Mystery misses in a 1970 472

[mikanystrom]

Hello club members,

I have an issue with my 1970 De Ville Convertible that has me ... actually stumped.

I bought the car with (I believe) 149,000 miles on it in 1997 and have driven it frequently since then, and now it has 275,000 miles on it.

The bottom end of the engine has to my knowledge never been opened.

A few years ago I had a head job done by my local mechanic, not a Cadillac expert but good, hard-working people that work on all sorts of old cars.  The travails of this rebuild are elsewhere on this site---turns out that I got a misground camshaft (MTS "Stealth" I think it was, or maybe a #5... doesn't matter much) and wound up replacing it with I think a Comp Cams camshaft that was about the same, a bit hotter than stock, nothing crazy.  Valve springs and oil seals upgraded at the same time.  Had the transmission replaced with a '67 Buick VP that I almost always drive in "low stall", which is a big higher than stock (Super Turbine 300 converter on it).  Higher than stock upshifts, and harder shifts.  Really wakes up the car without turning it into some sort of wannabe '67 GTO.

Ignition is Pertronix.  Spark plugs are Champion ... N11YC.  Pertronix transistorized breaker inside a regular Cadillac points-type distributor, Petronix coil.

I have driven the car a few years with the above configuration and it has run great.  An ever so slight ping at full throttle, otherwise no issues, good gas mileage, excellent power, ...

So what has happened...

Two weeks ago I am driving from Los Angeles to San Francisco, roughly, and the car runs great as always.  I drive 80-85 mph on I-5 through the San Joaquin Valley, moving with faster traffic, but nothing crazy.  Weather is ~70F and hazy.  Stop a few times for gas and biological needs, no issues.  Gas mileage seems normal, car runs great.

When I get to my first longer stop, in Fremont (just north of San Jose), I take the freeway exit and immediately notice the engine is running rough.

I am only 1 mile from my destination so I drive on and stop and check, it didn't feel too serious.

Engine is idling poorly, and vibrates when you give it gas at low speeds.  At high speeds (70+ mph) it feels ALMOST normal, you can notice a bit of a shake if you really pay attention.

In short, feels like I'm running on 7.  No big deal, right...

Stop and check, sure enough.  Rough idle.  I remove the ignition wires one at a time from the distributor.  Engine runs rougher as I remove each wire, EXCEPT #7 (and I discovered later #6 also seems maybe not to affect the idle).  It especially runs rough when I remove the wires from the neighbors (on the distributor) of #7 (nos 2 & 8 I think).  Note that #6 is the opposite in the firing order from #7 (it is in the same place in the rotation but on the next/previous revolution of the engine, 360 crankshaft degrees/180 camshaft degrees away from #7).

I replaced the plug for #7.  No effect!  Autozone gave me an RN14YC for some reason.

There is good spark between the distributor terminal and the wire for #7 when it is held close by. 

There is good spark on #7 when I lay the plug on the block and hook the wire to it, I can at least see it in broad daylight.

Same checks for #6, same results.

I tried swapping the wires for #6, #7, same results.  Not with each other (doh!) but with their neighbors.  No difference.  Still bad on nos. 7 & 6.

OK... I have an upgraded camshaft... valvetrain problems?

Take off valve covers, all the rockers are rocking normally.  Everything is roughly equally tight or loose as all the other cylinders (checked both sides for #6 and #7).

Replaced distributor cap.  No change.

What the...

Maybe a valve came apart, engine was hot and it pings a bit, right?  Went to Harbor Freight, first compression tester had threads too short to catch the spark plug threads (grrr, they charged a restocking fee).  Second, "Deluxe", kit works, and compression is

1. 182
2. 190
3. 182
4. 184
5. 191
6. 189
7. 183
8. 199

Numbers look a bit high, maybe?  Cheap tool?  But nothing that makes me suspect a mechanical failure.

I have just moved so I am not sure where my timing light is, I don't want to pull the distributor unless 100% necessary... and I AM getting spark on all 8.  The only thing I haven't really checked is the Pertronix unit.  I know sometimes the magnets can fall out.  But then.. I wouldn't get any spark at all on 6 and/or 7 would I???

OK... a few more information points.

I did replace the spark plug on #7 in San Jose.  I get back to LA and after ~500 miles the spark plug is dark.  Not "bad dark" but dark enough to show that there is some sort of combustion going on in the cylinder.

Gas mileage on trip back was 13 mpg.  That is if anything better than normal on California gas, but OK so I was babying it a bit since it is not running perfectly, really tried to keep it from kicking down to 2nd to pass and accelerated slowly, but still drove at a reasonable ~70mph.  Really doesn't seem like I am dumping 1/8 or 1/4 of the fuel down the tailpipe.  But the car does smell a bit of gas when idling.

It sounds "wrong" idling but if it is running on 6 or 7, I think that's probably normal?  There is a slight lifter clickety-clack---I suspect it always did that, but don't remember for sure.  I didn't pay attention till I had an issue!

Grateful for any ideas or comments...

     Mika

I am really baffled.. any ideas?

[wheikkila]

Sounds like you could have a vacuum leak effecting the two cylinders.
Thanks Wayne

[signart]

Yep^^^^^

[V63]

My experience with pertronix is verify magnets have not fallen out of trigger ring on rotor. Also verify the air gap as outlined in their instructions which requires the distributor to be removed and shims inserted. Moreover, the pertronix requires FULL 12 volts and not reduced thru factory resistance wire .

