Diagnostic Challenge!

[Matt Harwood]

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.

[joe_padavano]

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.

[TerryB]

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.

[m-mman]

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.

[joe_padavano]

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.

[joe_padavano]

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).

[Bloo]

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 June 25, 2020 by Bloo (see edit history)

[edinmass]

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.

 

 

 

[auburnseeker]

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. 

[nearchoclatetown]

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. 

[Bloo]

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

 

[misterc9]

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

[Matt Harwood]

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. 

[Frank DuVal]

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....😉

[edinmass]

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. 

[joe_padavano]

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?

[joe_padavano]

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.

[Matt Harwood]

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?

 

  

 

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...

[joe_padavano]

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?

 

  

 

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

[Matt Harwood]

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 June 27, 2020 by Matt Harwood (see edit history)

[61polara]

 I think you found your problem.  Change the cap,

[edinmass]

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

[Bloo]

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.

[Stude17]

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. 

[PFitz]

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 June 27, 2020 by PFitz (see edit history)

[JACK M]

I think I can see a track running out to the bottom of the cap.

Then at the end of that track there is evidence of arcing where the cap sits on the distributor.

I might also see a crack between near that inside post and the one to its right in the picture.

I can only see this on the dirty (I assume before) picture.

I say cap.

[joe_padavano]

Matt,

 

I don't know how long it will take to get the replacement cap, but apparently the difference between the tripower cap and regular Delco window caps is not the diameter, it's the location of the point adjustment window. The tripower cap has the window right next to one of the mounting screws, whereas the normal cap has it more centered between the two. Apparently you can use the conventional cap on a tripower car, you just can't access the points through the window. If the correct replacement takes a while to arrive, you might try a new conventional cap temporarily to see if the misfire goes away.

 

Top photo shows clocking of the window on a tripower cap, lower photo on a conventional cap. The clocking difference is about one tower.

 

 

[Studeous]

So I’m sitting here thinkn anybody knows the caps are all the same. I love it when I learn something new. Seems that happens on this forum a lot. Thanks!  Where does a body find such an unusual part? 

[joe_padavano]

15 minutes ago, Studeous said:

So I’m sitting here thinkn anybody knows the caps are all the same. I love it when I learn something new. Seems that happens on this forum a lot. Thanks!  Where does a body find such an unusual part? 

 

I had exactly the same thought, until I did some research. If the different window location surprises you, this one will blow your mind (hint, that's for a Corvette dual point distributor). 😉

 

[Studeous]

Remember those, since you mentioned it. Thanks!  In high school I craved a brown delco remy distributer cap for my 55 chevy. Never did get one. Dont remember if too expensive or too hard to find........or I just didn’t know how to find one. 

[Matt Harwood]

When I saw the carbon tracking on the cap, I just did a search for a "1959 Cadillac distributor cap" and found several for $35. Nice! But then I saw one that was about five times that much. That's odd, what's the difference? I did some additional reading and found several posts on the CLC message forum that said the Tri-Power distributors were smaller to clear the rear carb. They also mentioned the different location of the points window. With that knowledge, I checked with several vendors specializing in these cars and sure enough, all of them had reproduction Cadillac distributor caps whose fitment said "WILL NOT FIT 1958-1960 TRI-POWER." In fact, it appears that all Cadillac distributor caps from 1956-1974 are identical EXCEPT the Tri-Power caps.

 

Now whether the diameter is different or it's just the window, I don't rightly know, but I didn't want to make my #1 most common mistake and buy the wrong one even if it was a lot cheaper. We'll definitely find out when the presumably correct one arrives this week. I'm in no real hurry to get this car running, we have plenty of other cars to service, so whenever it shows up we'll give it a try. But knowing it may just be the points window, I may compare it to or maybe even try the cap from the 1963 Cadillac I have parked a few bays over...

[TerryB]

Does the tripower cap require its own unique rotor due to the difference in the cap from the others?  This is the type of question that pops into my mind at 2am when I should be sleeping.

[joe_padavano]

1 hour ago, TerryB said:

Does the tripower cap require its own unique rotor due to the difference in the cap from the others?  This is the type of question that pops into my mind at 2am when I should be sleeping.

It would if it were actually smaller in diameter, not if the difference is just location of the window.

[TerryB]

1 hour ago, joe_padavano said:

It would if it were actually smaller in diameter, not if the difference is just location of the window.

Yes, that is why it came to me, if the cap difference is diameter then the rotor would have to be smaller too and be different from the non tripower cap.  I watch reruns of Columbo on Sunday nights and often have to ask “just one more question” as a result of that.

[Frank DuVal]

On 6/26/2020 at 8:56 AM, joe_padavano said:

A 1980s Subaru still had points?

 

Memory not as good....

 

1979 Subaru.

 

Interesting, Tercel kept points until 1982.

[joe_padavano]

13 minutes ago, Frank DuVal said:

 

Memory not as good....

 

1979 Subaru.

 

Interesting, Tercel kept points until 1982.

 

I'm actually surprised that a 1979 Sube had points. The whole reason why automakers went to electronic ignition systems in the early 1970s was to comply with EPA requirements that the car meet emissions standards after 50,000 miles without a tuneup. I'm guessing that any point-style distributor that met that requirement must have been using the points as a low-current switch to trigger a CDI box or something like that so that even pitted points would still fire it.

[TerryB]

I think the 1980s Triumph TR8 with the aluminum V8 based on a 1960s Buick design had points too.  I was looking at buying one back then and the points distributor was a surprise to see.

[joe_padavano]

4 hours ago, TerryB said:

I think the 1980s Triumph TR8 with the aluminum V8 based on a 1960s Buick design had points too.  I was looking at buying one back then and the points distributor was a surprise to see.

 

Sorry, no. The US-spec TR8s came with electronic distributors.

[Bloo]

There was nothing unusual about points in imported cars in the late 70s. More unusual in 80-82 but there were still some around.

[joe_padavano]

7 hours ago, Bloo said:

There was nothing unusual about points in imported cars in the late 70s. More unusual in 80-82 but there were still some around.

 

I know that in the 1970s the EPA exempted some low-volume manufacturers from complying with all the emissions requirements. I don't know if that applied to the 50K mile without a tuneup requirement or not. I'm still wondering how points would go 50K miles without servicing. That's also why automakers went to high voltage coils and expensive spark plugs.