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Shakercars's 1972 Trans Am

2019 August
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Recently replaced rear main seal leaks with increased rpm.  Best fix? 

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1 hour ago, Geedubb said:

Recently replaced rear main seal leaks with increased rpm.  Best fix? 

How did you replace it the first time? More than likely that’s where to start. Did you pull the engine? Or just drop the pan & loosen the crank? 

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Had rope seal and began leaking, then recently replaced with new 2 piece viton by instructions.  Pulled engine.  Mechanic did work. Does not leak at idle, only when increase rpm.  Suspect old seal was ok and that I have increased crankcase pressure at higher rpm than idle. Really not sure.  Considering addition of vacuum pump.  PCV works.  Appreciate info on best solution.  

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57 minutes ago, Geedubb said:

Had rope seal and began leaking, then recently replaced with new 2 piece viton by instructions.  Pulled engine.  Mechanic did work. Does not leak at idle, only when increase rpm.  Suspect old seal was ok and that I have increased crankcase pressure at higher rpm than idle. Really not sure.  Considering addition of vacuum pump.  PCV works.  Appreciate info on best solution.  

Well that’s a possibility, but easy to check! Measure the blow-by. Or dump the PCV and put in breathers on both valve covers.

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Do you know of a fairly inexpensive but effective device?  Guess I could block oil port in valve cover and connect to pcv grommet at intake?  Interesting that no oil accumulates at oil spout breather.

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3 hours ago, Geedubb said:

Do you know of a fairly inexpensive but effective device?  Guess I could block oil port in valve cover and connect to pcv grommet at intake?  Interesting that no oil accumulates at oil spout breather.

An accurate as well as inexpensive test is a leak down test. This may be preformed in different ways, but all serve the same purpose. Put air in each cylinder with all valves closed and see how long that cylinder holds pressure. There is a tool that allows you to replace the spark plug, connect a compression gage and an air chuck. With this arrangement you bring each cylinder, one at a time, to TDC. This closes the valves for that cylinder, add 150 psi, close the air port off, read the compression gage and wait to see how long it holds pressure. This tells you what your blow-by is in each cylinder.

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Wow, sounds great but suspect it is beyond my ability to do.  I did check compression in each cylinder and they are within 8 psi of each other.  I have read about hot rod engines that have to have significant vacuum pump to decrease crankcase pressure.  There are expensive electrical flow meters beyond reasonable price (more than rebuild).  Engine was rebuilt about 25,000 miles ago (1985).  Only recently started leaking problem.  I have recently replaced power brake booster and 4-wheel disc brakes.  Noting that the brakes work great but would new booster contribute to problem?  Got my license in this one family car in 1968 and have owned it since 1974.  I need to figure this out.  Thanks for your feedback.

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5 hours ago, Geedubb said:

Wow, sounds great but suspect it is beyond my ability to do.  I did check compression in each cylinder and they are within 8 psi of each other.  I have read about hot rod engines that have to have significant vacuum pump to decrease crankcase pressure.  There are expensive electrical flow meters beyond reasonable price (more than rebuild).  Engine was rebuilt about 25,000 miles ago (1985).  Only recently started leaking problem.  I have recently replaced power brake booster and 4-wheel disc brakes.  Noting that the brakes work great but would new booster contribute to problem?  Got my license in this one family car in 1968 and have owned it since 1974.  I need to figure this out.  Thanks for your feedback.

Ok, let’s back this up for a minute. First it is possible that you don’t have to much blow-by! If you don’t have oil depositing at the oil fill, the PCV or valve cover gaskets, but only a rear seal leak, than that doesn’t mean crank case pressure. 

First, what was the condition of the crank journal where the seal runs? The seal could have been put in wrong or nicked. If the journal was polished for the new seal that needs done properly, otherwise that can actually induce leakage as rpms increase.

Finally, you need to step through this step by step. First find a rubber hose that fits over the dipstick tube, connect the other end to a pressure gage. If the reading is under a pound or less the problem is the rear main. 

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14 hours ago, Last Indian said:

Ok, let’s back this up for a minute. First it is possible that you don’t have to much blow-by! If you don’t have oil depositing at the oil fill, the PCV or valve cover gaskets, but only a rear seal leak, than that doesn’t mean crank case pressure. 

First, what was the condition of the crank journal where the seal runs? The seal could have been put in wrong or nicked. If the journal was polished for the new seal that needs done properly, otherwise that can actually induce leakage as rpms increase.

Finally, you need to step through this step by step. First find a rubber hose that fits over the dipstick tube, connect the other end to a pressure gage. If the reading is under a pound or less the problem is the rear main. 

I agree with Last Indian.  Esp when you said there is no oil collecting around exit points and all cyls are within 8lbs of each other.  Take it back to who replaced the rear main.  They messed up somehow. 

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Great info!  If car idles for 20 min, no leak.  Only when rev up and then immediate leak, stops back at low rpm or idle.  Nice idea about the dipstick tube.  I will look into it.  Mechanic claims followed instructions and that crank looked good but makes you wonder.  Thanks, good info from all.

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