Dear Gents, just had time to dova compression test on the 429 engine by warming up the engine and removed all plugs and went through cranking without pouring oil into the cylinder since I want to determine if the carb is set incorrect or if the engine is getting tired and got horrible results: driver side psi starting from front 135, 95, 130, 190, passenger side beginning from front 180, 190, 200, 140.
what should be the next steps to determine if its just stuck valves or major piston/cylinder issues? I assume a healthy engine holds around 200 psi ? Thanks in advance for your help

Views: 276

Reply to This

Replies to This Discussion

Put a squirt of oil (about a teaspoon) in the low cylinders and rerun the test.  If the compression boosts then the rings or cylinder walls have leakage.  If no boosts then likely a valvetrain issue and you can move to the next tests.

Don't be as concerned with overall psi if they are all over 100 psi.  More important is that they be within 10% of each other.

Thank you for that helpful advise. I will try this week to do a compression test with oil. Do you have recommendations which oil to use? Hopefully it will be a valve issue since I don't like to pull the engine. The car was not driven often in the past 20 years. 

Doesn't matter what oil, whatever you are running in the car would be fine.

Next step would be to perform a leak down test.  You can usually rent a leak down tester from an auto parts store.

Important that you get each cylinder at top dead center.  You will probably have to source a bolt for the center of the crankshaft to allow you to rotate the engine by hand.  Unlike many cars, ours have a cork plug in the crankshaft threaded hole rather than a large bolt.

Cylinder Leak Down Testing

Since you said the car has been sitting a long time, it's possible some of the rings are gummed up and stuck in the ring lands.  Might be worth squirting some Marvel Mystery Oil into each cylinder and letting it sit a day or two.  Turn the engine over to expel the excess oil out the plug holes and then run the compression test again.

A rust buster type  penetrating oil ( PB Blaster-is  best  ) would  be good in the cylinders to help loosen the gummed up rings of an engine not run much in last 20 years ?

Easy to spray into the cylinders 

Very good chance the rings  are gummed up and can come back to life with some  initial lubrication ( in your compression test ) — an engine oil flush - ( to get the lower rings loose from inside the crankcase  ) and some  new miles on the engine ( doing  two early oil changes ) 
Sure worth the time and effort to hopefully bring the engine compression back to life and improve performance 

Marvelous Mystery oil is a thin oil that will get into places a thicker oil cannot  reach which helps  a lot to loosen parts in tight confined places and increases compression in cylinders 

My Dads airplane mechanic used the Mystery oil in my Dads airplane engine to bring up the airplane engines compression to spec —to be passing his  the yearly required complete airplane inspection —called an annual —in the airplane world 

MMO was recommended  by my bike shop when the low end of my Triumph motorcycle engines  bearings got so tight it was hard to kick start the bike 
Kicking the bikes engine with MMO in the crankcase a good number of times until it loosened up corrected the problem to my  wonderful surprise 

The lubrication should loosen up the rings and could improve your current compression numbers 

I would do the lubrication above before the leak down test because if the rings are gummed up you will not be getting good or better numbers until they are freed up 

I always use —MMO in my gas tank  ( 50 years ) to lube the fuel system and to oil the valve guides and top of the piston rings which is called a —a  Top oil 

The only way that any oil gets to the valve guides ( intake mostly ) and top piston rings ( some gasolines have Top Oil in them - Exxon - Marathon Stp )  

MMO has been around for 100 years which does speak for itself 


Out of curiosity, how much MMO do you add to a full tank of gas?

I know that one of the front bumper bolts will thread into the harmonic balancer bolt hole ( where the caulk plug came originally stock ) —but not sure which one it would be —if that is the way you desire to be turning the engine over 

Someone might add to that thought for the bumper bolts location you might need 


Thank you both for your helpful advise. Today I had time to do the low cylinders with a bit of oil. The pressure just increased by 15-20 psi so not much at all. It has to be a valve issue. What is notorious on the 64? Stuck Springs or worn Out seats? The engine is running pretty silent at idle..
Regards from Austria!


