1962 Thunderbird - Leak Down Test Results

[Leon]

So, I need the advice of people much smarter than me. I have been having problems with white smoke (smells like gas) blowing out the exhaust and overflow tube. I performed a compression test on all cylinders. Seven of the eight cylinders were within 10% of one another (135-150 psi cold). The fourth cylinder was at 105 psi. The car has 93K miles on it.

I added an entire can of seafoam to about 3 gallons of gas, put some in the oil and sprayed an entire can into the carburetor. Followed all the directions. The compression in those that were at 135 increased up to much closer to 150 psi. The fourth only went up to 110.

So, I performed a poor man's leak down to see where the leak is while cylinder 4 was at TDC. No bubbles at the head or in the coolant, so I am pretty sure the head isn't cracked or that there is a leak at the gasket. There was air escaping out the exhaust and out of the oil filler cap. The air coming out of the oil filler cap smelled like gas.

So what are your thoughts, do I just need to get the heads rebuild or do I need to go for a whole engine rebuild.

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[64ZCODE]

Hi Leon. White smoke could be excess fuel from mixture too rich. Could also be water getting into the cylinders but sounds like you've ruled that out. Engines prior to about '69 didn't have hardened valve seats so unleaded fuel can cause eroded valve seats hence cylinder compression leakage. I would recommend going after why it is burning rich which most likely has to do with the carb, for example stuck float bowl needle valve resulting in gas being dumped into the throat of the carb. The 4 barrel carb rebuild kits are not expensive. Good luck!

 

[doug7740]

So, I need the advice of people much smarter than me. I have been having problems with white smoke (smells like gas) blowing out the exhaust and overflow tube. I performed a compression test on all cylinders. Seven of the eight cylinders were within 10% of one another (135-150 psi cold). The fourth cylinder was at 105 psi. The car has 93K miles on it.

I added an entire can of seafoam to about 3 gallons of gas, put some in the oil and sprayed an entire can into the carburetor. Followed all the directions. The compression in those that were at 135 increased up to much closer to 150 psi. The fourth only went up to 110.

So, I performed a poor man's leak down to see where the leak is while cylinder 4 was at TDC. No bubbles at the head or in the coolant, so I am pretty sure the head isn't cracked or that there is a leak at the gasket. There was air escaping out the exhaust and out of the oil filler cap. The air coming out of the oil filler cap smelled like gas.

So what are your thoughts, do I just need to get the heads rebuild or do I need to go for a whole engine rebuild.

Leon,

You stated that you have a problem with white smoke that smells like gas blowing out the exhaust and overflow tube. You performed a compression test and the results were 135 to 150 psi on seven of the eight cylinders, however cylinder number four had a reading of 105 psi. After spraying an entire can of Seafoam into the carburetor, the compression increased closer to 150 psi in seven of the eight cylinders, but cylinder number four only increased from 105 psi to 110 psi. From what you told us these are my thoughts:

I believe the compression specification on your engine is 180 psi + or - 20 psi. A variation of 20 pounds from specified pressure is satisfactory; however the compression of all cylinders should be uniform within 10% of each other, and cylinder number four is clearly not. By spraying an entire can of Seafoam into the carburetor, you performed a wet compression test and that’s why your compression increased in seven of the eight cylinders. That would indicate that there is a compression loss through the rings into the crankcase and that’s why you could hear air escaping out of the oil filler cap when you did the leak down test. If when you performed the leak down test on cylinder number four you could hear air escaping out the exhaust, that would indicate that you have a leaking exhaust valve and would also explain why the compression didn’t increase after you sprayed the Seafoam into the carburetor.

I was wondering if you took a look at the spark plugs when you removed them from the engine, because the color of your spark plug is a valuable piece of information and can tell you how well your engine is really running. By examining the insulator firing nose color, you can determine a great deal about your engine’s overall operating condition.

A light tan/gray color tells you the spark plug is operating at optimum temperature and the engine is in good condition. A dark coloring, or heavy black deposits can indicate a rich condition, low compression, or overly retarded timing.

I hope this information will help you determine the cause of your problem.

doug7740
1955 Thunderbird Blue

 

[Leon]

Appreciate the reply. I actually remeasure the pressure a week after doing the cleaning. Did a little more to clean out the carbon and is running without blowing smoke (well until my heater hose burst while driving home). The compression that i reported was cold, so it would probably be 170-180 hot

Spark plugs were black, but now have a light tan.

 

[Leon]

Hi Leon. White smoke could be excess fuel from mixture too rich. Could also be water getting into the cylinders but sounds like you've ruled that out. Engines prior to about '69 didn't have hardened valve seats so unleaded fuel can cause eroded valve seats hence cylinder compression leakage. I would recommend going after why it is burning rich which most likely has to do with the carb, for example stuck float bowl needle valve resulting in gas being dumped into the throat of the carb. The 4 barrel carb rebuild kits are not expensive. Good luck!

Rebuilding the carb is the first thing I did! It was quite fun. Just took off the top of the carb, the floats are working as they should. White smoke is slowing down, thankfully.