Backfire through throttle body
 

I have a '90 B250 with a 5.9. As I accelerate I get an occasional back fire in the throttle body. It did this same thing last fall,I found that it had a bad injector,so I replaced them both. Throttle body gasket looked pretty bad so I replaced it as well. As well as the TPS and the coil. It has ran fine up until a couple days ago when I got the first back fire. Now they are getting much more common again.I am noticing a major drop in fuel economy,but its not running as rich as it did last time (yet). Any ideas from the Mopar guru's?

The two most common reasons for backfiring is the timing chain has jumped a tooth or the fuel pump is not putting out enough pressure.

Thanks. Would a jumped timing chain back fire be more consistent? Mine is kind of sporadic,only seems to happen in the first 3rd of throttle at a slow acceleration.

Do a search in the forums here, I typed in your description (Backfire through the throttle body) html Here's an example of one thread in 2nd gen trucks (this one being a 95 ram)
https://dodgeforum.com/forum/2nd-gen...-and-more.html
Some of the threads related to your own description have some good info for help. As Alloro said, it could be timing, slack in chain, fuel pressure issue or as other threads have suggested... pre cat 02 sensor failing, cat converter clogged, engine coolant temp sensor sending bad signal etc.
Read a couple 3-4 that ended with it was a failing crank sensor that fixed the issue, one I think was the pre cat 02 that fixed it. They recommended using oem mopar or delphi rather than aftermarket for replacement. I'd say read through a few and find your similarities of the issue and their solution.
Have you checked for any fault codes using "the key dance" http://www.allpar.com/fix/80s-codes.html

Jfloors thanks for your help. I have checked codes and nothing but the same old trans code that I've had since I've owned her is there. But i just took a peek under the van and I don't see a O2 sensor anywhere in my exhaust. Is it possible that a '90 doesn't have one? What would be the best way to check for a clogged cat?

Issues like this are rarely consistent due to variables like temperature, atmospheric pressures, amount of fuel in the tank, just to name a few.

Check your exhaust again, probably have to crawl under the van. O2 sensor will be between the engine, and catalytic converter. It's gotta be there somewhere, otherwise, the computer would be whining about it.

Yeah, if your van sits low it'll be difficult to see and get at but the sensor should be there like HeyYou stated. Mine has plenty of clearance to roll around underneath, you may need to get yours up on ramps. Here's one of quite a few videos for testing the cat.
Do a search for How to test 02 sensor once found too before just buying one, as it may not even be your issue. It's easy n all you need is a basic inexpensive multi meter, Harbor freight or Walmart will have something that will do.

Hey good luck

Hey thanks JFloors, found the o2 sensor on the driver side manifold. I'll be testing the cat and sensor tomorrow morning. I cant drive it any longer like this. It seems to run great early in the morning while the outside air is cool. I dont know if that means anything,but it didn't backfire till it got warm outside.

Thanks for all of y'alls help. I'll keep everybody posted.

Welp........... I tested the Cat and it was good. Installed a new 02 sensor,no change. Brought it to my mechanic to test the timing and fuel pressure,both are good. I'm at a loss again.