[DeVille68]

yes, definitely check the vacuum, I mean disconnect every vacuum hose from the engine: transmission, power brake, air condition etc.
I had a lean nr 7 and the cause was that the vacuum fitting on the nr 7 cylinder was providing extra air because the air condition master unit had a huge vacuum leak. You can use a small hose (for example a vacuum hose and hold on end close to your ear and with the other end you can check around the carb base to hear if you hear air being sucked in)

Then also sway the coil, I had some rough and hard idle because of a bad coil. I did check out oke with my volt meter but would leak internally as soon as the voltages were high enough.

You might also want to verify TDC, maybe the timing chain stretched a bit or jumped a teeth.

Best regards,
Nicolas.

[Dave Shepherd]

Get a non contact laser thermometer,  engine  warmed up check the temps  at  each  exhaust port at the manifold. Should be within about 50 degrees  of each other. This should pinpoint the suspect cylinder.

[James Landi]

I worry about the Petronix ignition aftermarket parts as well.  I've chased a couple of failures--- and returned to standard points... they, too, have been known to fail- especially with condensers, but if you're like me, just getting the engine to run right is really important... your compression test for a 50 year old engine is something to celebrate... so the advice regarding vacuum leaks as suggested above, (check your carburetor gasket for leaks)... loose nuts, spray some wd 40 at those gaskets to check for gasket leaks....  might not be ignition, but I always start with the easy things to eliminate (vacuum hose leaks --- there's that vacuum choke release pod on the side of your carburetor that's been known to create some big issues--- made of plastic, with a very short vacuum hose and subject to failure.)   Keep us in the loop.... hope this helps, James

[TJ Hopland]

Vacuum leak is my first thought too.   Does a 70 have a vacuum port on the #7 intake runner like slightly later years do?   If it does I would be looking real close at everything that is attached to it.

[Cape Cod Fleetwood]

Sounds JUST LIKE what happened to me recently.
There's a vacuum manifold (plastic thingy) on the firewall, mine cracked.
Has 1 line in and I think 3 out. Its black and plastic, I'd check that....

[Big Fins]

That 'thingy' directs vacuum to the A/C controls. It does have one main 5/16" line in and 3, 3/16" lines going out to various servos and controls inside the firewall.

[mikanystrom]

Hello everyone,

and thanks for all the suggestions.

But.... no, still running poorly, mysteriously.

I did the following... I disconnected the vacuum consumers from the #7 port and capped the port.  The distributor advance is connected to the carb (manifold vacuum fitting on the carb) and looks good.  Power brakes are working well and hose looks good.

Vacuum at idle is ~15 inches of mercury and fluctuates at the r.p.m. rate, I assume from a cylinder missing.

I have had issues with Pertronix before, so to rule it out... got a rebuilt AC delco distributor, points type, put that in.

Replaced all the plug wires.

And the plugs.

And the cap.

And the rotor.

Still the car shakes when you give it gas.  It shakes worst at low rpm and medium power.

I sprayed starter fluid all over the top of the engine and didn't hit any immediate culprits.

Remember that this issue came on suddenly in connection with a long freeway cruise... it's not a gradual deterioration or something that happened in storage.

I'm really at a loss to explain what's up.

Oh yes one cylinder (#6) did seem cooler than the others when I tried the IR thermometer.  110C vs 150C after idling for a while.  There were differences between the other cylinders but not as much.

OK there are a few other things I can do... disconnect EVERY vacuum consumer.. not much left but still, mess some with the coil... hmm.

The issue doesn't come and go, though, it's always there, it's very obvious, nothing at all subtle, and doesn't affect all cylinders, just one or maybe two....

Given all of the above it almost feels like one exhaust valve is stuck closed.  But I suspected that early on, and took off the valve covers, and the rockers seem to move normally.   Argh!!!

[wheikkila]

Have you put a vacuum gauge on your engine? I think you should.
       Thanks wayne

[mikanystrom]

vacuum gauge.. if the link works

https://photos.app.goo.gl/A8EyYwJC8HbmeoHE9

[mikanystrom]

more video...

even cylinders

https://photos.app.goo.gl/QoHU56ngacDRxUsaA

odd cylinders

https://photos.app.goo.gl/SatJEhooHfh7SYdu6

[The Tassie Devil(le)]

I know it is hard to tell, but the last rocker arm on the Left side head appears to be doing something odd.   Could just be the video though.

Bruce.

[James Landi]

One important "tell tale" as follows:  All of the push rods should spin when the engine is running.... if a push rod is not spinning, the rod might be bent and (or) there's abnormal wear on an important part of the valve train for that cylinder.  Again, if your car's engine has a vacuum break on the side of the carburetor, they have been known to cause rough running, and because t he vacuum hose is so short and the plastic pod "looks" fine, that part is often overlooked.   Hope you find the culprit.   James

[hornetball]

Check the bolts securing the AC compressor to the intake manifold.  On the 472/500, one of those bolts goes all the way through and can cause a vacuum leak/miss.  Not saying that's your issue, just something unique to these engines.

[chrisntam]

That vacuum gauge is sure bouncing around.  Here's a link to a page that helps tell what the engine is doing based on those readings:

https://itstillruns.com/erratic-fluctuation-vacuum-gauge-during-idle-12227649.html

[dn010]

I posted this for someone else who was having engine trouble. Between this and the link above, you should get an idea of what is happening on the gauge.

[wrench]

So, if I read this correctly, you are running points again now?

Was the dwell reading steady?