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I have a 1998 Oldsmobile 88 with the N/A series 2 3800.

I am trying to wrap my head around a possible vacuum leak I highly suspect I have. The car drives fine and is responsive and the only thing I can say as far as me suspecting a problem is at idle it slightly stumbes very lightly. Engine rpm remains consistent, no drop outs or misfires, just a very light stumble I initially chalked up to being caused by it being older engine. I smoke tested my intake through the hose barb fitting on the throttle body where the FPR and evap purge valve vacuum line attaches to. I found smoke coming out of a hole in the block. I have attached a pic for reference.
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This hole is located right under the engine coolant temp sensor, and is where the transmission bolts to the engine. From previous research, I have found that this hole was used at the factory to hold the engine block during assembly and is non functional for anything on the engine. So its not something like a missing freeze plug or anything like that. It came from the factory uncapped as shown.

The problem is I can't wrap my head around how I'm getting a vacuum leak from this hole. My long term fuel trim numbers start out at around +13 at idle and any slight amount of throttle makes the numbers drop drastically close to zero. So it pointed me in the direction of a classic vacuum leak. I have recently replaced all upper and lower intake gaskets along with a new manifold, every o-ring for pcv/map/throttle body etc, and the smoke is only coming out of that hole in the block. For what its worth I also have great engine compression numbers on all cylinders and no coolant or oil loss so I do not suspect its coming from a crack in the head or block.

My guess so far is possible lower intake gasket leak through the blind bolt holes in the block or maybe a vacuum leak at a transmission module? I'm pretty sure there's no vacuum control on the 4t65e though as its electrically controlled. At least not any I've ever seen. And I cant see how a LIM gasket leaking could expose manifold vacuum to the outer shell of the block...

Any insight is greatly appreciated.
 

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The leak isn't from that hole. It only opens up to the transmission bell housing and the engine's rear cover (see photo below I just took of mine on the stand). Unless you're seriously pressurizing your crankcase and blew a leak into the rear cover/main, it's going to be from somewhere on the intake manifold or vac lines on top.

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Thanks for the reply and the picture. Puts me at ease knowing there isn't anything behind there. I used a diy smoke machine and regulated it at around 2 psi so I think it comes down to being the LIM like you suggested. Sucks to have to redo the job and tear it down, but at least its nothing major. I did buy the fel-pro set so I'm kinda surprised. This time I'll probably use some permatex high tack spray gasket to set them up before I drop the manifolds in.
 

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Before you do any disassembly, I'd work to really isolate it, first. Use the "hose stethoscope" trick I mentioned in a recent post, where you use a length of hose to search for a significant leak sound around the top of the engine. It will become VERY clear where that leak is occurring, and that'll limit how much you have to un-do. Check lower injector o-rings, all vac connections, etc., as something probably didn't seat/seal properly or possibly got torn/mis-aligned. Last resort would be pulling the big parts off to re-attack.
 

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Before you do any disassembly, I'd work to really isolate it, first. Use the "hose stethoscope" trick I mentioned in a recent post, where you use a length of hose to search for a significant leak sound around the top of the engine. It will become VERY clear where that leak is occurring, and that'll limit how much you have to un-do. Check lower injector o-rings, all vac connections, etc., as something probably didn't seat/seal properly or possibly got torn/mis-aligned. Last resort would be pulling the big parts off to re-attack.
Will do. Definitely want to limit having to redo the entire job.
 

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Before you do any disassembly, I'd work to really isolate it, first. Use the "hose stethoscope" trick I mentioned in a recent post, where you use a length of hose to search for a significant leak sound around the top of the engine. It will become VERY clear where that leak is occurring, and that'll limit how much you have to un-do. Check lower injector o-rings, all vac connections, etc., as something probably didn't seat/seal properly or possibly got torn/mis-aligned. Last resort would be pulling the big parts off to re-attack.

Unfortunately it turns out it really is leaking from that part of the block. So it must be either a bad rear main seal or a bad cam seal. I smoke tested it again and saw the smoke come out of the starter housing, and it definitely was coming out of that hole. Really odd that I don't have any oil loss so maybe its the cam seal, but I figure its getting through the pcv system and leaking out one of those seals in the crankcase.

Guess I'm just gonna have to live with it for now. I'm in no position to pull the engine and trans to replace those seals. If it starts leaking then thats a hit I'll have to take and have to fix it then. I've just done extensive work to the car as it is. Replacing all of the motor mounts and replacing every failure prone part in the valve body/channel plate of the transmission has me reluctant to do anything that major as of now. That was bad enough doing all those repairs in a driveway on jackstands lol. Thanks for your help though. It definitely was a head scratcher for a second.
 

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Very odd. Probably right with the PCV guess. I'd have to think harder about the internal pathways to have any top-end thoughts on it, especially since I don't usually tinker with N/A plenums in stock form.

Hopefully, you hear some other thoughts from folks here and elsewhere.
 

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Another update just for anyone interested. I suspect the leak is coming from a gasket that goes to what looks similar to something like a timing cover. I figure its just a cover for the seals to ride on. I found a video with a N/A 3800 that shows this gasket going bad. It would make more sense that it would be leaking from that gasket vs the cam/crank seal since I don't have any oil leaks. The video can be seen here for reference. Leaking gasket
 

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There's a "rear cover" behind the flexplate that you see in my photo, which I've never personally removed on the dozens of builds I've dealt with. I have heard of some leaks in those due to various reasons, and I know Series 3's had a change that requires extra care to properly seal the different gasket type.

If that's caused a leak/opening and is letting your PCV draw un-metered air that the MAF doesn't register, it could be similar to a traditional top-end leak. Be sure to hunt for all "simpler" solutions before possibly having to address this bigger engine-pull repair, obviously.
 

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Yeah I'm confident its either leaking from that gasket or one of the seals. Either way for now I'm going to leave it as is because I'm not in the position to do that sort of repair where I am. When I fixed the other vacuum leak I found I have the ltft down to +5 which I'm happy with for now. It'll be nagging me for sure that I know it's there, but it's a job for another time.
 
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