My 1990 just got a new heater core this weekend. It would have been a quick and easy job, except that the new core I bought was a different part. I fired up my old '76 Lemans and ran back to O'Reilly to swap it. I took in the leaky one for comparison. He said that my car is actually a 1991. At least that'* what the heater core catalog suggests. The leaky one looked just like the 1991.
How do I determine the build date? What other parts are different from a 1990 to a 1991 LE? Was the guy at O'Reilly just trying to save face because they sold me the wrong part?

VIN is L, title says 1990. I didn't see the parts manual in the store so it makes me wonder. I like the store, good prices, I seem to get the wrong part about half the time. How do I determine the build date?

How much different can a 90 LE be from SSE for a heater core I recently changed mine and got it from Canadian tire just asked for a 90 sse no problems .Try another place and see what happens.

i was about to have a fit when you said vin L (vin L is the l27 engine which our generation bonnes didnt get). uh the build date should be on the sticker on the drivers door.

The eighth digit of the VIN shows the engine code, which is C. The tenth digit, which is the one we were discussing (see previous posts in this thread) is L, which signifies a 1990 model year car.
The sticker on the driver'* door says May of 1990. I guess that was late enough in the model run to start phasing in the new stuff. I wish I could have used the 1990 one because the lower nipple on the core was angled up slightly. The mechanics at the dealership must have complained that it was too easy to reach this way so they changed it. The other difference was that the 1990 was noticably thicker than the 1991.
Just noticed that the owners manual is for a 1991. All the paperwork says 1990.

What is your build date? I think that will be the difference in our cars. If my theory proves true, yours will be sometime before May.
Didn't you pay like $120 for yours? Mine was only $40 US. I don't know what the exchange rate is like...
Like I said, I like the store, the prices are usually much better, they have a large warehouse/store in town, and for the most part the staff is reasonably intelligent. I have found them to be the best in my area based on my personal experience with other makes and models of cars over the past 25 years that I have been working on my own vehicles. Getting the wrong part isn't always the fault of the store or parts clerk. Sometimes they are numbered wrong at the point of manufacture, and as is probably the case in my... case Pontiac, in their infinite wisdom saw fit to change heater core styles in the middle of the model year and didn't bother to tell anyone. Or maybe they just ran out of 1990s toward the end and started using 1991s because that was what they had laying there handy. At least the SOB doesn't leak anymore. Now we're having short-sleeve weather

Build date is on the sticker inside the drivers door..