A good machine shop will help you sort out what you should do and replace on the heads 

Your country is know for outstanding mechanical engineers in many fields and 

Be especially careful to —-Not put the rocker arms on backwards — as that has happened much to often with professional engine builders with our year Cadillac engines 
i would have the heads CCed  ( make all the head chamber volume  the same size ) and do some smoothing of the fuel and exhaust passages to remove some casting roughness that was noted when they cut up a head on this site a short time ago - but that’s just what I would be doing and not for everyone 

Having fresh heads on the engine is a  very good thing !

If your  rings are gummed up  from limited use using an engine flush for that purpose and a few oil changes putting some fresh miles on the car could show some improved engine compression and performance 

Time will tell on the last suggestion 

Hello from Florida USA !



  • Add Photos
  • View All


63 Hydra-Matic won't shift out of first

Started by Norm Waters in 1963/64 Cadillac Specific Discussion. Last reply by Tony and Ginny 429 3 hours ago. 3 Replies

Hello everyone. I had a perfectly good shifting Hydra-Matic transmission until I elected to fix the leak at the dip stick tube. I just did a quick seal with gasket maker since my current tube is…Continue

Rear brake lock up when cold

Started by Cliff Johnston in 1963/64 Cadillac Specific Discussion. Last reply by Tony and Ginny 429 3 hours ago. 1 Reply

Hi everyone, ever since getting the car back on the road, the rear brakes grab when they are cold and I’m moving at slow speed.  Once I get along the road and stop they are fine. I see some posts…Continue

Vacuum advance

Started by Thomas Yarri in 1963/64 Cadillac Specific Discussion. Last reply by Thomas Yarri 15 hours ago. 4 Replies

Good afternoon to all my fellow Cadillac owners! I have a question about the vacuum advance on my car. The car is a 64 without a/c and a Rochester carburetor. I noticed that I have vacuum at the port…Continue

Blog Posts

1963 series 62 convertible transmission issue

Posted by Matthew VanDeWater on June 22, 2021 at 3:03pm 1 Comment

Hello I've been restoring a 63 for way to many years. Down to almost done, but have issue with trans linkage, when column in park shift rod 2 inches short. everything original. been trying to find picture of manual rod hooked up from under car to see position of linkage. was wondering if anybody has access for pictures. I wanted to see if my manual linkage is bent  wrong out of the trans if somebody dropped the trans or something. Doesn't make sense why it doesn't work. Thanks for any input…


New vehicle in the club

Posted by Patrick Westphal on June 17, 2021 at 4:00pm 1 Comment

I was successful in getting a vehicle from the New York contact I talked about at the last club meeting. I am still trying to figure out how to post a picture of the vehicle that is tied with this comment so if you see a random picture of a black convertible in a yard, that's me haha.  …


Torque converter grab when idling in Drive

Posted by Cliff Johnston on June 14, 2021 at 8:47pm 2 Comments

Hi Everyone, been quite a while since i have been on here.

I have had an issue with the idle on my 64 Coupe Deville ever since i have had it.

With the Idle set high enough that it is smooth, but still a nice idle, the RPM seems only slightly too high for the stall of the Torque Converter. And therefore it tends to sit there and grab and lunge a bit. So until now, i have been putting the car in Neutral at the lights to stop this happening.

I've had the car dyno tuned and…


Can you help? Driveshaft Question....

Posted by Chris on May 30, 2021 at 8:55am 1 Comment


Newbie to the group here - we have already found tons of invaluable information on the board, so thanks! We are restoring a 1963 Series 62 Convertible. We are wondering if anyone knows of a source to buy the propellor yoke flange (that bolts to the pinion flange) for the driveshaft? One of our ears partially broke off. Not sure if we can buy just the yoke flange as an aftermarket part, or if it has to be rebuilt by a welder/fab guy? Maybe a salvage yard for a source? Again, any…



  • Add Videos
  • View All

© 2021   Created by Jason Edge.   Powered by

Badges  |  Report an Issue  |  Terms of Service