only ran on three cylinders. Quite rough and down on  power....😉

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Matt.......send me the distributor and I will put it on my machine free of charge.........and the coil.......we can eliminate primary ignition fault.......thus if ignition it must be in the secondary. 

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8 hours ago, Frank DuVal said:

I also have run into a flat lobe in the distributor. it was on an early 80s Subaru, so it only ran on three cylinders. Quite rough and down on  power....😉

 

A 1980s Subaru still had points?

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14 hours ago, auburnseeker said:

If you suspect a poor plug ground You could screw the plug in, if possible  connect  a dvom from the metal part of the plug to a good ground and see what the resistance is.  That would atleast eliminate that. 

 

Also a good suggestion. The "poor conductivity between plug and head" failure mode was one I considered, but since Matt has removed and replaced the plug, that's less likely. Agree that it's an easy test, however.

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Continuity is good, so the block is grounding the plug when it's in there.

 

Here's the distributor cap, which honestly doesn't look all that bad. A close-up of cylinder #2 shows maybe a small track mark, but serious enough to kill spark?

 

1004001657_2020-06-2615_41_20.thumb.jpg.a43fc52de16424bcb22a189cdf7692e8.jpg  1085598441_2020-06-2615_41_30.thumb.jpg.bde81e370208354791008561a1e3cb44.jpg

 

All the lobes in the distributor are there (there are only four and they're quite visible).

 

We'll try the new cap and see, but I'm not sure this one is bad. Hmmm...

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26 minutes ago, Matt Harwood said:

Here's the distributor cap, which honestly doesn't look all that bad. A close-up of cylinder #2 shows maybe a small track mark, but serious enough to kill spark?

 

1004001657_2020-06-2615_41_20.thumb.jpg.a43fc52de16424bcb22a189cdf7692e8.jpg  1085598441_2020-06-2615_41_30.thumb.jpg.bde81e370208354791008561a1e3cb44.jpg

 

Can't tell from that photo, but it sure looks suspect. To educate myself, how is that different from a common Delco window cap?

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Posted (edited)
3 minutes ago, joe_padavano said:

 

Can't tell from that photo, but it sure looks suspect. To educate myself, how is that different from a common Delco window cap?

 

Smaller diameter to clear the Tri-Power air cleaner. Unique to the 58-60 Cadillacs with Tri-Power. 

 

 

Edited by Matt Harwood (see edit history)
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 I think you found your problem.  Change the cap,

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If it was the cap......you could have used an oscilloscope to check for high KV’s..........another useful but seldom used tool. 

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If you find a problem with the cap, also be sure to ohm-test your wires, or at the very least the one on the cylinder that was bad, even if it runs OK.

 

Tripower?! Well... thats an interesting wrinkle. I suppose the closest barrel might not be feeding fuel. My money is still on a secondary ignition problem.

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Have you checked the valve clearance on number 2 cylinder to see if they are tight?  I know that this does not sound probable due to the good compression, however, it may be that they are just at the point where you can good a good compression reading but as the engine heats up they close up.  Just a thought. 

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Posted (edited)

Had a 31 Pierce 8 cyl. with a bad miss. Went mad checking everything and finding nothing.

 

Finally called my brother, a former mechanic, and electrical engineer, he brought his oscilloscope. Turns out it was the cap shorting out. No carbon track inside or out. We replaced the cap with a NOS cap the customer had and it ran fine.

 

We started cutting open the bad cap and found it had a carbon track inside the cap material. During the molding process there was a slight gap between layers of cap material and that created a pathway for the internal short.  

 

Paul

Edited by PFitz (see edit history)
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