Hello,
Maybe you guys can help me with my frustrating engine problem on my caddy. It's a 1970 Coupe with a 472 ci. V-8 and has about 115,000 miles on it. The engine has been freshened up quite a bit since I got it including a top end re-build, full tune up, new exhaust system, etc. Recently I noticed that the engine has begun stuttering. It still starts perfectly, but seems to have this skip and shakes throughout the throttle range like it has dropped a cylinder. It does it in neutral and while under a load, it never goes away. I did a compression test, it has ample in all cylinders. I put in a new set of lifters thinking maybe it had a collapsed lifter, but that didn't change the problem at all. I checked the cap and rotor, and wires (all fairly new) and it has ample fire throughout. I replaced the plugs, which were black with carbon, but it had no effect on the problem at all. What am I missing here? When I changed the lifters none of them looked overly cupped or deformed on the bottoms so I really don't believe it's the cam. Could it possibly be a rocker or pushrod? The tips of the rockers where they contact the valve stems did look well worn. The pushrods looked straight from what I could see. I looked all over and didn't find any sort of vacuum leak or anything around the crb or intake. I'm at a loss. Any suggestions would be really, really appreciated at this point. THANKS!!

Check your timing first,then go from there.Did you do the firing order correctly after the top end rebuild?

I will check the firing order but I am fairly certain it is correct. The top end re-build was several thousand miles ago. The car was running perfectly until it just started this shuddering business right out of the blue the other day. It's really got me scratching my head.

See,if it's a bad lifter,a bad pushrod or a bad valve you'll hear a loud tick,and possibly a knock while it's running.Might want to check for vapor locking in the fuel lines too.Now,I'm guessing when you say shudder it's like when you drive a manual transmission and try to take off on a higher gear at too low of an rpm?Only way I can describe what you're trying to describe.Also,might want to get rid of your points and drop in a pertronix unit.Points are a bitch man.With a Pertronix,no more having to change points and condenser,or gapping points.But with the way you described your plugs,sound like you're burning a lot of fuel and running a hell of a spark,so might want to definitely look at the timing,and possibly air/fuel mixture at the carb.Also,run some Sea Foam through the fuel and oil,which will get rid of a lot on nasty crap in your system.

Any car that old needs the timing chain changed, did you? Regardless of miles, the sprocket has plastic teeth that can wear or crack & fall off at any time.

You can check the chain by removing the distributor cap and rocking the crank by hand. If you move the crank 6 degrees before the rotor starts to move, the chain is shot. More is impending disaster. A roller chain replacement is best. Bruce Roe

It's always the ignition points.

I'm with Al. If the rubbing block on the points is worn a bit, the point gap may be too small, working OK on some cylinders but not all. Just a touch of run-out in the distributor shaft with points too close can make it run strange. Check the gap.

Wow,
Thanks for all the suggestions guys, some of these I would not have thought of. To describe the "shudder," when the engine is at idle it looks just like someone has it by the manifolds and is rocking it from side to side very quickly, you can feel it vibrate throughout the car. When you rev the engine, as the RPMs drop back to idle it gets worse for a moment and then goes back to a steady shaking. Under the hood it is quiet, there is no ticking or knocking at all, but if you hold the throttle part way you can hear a faint ticking through the exhaust which mirrors the shaking. I didn't fool with the points/condenser when I did the engine re-build as the car was running well prior. I have never changed a set before so can someone tell me what to look for if they are bad and how to go about replacing them if they are? I know some engines do strange things if the points are out of whack so I think I will try that approach next and see what happens. THANKS!!

You might consider installing a Pertronics electronic ignition kit in your point type distributor. It will have great spark, and you will likely never have to fool with it again. They are available in many auto stores and online, at places like Summit Racing, Jegs etc. The spark plugs will last longer also, due to the high frequency type spark they put out.

Dude, when I ran points cars I had to change them about every year. They would start running bad and I would think the engine is getting worn out. Put in new points and it would feel like a new car again. That's why I refuse to use them anymore; they just don't last long. I convert everything to electronic ignition now. If your points haven't been touched then it will run like new with a new set (for a little while).

You are right Al. I seldom got more that 10 or 12 K miles on a set of points in the old Chevy small blocks.

