Hello my name is Jerry. I have a few questions regarding a 98 Buick LeSabre 3.8 L engine.
Could I swap out both the upper and lower intake manifold with everything still in tact such as injectors and etc..
If I have another engine on hand that I could swap out with?
I have the original engine that was swapped out from this car that had some problems and was stored for part. I thought maybe I could swap out both the lower and upper manifolds if I unattached what was necessary to swap them. The lower intake has a big crack running along its side just underneath the upper plastic intake. Since the other engine looked in good shape I would swap out along with new gaskets. Is this possible or even a good idea to try? Thanks for any help guys

So long as it has everything the other has. If it has extra sensor it can be ignored but it must have everything else or be able to be brought over.

I will double check everything to make sure.
I have been reading the posts about 3.8 L36 and I see there is a series 1 and series 2 engine. How can I find out which series my engine is? And do the parts interchange between the 2 ?

Not off the top of my head. Call a junkyard.

The 1998 LeSabre came with the Series 2. If the original engine was swapped out for another, and the second engine worked without changing out the PCM etc., then the second engine is a Series 2.

The 3800 Series 1 naturally aspirated engine was the L27, roughly 1991-1995
The 3800 Series 2 naturally aspirated engine was the L36, roughly 1996-2003
The 3800 Series 3 naturally aspirated engine was the L26, roughly 2004-2008

. . . and while you're fiddling with all of this, I'd suggest taking the upper and lower manifolds apart, inspect, clean up, re-assemble. The upper intake gasket is one of the parts likely to get tired later in life. You should also inspect the EGR passage that is next to the coolant passage on the upper intake. Make sure it isn't about to fail and let coolant in. Also, replace the fuel pressure regulator if the current one is over five years old.

These cars generally have just about everything except the engine fail before the engine does, unless the upper intake failed. It'* surprising to hear of so many problems on two of them.

Thank you Sir for your time and valuable information that helped me better understand the differences in this series of engine.
I have no prior knowledge or experience with Buick engines and or their terminology. I may make you laugh at some of the questions I ask, so bare with me please.
I’m guessing this particular problem was inherent with the 3.8L with plastic upper manifold? I could see myself misdiagnosing it for a blown head gasket right away.
I will follow your advice and recommendations for this job, once I get started.(hopefully this week) I’m sure I’ll have a few more questions before I finish. I’ll post and update as I progress. Thanks again for everyone’* help.

With the help from this Forum, I was able to successfully replace the lower intake manifold gasket, new upper intake manifold/ plenum and gasket, along with the fuel pressure regulator and new lower temp 180* thermostat. I was hesitant about the lower temp thermostat, because of the pcm, but I figured I could easily replace it with the 195* if suggested to do so.
Most of my time was spent cleaning the mess that the leaking upper manifold created. Everything was covered with the “black glue” as I called it, had to be cleaned, including the old loctite from all threads: internal and external.
Since I already had the other lower intake manifold from the prior engine, cleaned and ready for install, it went together fairly simple. I followed everyone’* advice and read many posts about “what not to do” beforehand, which helped with fear and anxiety prior to starting a new unfamiliar job.
The new parts were purchased of eBay. I went with metal/rubber lower intake gasket and a brand new upper plenum with gasket. Lower Gasket was $55 and the plenum with gasket was $52, fuel pressure regulator $16, thermostat $5
Thank you everyone for your time and especially your advice. Without it, I would had to paid someone for these repairs.

Get er done. Good job.

Glad you got it sorted out!

For what it'* worth, I has a 180 degree thermostat in my 1997 LeSabre for like 40,000 miles. All that I noticed changing from that was the fuel economy went down about 10% and otherwise the car acted about the same. I put a new 195 degree thermostat in it and fuel economy has gone back up. Watching the gauge, I saw a lot more variation in indicated temperature and I didn't like the idea of even more heat cycling that normal. It would go from a few ticks below the old "normal" to a little above before fans would kick in, where before it would only vary from "normal" to slightly above. Maybe I would have gotten better results if I had set the fan-on temperature lower, but the juice wasn't worth the squeeze for me to change stuff around more than that.