I have a 1998 Oldsmobile 88 with the N/A series 2 3800. I'm posting in the Buick section because this engine setup was in many Buicks and this section seems to get more traffic.

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 stumbles 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 an 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.




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'* 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.

Welcome to the forum.
Not to worry we see all the posts, and have moved this into the Olds section.
Based upon a stumbling idle, I would start by trying to clean or replace the IAC- Intake air control.
It controls the idle and gets really dirty with carbon. See how the car runs after cleaning.
See this thread: https://www.gmforum.com/normal-maint...2/#post1597076

I agree with Soft Ride ^^^^^^^ , that'd be a good place to start.

Regarding the smoke issue, I can't imagine how it would get through the modulator all the way to there. Otherwise I'd be guessing.

For anyone that may have this same issue, I wanted to give an update that I did conclusively find the problem. Unfortunately it is leaking from that part of the block. I've posted this issue in other 3800 forums and a member was kind enough to take a pic of a 3800 block he had on an engine stand. It gave me a better idea as to what is behind that hole. The only places it could be leaking from is the rear main seal or the cam seal. So one or both of those has a leak. My guess is when I smoke test through the throttle body, its going through the pcv system and leaking out the crankcase through those seals. I'm leaving it as is because I'm in no position to pull the engine and trans to replace those seals right now.

Technically with the pcv valve being a controlled vacuum leak, it really shouldn't matter that much other than maybe leaning out the crankcase gasses the system pulls into the intake since it now pulls a bit of air. Figure it will only be a real issue if it starts leaking oil which luckily it hasn't so far. I did find another vacuum leak before posting this issue so that may have been the cause of my original high ltft numbers. I'll give it time and drive around to see if those numbers correct itself. Either way thanks to everyone that replied, and I will look at the IAC valve counts on a scan tool. When I redid the upper end gaskets I cleaned the TB and all that out so it shouldn't be dirty, but it could be sticking or not adjusting properly. But yeah, just wanted to give an update in case someone else with this really odd issue finds this thread and gets some insight.

Thanks for the update. I still can't imagine how smoke gets there. It occurs to me that if there'* a crankcase leak there, then if the leak point is experiencing vacuum, unfiltered exterior air would be getting in.

Yeah I was thinking the same thing. I try not to think about it for now haha. My ltft numbers went down to about 5% at idle after I fixed the other vacuum leak so that seems to have been corrected. I was doing some more research and it seems that on 3800 blocks there is what looks like a cover similar to a timing chain cover that you would see on the other side of the block. It has a gasket like one too. I'm not sure if its on both N/A and supercharged blocks but video shows it on a N/A 3800.

After watching the video I highly suspect that is where it is leaking out of. Can't comment on what the guy in the video is saying about them commonly blowing out. Nonetheless its pretty interesting to think how many 3800s may have this gasket compromised. Theoretically if the tear is high enough and small enough it wouldn't leak oil to give any indication anything was wrong. Really annoying that its a pretty simple fix but after replacing the motor mounts/front end (which requires you to pull the entire sub frame on this car in particular), and rebuilding the valve body/channel plate to fix the known long shift and TC shudder issues on jack stands in a driveway I'm just not going to deal with it right now. I just had a wheel alignment on top of it. I suspect this issue has been there a while. I'm not the original owner and it wasn't necessarily maintained well prior to me owning it. If anything I'm glad this is documented for someone else who may run into this odd issue.

I think if I smoke tested mine someone might call the fire department. I replaced gaskets for leaks etc. but never touched the hard-to-reach ones except for replacing the UIM over 100,000 miles ago.

Funny you say that. I always think someone is going to think the car is on fire when I break out the smoke machine. The only gaskets I've heard that are recommended to be replaced besides the UIM and LIM are the valve cover gaskets. Apparently like the plastic intake egr design flaw, the 3800s had another flaw where leaking valve cover gaskets allowed oil to spill on the exhaust manifolds and start fires. Never seen it or known of anyone who had it happen to them, but I know I've read about it a few places. Looking at the back valve cover I can see how its possible.

Yeah, they commonly leak. I can't see one catching fire unless the leak is really bad or the oil really crummy. I might happen to know somehow that it can leak for quite a while with the only consequence being oil smoke smell coming in the vents when idling.