Well here is the disheartening update for today. I opened up the distributor and looked at the points, which I actually had put in when I first got the car, just forgot about it. The block that rides on the distributor was obviously worn and had decreased the gap. I re-gapped them, started the car, no effect on the shaking. I then took them out, cleaned the point surfaces themselves, which were a little bit burnt, tried it, no effect. Then I went to the store, got a brand new set, tried them, no effect. I also played around with the gap a little just to see, which didn't do anything either. I checked the timing and firing order, all are spot on. The only thing that has changed is that the exhaust smokes ever so slightly, which it didn't do before, and you can hear the ticking and sputtering in the exhaust more prominently now it seems. Am I wrong for wanting to stick a rag in this thing's fuel tank and light it? Any more ideas gentlemen?

What about checking the timing chain? Bruce Roe

Man,better hope your crank isn't screwed.Vibration and noise makes me suspect the crank,but take it to a mechanic instead of asking all of us.

Bruce is likely right about the timing chain, with the plastic teeth being on the edge.
Maybe you should take a look at the rocker arms. Even though you did a top overhaul a short time ago, the stock rockers are a weak point in these engines.
I had one that the bolt that holds a pair of those came loose. Shouldn't be too difficult to take a look. We are still trying to get some ideas here:)
Did you replace the coil? Heat related problems could affect the coil on some ignitions with points. The old style coils were oil filled, and prone to heat problems as I remember. The spark is strongest at idle, because of longer coil saturation, and any carbon tracks on the coil tower or anywhere in the cap will affect the way it runs.
You could also start it up after dark, and see if any stray arcing is evident in the ignition system.
If you suspect an intake manifold leak anywhere, like at the cylinders that are affected, dribbling a few drops of gasoline along the manifold gasket edge will bring RPM up if there is a vacuum leak.

When you start hearing loud ticking in the exhaust it is either a burnt valve or a sticking valve. Since the new points didn't make any difference it is sounding that way to me. It is nice to adjust the points with a dwell meter so you can hear how the engine responds. You said the top end was rebuilt and the compression was decent in all cylinders, That kind of kills my burnt valve theory. I guess I'd have to hear it. A burnt or stuck valve will make a loud ticking echo all the way down the exhaust. I have removed some very loose timing chains over the years out of engines that were running fine. I don't think they cause driveability problems until they jump a tooth and quit altogether. It's a stumper.

Man,I'm still thinking problems at the crank,which might also have to do with the timing gears/chain

I would agree with the abrupt failure of a timing plastic gear. They almost always fail when the engine is turned off for some reason, at least in my experience.
This problem is really getting interesting.

I still think an inspection of the valve springs and rockers is a good idea. Pulling the coil wire and rolling the engine over without starting will be a way to look-see how the rockers and springs are working, and if there is ample lift etc.

Didn't get a chance to play with the caddy much today but for what it's worth I took a look at the distributor rotor with the cap off while I turned the crank pulley with a wrench. There doesn't seem to be any noticible play between the two, a tiny hesitation, if anything. I suppose it's still possible it jumped a tooth or something if the gears are plastic as you say, but there doesn't appear to be any play in the chain. I also thought it important that I mention that I didn't do a COMPLETE top end re-build. The engine had blown the head gaskets and was taken down to repair that. The heads were shaved 30 thousanths to straighten them but I did not do a valve job. The valves, valve springs, rockers, and pushrods are all original to the car with 115,000 miles on them and it is making a noise just as Al mentioned, a loud ticking, not from under the hood but "echoing down the length of the exhaust." What would be the remedy for a sticking or burnt valve? Pull the heads and do a complete valve job?

Didn't get a chance to play with the caddy much today but for what it's worth I took a look at the distributor rotor with the cap off while I turned the crank pulley with a wrench. There doesn't seem to be any noticible play between the two, a tiny hesitation, if anything. I suppose it's still possible it jumped a tooth or something if the gears are plastic as you say, but there doesn't appear to be any play in the chain. I also thought it important that I mention that I didn't do a COMPLETE top end re-build. The engine had blown the head gaskets and was taken down to repair that. The heads were shaved 30 thousanths to straighten them but I did not do a valve job. The valves, valve springs, rockers, and pushrods are all original to the car with 115,000 miles on them and it is making a noise just as Al mentioned, a loud ticking, not from under the hood but "echoing down the length of the exhaust." What would be the remedy for a sticking or burnt valve? Pull the heads and do a complete valve job?

Yeah,a complete valve job is usually the solution.Now,if there's vibration,take it to a shop and have them inspect your u-joints,because a lifter or valve isn't going to make your motor shake.

