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.
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.
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.
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.