Welcome to 2CarPros.
For you to get a backfire through the intake, most likely the fuel mixture is too lean. I know that doesn't sound right because you replaced the pump and filter, but it can be a vacuum leak causing it, especially if you blew the air cleaner off.
Do me a favor, here is a link that explains in detain causes of backfiring and how to fix them. Take a look through this link. I will explain what to look for and how to do it. If it doesn't help, let me know and we'll work together on it. I will tell you that I would check both fuel pressure and for a vacuum leak first. I would also strongly recommend checking for diagnostic trouble codes.
This vehicle has what is called and OBD1 system. To retrieve trouble codes that could point us to the exact problem is very easy to do and doesn't require a scanner. Instead, all you will need is a 3 inch jumper wire or even a paper clip.
I am going to list different links to help. Take a look through them and let me know what you find. Also, if you find a diagnostic trouble code when checking, let me know what it is.
https://www.2carpros.com/articles/engine-backfires-while-running
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator
Here is the link for retrieving trouble codes on this vehicle:
https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method
Please keep in mind, there could be other things causing this to happen. Components such as a bad crankshaft position sensor or camshaft position sensor can do it too, but that's why I need you to get codes.
Let me know if you have other questions or need help.
Take care,
Joe
SPONSORED LINKS
Wednesday, June 26th, 2019 AT 8:32 PM