Sounds like a compression test is what's needed if you think a valve is burned or sticking. Too bad you didn't do the valves when you had it down for the head repair.

Well just wanted to say that I really appreciate all the help and suggestions that you all put forward on this, it is most appreciated. Unfortunately, I think this is where I have to draw the line. With all the money that I have invested into the caddy, I am really loath to think about selling her but at this point I am so frustrated by the prospect of possibly having to pull the motor back apart that is likely whats going to happen. I think I just have too many irons in the fire right now, as I also have a 1969 Ford F-100 Ranger I am restoring as well. Thanks again for all your efforts and expertise fellas. If anyone out there might be interested, please feel free to contact me at: theadirondackhermit@hotmail.com

Since these engines weren't meant for unleaded fuel, the heads and valves supposedly erode and don't last very long. I personally haven't driven any old engines that far for that to happen. It sounds like that's what may have happened to your engine though If that is the case it really should have at least one cylinder with low compression. I'm surprised the machine shop that worked on your heads didn't try to sell you on a good valve job with hardened valve seats to make it unleaded compatible. He missed a marketing opportunity there. The car would probably sell for a better price if it had a valve job with hardened seats. I think I would have a hard time pulling the heads off unless I definitely had a dead cylinder but it is just a matter of time anyway.

Funny, thats the same thing my Dad said when I put the question to him. He is an old gearhead from way back and told me that the gas today is basically "piss" compared to the higher octane leaded fuels which these older high compression engines were originally designed for. Just as you mentioned, over time the newer unleaded fuels supposedly can deteriorate the valve seats as well as other components. Anybody else ever have this happen to an engine or hear of it?

The lead in fuel builds a coating on the valve faces and seats, which helps them last. Without lead for a break-in, the valve may get a bit of a groove or seats may get hammered a bit. I get by fine on Chevy small blocks that are old engines, without hardened seats. Don't know how a Caddy holds up.

Just wanted to give all the guys who helped me out here a little update on this car. Sold the '70 Coupe DeVille, hopefully to someone who is going to give her the teardown she needs. I have been missing her though, I mean what is a man without a Caddy? So lucky for me this past week I happened upon the BARN FIND of the century!! A 1968 Calais with 53,000 original miles! The car had been sitting in a garage about 10 miles away from me since 1975!!!! Still has the original bias ply tires even and not a speck of rust anywhere!! Even the interior is immaculate. She has one brake line that needs replacing and has a slight lifter tick, but runs like a champ! Anyone have any tips for getting out a lifter tick? I'm guessing it's because the engine wasn't run in so long that one probably lost it's prime. Anyway, I'm beyond happy with her and everything has worked out great so far. Thanks again for all the help.

Also forgot to add that the red engine temp light stays on all the time for some reason. Even when the car is cold and not running. The gauge says the temp is fine so why would the idiot light stay on like that? Any ideas?

After reading this thread over again, I think the '70 might have had a bad coil too like Bill suggested. A compression test would have ruled out the stuck/burnt valve. I did have a brand new chrome coil fail on me once and the car ran so terrible I thought it was dead and parked it. A couple years later I went to put that new chrome coil on another car and it ran terrible too. I put a new coil on the old car that I parked and it ran like a champ. I always thought a coil either worked or didn't work but I found out they can barely work and make an engine run very badly. Too bad we didn't try that before you sold it. I love a '70 Cad. The echo down the exhaust pipe did sound like a burnt valve to me though. Good luck with the '68.

You will likely change the oil and filter on the '68, and when you do, add a 1/2 pint of Marvel Mystery Oil to the oil. It will likely un-stick the noisy lifter, and any sticky valves. I use it in airplane engines all the time, and nothing else seems as good.

I will definately do that, sounds like it would probably do the trick. I know that marvel mystery oil has a reputation of doing amazing things. I tried unplugging the coolant temp sensor in the intake and that stupid light still stays on, even with the keys not even in the ignition. The only time it goes out is when you unhook the battery. Weird. Could it possibly be a fuse or something? The accompanying gauge seems to work fine.

I believe the coolant temp light is switched by a different sensor than the gauge sender. Could even be a light switch /sensor wire shorted to ground etc.
Look around for another sensor in the head or block etc. If you lift the sensor wire you mentioned, the gauge should move, up or down etc, which will verify that that sensor is for the gauge.