Where's the blue smoke coming from?

[moparlee]

My '71 Challenger street car with a 340 has a puff of blue smoke under a slight load (normal acceleration). It comes from the right bank. Probably rings or valve seals or both. I would like to diagnose it little first before I pull it and do a tear down. With a compression or a leak down test how do I tell if the problem is ring blow by or a valve not seating? If the tests check out ok, can I assume then that only the valve seals need replacing? Do I need to do both a compression and leak down test?

The motor was completely rebuilt probably 20 years ago, but only has a few thousand miles on it. Oil pressure when hot is 40 at idle and 70 at cruising speed.

 

[74 challenger]

you might of started your engine with a dry cylinder from sitting, a ring could of froze .You said only 2000 miles on a engine built 20 years ago. How much do you start it? A valve seat could of dry rotted also. If you do a compression test and find a bad one do it a couple times.In doing it more then once the valve will not be in the same position . Im not a engine pro so this is my opinion . I think you really cant figure it out without tearing the engine down. You can locate the area but not the exact problem. Look at the cylinder wall if thats good have the head checked. Good luck and let us know.

 

[moparleo]

How long has the engine run since running it after 20 years? If it is very low, try some Marvel Mystery oil in the cylinders with a fresh set of Champion plugs. Let the oil soak in for a few days. About a tablespoon in each hole. Also check the umbrella seals on the valves. They harden and crack with age. Does it continue with driving or just after you start it up ? If it is only at start up, it is most likely the valve guide seals. These can be replaced on the car without engine disassembly.

 

[moparlee]

The engine has not run very long over the 20 years with only a cruise or two each year. It actually didn't run at all for a 5-6 year stretch but I did prime it before re-starting. The smoke does continue after driving, so not just on start up. I will give the Marvel Mystery oil a try and also take a look at the valve seals. It is winter in Michigan so I won't be driving it until spring.

Thanks for the tips Chuck and Leo

 

[Juan Veldez]

A wet versus dry compression check will tell you if the leak is from the rings and which cylinder(s) is/are having issues. Remove ALL the spark plugs! Do the dry test first, then the wet test. Engine cold or hot, but be consistent for both tests. If the compression gets better on the wet check, it's typically a ring problem, sometimes a piston problem. A wet test will not help valve issues.

I know there is something else that it would tell you if the numbers are different doing the test cold and then hot, but I forgot what is was. Someone help me out!?

A vacuum test that produces a fluctuating reading at idle is typically a sticking valve, at increased engine rpm, it would be a leaking valve. Could be a leaking head gasket or weak valve spring too, probably not those though.

To all you guys that park your ride for extended periods of time. A start up, back up and pull back into the garage once every one or two months is good enough to keep all the seals and gaskets lubed. Especially for carbs and axle tube seals! A spin around the block is better, but at least start it and move it 10 feet back and forth.

Let us know the results of the tests - good luck.

 

[sheetmetaldan]

A wet versus dry compression check will tell you if the leak is from the rings and which cylinder(s) is/are having issues. Remove ALL the spark plugs! Do the dry test first, then the wet test. Engine cold or hot, but be consistent for both tests. If the compression gets better on the wet check, it's typically a ring problem, sometimes a piston problem. A wet test will not help valve issues.

I know there is something else that it would tell you if the numbers are different doing the test cold and then hot, but I forgot what is was. Someone help me out!?

A vacuum test that produces a fluctuating reading at idle is typically a sticking valve, at increased engine rpm, it would be a leaking valve. Could be a leaking head gasket or weak valve spring too, probably not those though.

To all you guys that park your ride for extended periods of time. A start up, back up and pull back into the garage once every one or two months is good enough to keep all the seals and gaskets lubed. Especially for carbs and axle tube seals! A spin around the block is better, but at least start it and move it 10 feet back and forth.

Let us know the results of the tests - good luck.

If I drive 10 feet back and forth in my garage 8 feet of car will be sticking out each end of my garage! :tongue3:

 

[moparlee]

A wet versus dry compression check will tell you if the leak is from the rings and which cylinder(s) is/are having issues. Remove ALL the spark plugs! Do the dry test first, then the wet test. Engine cold or hot, but be consistent for both tests. If the compression gets better on the wet check, it's typically a ring problem, sometimes a piston problem. A wet test will not help valve issues.

I know there is something else that it would tell you if the numbers are different doing the test cold and then hot, but I forgot what is was. Someone help me out!?

A vacuum test that produces a fluctuating reading at idle is typically a sticking valve, at increased engine rpm, it would be a leaking valve. Could be a leaking head gasket or weak valve spring too, probably not those though.

To all you guys that park your ride for extended periods of time. A start up, back up and pull back into the garage once every one or two months is good enough to keep all the seals and gaskets lubed. Especially for carbs and axle tube seals! A spin around the block is better, but at least start it and move it 10 feet back and forth.

Let us know the results of the tests - good luck.

Thanks Juan for the compression test process. I had forgotten about the wet test. Was it the leak down test you were trying to think of? I have never done one, but it measures the percentage of pressure loss in each cylinder.

You are right on in making sure we start up these engines on a regular basis.

 

[Juan Veldez]

Thanks Juan for the compression test process. I had forgotten about the wet test. Was it the leak down test you were trying to think of? I have never done one, but it measures the percentage of pressure loss in each cylinder.

You are right on in making sure we start up these engines on a regular basis.

Yea, I think your correct, it's for a leak down test.

 

[Juan Veldez]

If I drive 10 feet back and forth in my garage 8 feet of car will be sticking out each end of my garage! :tongue3:

Thanks for making me laugh! I hope you have a garage door on both ends then. Ya all know what I meant, right?

 

[moparleo]

Just one more thing on the monthly start ups. If you have a torqueflight transmission in your ride, after start up, engage the parking brake and foot brake, then put trans in nuetral and just let her run for a bit before putting into gear. The reason for this is the front pump does not engage in park. This is why you check the fluid level in drive or nuetral. Also why when you start your car and just put it in reverse or drive with out a warm-up, there is a hesitation before the transmission engages. So to wrap this up if you are storeing for the winter,A. Fill the fuel tank and add your Sta-bil or whatever you use for storage. B, Check the condition and strength of your coolant before you put her up on jack stands or what ever you use to take the load off of the tires and suspension. C. Clamp the fuel line before the pump and let her run until it empties the bowls and dies. D, disconnect battery and remove from vehicle, connect to a battery Tender or something similar, of course you should wash,dry and wax her before you do any of this and cover her up with a good quality car cover for the winter. This will cover about 90% percent of your winter concerns for storing your car. Merry Christmas.

 

[Chryco Psycho]

1 thing that bit me was an intake gasket leak sucking oil up out of the lifter valley , could be that simple !

 

[moparleo]

Chryco is correct it is usually something simple, so don't ever by pass the obvious, it is right a lot of the time. Also ,you said a puff. Does that mean it doesn't smoke after the "puff"? Or all the time? Constant blue smoke could also mean a stuck pcv valve and poor drainback of the oil on the top of the cylinder head thathouses the PCV valve. Could be/ or not, never sure. Have fun. This is called getting experience. Takes a life time.

 

[Chryco Psycho]

It could be a simple as a bad intake gasket & sucking oil out of the lifter gallery

 

[moparlee]

Thanks everyone for the possible causes and what to look at. I will be checking out the simple and easy ones for sure before hopefully not having to wrench deeper into the engine.

The smoke does not seem to be constant, only under acceleration.