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Rear Main Seal Leaking After Engine Replaced

Last year, I had the unfortunate experience of having to get my engine replaced on a 2003 Infiniti G35 (only 121k miles, I got it used in '06, long story, I assume previous owner didn’t take very good care of it). I opted to have a local mechanic find a used engine instead of paying for a rebuilt one and he was able to find one that had ~60k miles on it so I went with it. Now it’s 9 months later and I have started noticing a burning oil smell while sitting in the car at a stop. I took it into a shop (not the one that replaced the engine, different tire and lube shop) and they found that the rear main seal was leaking pretty badly and that’s where the smell was coming from. They don’t do that kind of work so I need to take it somewhere else to get any repair done.

I feel like, if a second diagnosis also says RMS is the problem, that the shop that put in the engine should have given me a new RMS when they put in the used engine. Is it not something that would be visible and thus easily replaced when installing the new engine? If they did in fact replace it, it wouldn’t possibly have failed on its own after only 8 or 9k miles, would it? I don’t normally hearing about RMS problems until cars are well over 100k miles. They gave a 2 year warranty on their labor related to installing the engine; am I wrong to feel like fixing this should be covered under their warranty?

Thanks for the input either way!

My trans was never separated from engine and my seal does not leak. Your seal was “tweaked” in that u had trans removed. I would say installing the trans MAY have nicked the seal? But, u do not usually replace the seal during a motor swap. I assume the front crank balancer did not come off so they probably left that seal alone? I think the motor swap did play a small role in this leak. But it is debatable whether shop is responsible to change seal.

You bought a used motor, not a new one, not a rebuilt one. If the seal wasn’t leaking when the motor was installed why assume a new rear seal was needed nine months ago? The motor is now 10 years old (if it was from an '03 like your original motor) and has what 70K - 80K miles on it now?

Be happy you have a good running car, and focus on replacing the leaking seal. If you want to go back to the original shop that replaced the motor perhaps they will give you a break on the price of the job, but don’t expect them to do it for free.

If I was having an used engine or transmission put in, I would instruct the shop to replace the rear main seal unless it is one that can be replaced by dropping the oil pan. I would also replace the front seal on the transmission. Most customers are so upset about the cost of an engine or transmission that a shop is reluctant to mention anything that will add to the bill/

How much oil are you using (miles per quart)?

Thanks everyone, I’ll just have them take care of it and hope for the best.

Texases - not positive yet on the amount that is leaking, but my guess is about 2k/qt as that is when I noticed the smell and saw I was low on oil and put another qt in.

I would not trust the diagnosis of a tire and lube shop. To them, every leak looks like a rear main seal. when oil leaks from an engine, it tends to travel to the lowest point, which is just below the rear main seal so it looks like a RMS leak when it can be something else.

@nateha in my professional experience, burning smell = leaking valve cover gaskets

I have replaced some rear main seals over the years, but they usually didn’t drip onto the hot exhaust.

If the rear main oil seal is leaking the oil can drip onto the pipe that runs under the engine from the front bank of cylinders. If it is the rear main oil seal leaking, and it’s only leaking a quart every 2,000 miles, before replacing the seal, trying adding a high mileage oil to the engine. Such as Valvoline Max Life. These high mileage oils have additives that cause the seals to swell slightly thereby stopping oil leaks at the seals.


The car is at the new shop now and I’m waiting on their diagnosis of what is going on. The tire/lube place did say that they noticed a small leak from the valve cover gasket as well but they said the main issue looked to the be RMS; I’m very interested to hear what this shop says given the info from @db4690

Keep in mind what @Keith said.

I just heard back from the shop and they said valve cover gaskets are leaking as well as an o-ring on a cam position sensor on the driver’s side is leaking. He is apparently unable to tell if the RMS is leaking as well due to the other leaks. He wants to do those repairs then clean the engine and look at the RMS, all which sounds believeable to me.

I’m planning on tackling the valve cover gasket myself but what is this o-ring on a cam position sensor? Is this something I’m going to get myself into trouble with or should I be able to handle it?

Thanks again!

I think I answered my own question about the camshaft position sensor, so ignore that one. Looks pretty straightforward.

Anyone have any tips/things to watch out for when replacing the valve cover gaskets? It looks pretty easy but I know looks can be deceiving.

Good to hear it sounds simple. I always like to replace any gaskets or seals that can leak during an engine swap for good measure, even if they look fine. Also keep an eye on the PCV system. This is super simple and easy to check and the PCV valve is a 30 second replacement of a $10 part. A plugged PCV can cause oil burning and leaking.