Oil Leak

[Scoop]

I just got an oil change with a synthetic oil 10 W 40 with high zinc, before putting my 74 Challenger away for the winter. Soon after the oil change and there is a large oil leak just below the oil pan drain plug. The plug is tight but I'm thinking the leak is from the rear main seal? So my question is, if it is the rear main seal, can it be changed without taking the engine out? The Challenger has a 340 with a automatic 904 Torqueflite. It could be coming from the oil pan? I don't have a lift to under the car fully, so it just me trying to crawl under. From what I can find, the 340 was manufactured in 1968.

Thanks in advance for your help.

 

[moparleo]

First is to verify the leak. Can you access a lift ? You need to clean all oil and grease from the pan and lower block area.
Put oil leak detector dye in the oil and take for a short drive. put back up in the air and with a small black light or dye detector look for any traces of the dye. It will show where the leak originates from.

 

[Ricks72Chlgr440]

I just got an oil change with a synthetic oil 10 W 40 with high zinc, before putting my 74 Challenger away for the winter. Soon after the oil change and there is a large oil leak just below the oil pan drain plug. The plug is tight but I'm thinking the leak is from the rear main seal? So my question is, if it is the rear main seal, can it be changed without taking the engine out? The Challenger has a 340 with a automatic 904 Torqueflite. It could be coming from the oil pan? I don't have a lift to under the car fully, so it just me trying to crawl under. From what I can find, the 340 was manufactured in 1968.

Thanks in advance for your help.

Like Leo said, clean the underside the best you can to remove all oil that has leaked. Run the engine for about 10 minutes then stop and put jack stands under the front control arms and lift it as high as needed to crawl under. With a light look all around the underside for leaked oil. When you find it look higher to see where it came from. Synthetic oil was never designed for our older car engines so often what you're seeing is the result. Also, the 340 cars never came with a 904 automatic, only 727.

 

[mrmopar340]

Like Leo said, clean the underside the best you can to remove all oil that has leaked. Run the engine for about 10 minutes then stop and put jack stands under the front control arms and lift it as high as needed to crawl under. With a light look all around the underside for leaked oil. When you find it look higher to see where it came from. Synthetic oil was never designed for our older car engines so often what you're seeing is the result. Also, the 340 cars never came with a 904 automatic, only 727.

Yep on the 727.

 

[Scoop]

Thank you for your suggestions and help. The Challenger originally came with a 318. The 340 was put in some time ago, not sure when. Would the 318 come with the 904? Would the 904 be compatible with the 340?

Thanks again for your help.

 

[Chryco Psycho]

There are 3 possibiities , could be the oil pan , the rear main which can be replaced in the car or the oil sender by the dist leaking .
318/ 904 is the normal configuration , the 904 will work with the 340 for a while at least , the 904 is not a weak trans & can be used in racing behind 340/360 / strokers as it is lighter than the 727

 

[Ricks72Chlgr440]

Thank you for your suggestions and help. The Challenger originally came with a 318. The 340 was put in some time ago, not sure when. Would the 318 come with the 904? Would the 904 be compatible with the 340?

Thanks again for your help.

Yes, the slant 6 && 318's normally came with 904's. You can certainly keep the 904 behind your engine but be aware that the stock 904 does not have the strength needed to hold together behind a stout 340. With upgraded internals, the 904 is actually a coveted transmission due to its lightweight design & better 1st gear ratio.

 

[Scoop]

There are 3 possibiities , could be the oil pan , the rear main which can be replaced in the car or the oil sender by the dist leaking .
318/ 904 is the normal configuration , the 904 will work with the 340 for a while at least , the 904 is not a weak trans & can be used in racing behind 340/360 / strokers as it is lighter than the 727

Thank you very much.

 

[Scoop]

Yes, the slant 6 && 318's normally came with 904's. You can certainly keep the 904 behind your engine but be aware that the stock 904 does not have the strength needed to hold together behind a stout 340. With upgraded internals, the 904 is actually a coveted transmission due to its lightweight design & better 1st gear ratio.

Thanks very much for your time and explanation.

 

[Xcudame]

There is a fourth possibility that may have come about due to the synthetic oil. But I pray that it is not the case! The three places Chryco listed are relatively easy to fix and most likely. The fourth one is a porous oil gallery plug in the rear of the block allowing the oil to seep through the plug! Only way to fix that is by removing to transmission to get to the plug. Let me tell you, that was one hell of a nightmare locating that leak and it was only discovered with the dye and black light after many hours of trying to fix the leak to no avail. I remember bashing that plug with a sledge hammer to deform it before troughing it away in case someone found it and tried to use it! I wouldn't wish that nightmare on anyone!

 

[Challenger RTA]

Rope seal?
https://www.amazon.com/Lisle-27000-Sneaky-Pete-Tool/dp/B0002SRCJWhttps://www.youtube.com/watch?v=pXQw2luGYPQ&ab_channel=Eightbanger

 

[Scoop]

There is a fourth possibility that may have come about due to the synthetic oil. But I pray that it is not the case! The three places Chryco listed are relatively easy to fix and most likely. The fourth one is a porous oil gallery plug in the rear of the block allowing the oil to seep through the plug! Only way to fix that is by removing to transmission to get to the plug. Let me tell you, that was one hell of a nightmare locating that leak and it was only discovered with the dye and black light after many hours of trying to fix the leak to no avail. I remember bashing that plug with a sledge hammer to deform it before troughing it away in case someone found it and tried to use it! I wouldn't wish that nightmare on anyone!

Thanks for you help. I hope that is not the case. I'll have to check it out in the spring after it out of hibernation.

 

[Scoop]

Rope seal?
https://www.amazon.com/Lisle-27000-Sneaky-Pete-Tool/dp/B0002SRCJWhttps://www.youtube.com/watch?v=pXQw2luGYPQ&ab_channel=Eightbanger

I appreciate the link. Thanks.

 

[Surfcuda]

Hey Scoop, I recently had the same leak issue. Like Chryco mentioned in his post, the Oil Sending Unit was the culprit on my 1970 Barracuda. Run your finger around the Oil Sending Unit and if it’s got oil on it that’s probably your issue. Simple fix with new sending unit. Hope that’s the issue!🏝

 

[Challenger RTA]

Just to be sure I would pull the oil sending unit and reseal the threads. Make sure there is still contact for the unit to work.