Weird exhaust Popping when decelerating

[Big_Mark]

Like the title says, whenever I decelerate, whether under load or coasting, I hear a popping sound coming from, I think, my exhaust. 
Let me say, this has been happening the whole time I've owned this Jeep and i though it had something to do with compression because it happens when I downshift and use the engine to break/slow the vehicle. BUT the other day, I was feeling to lazy to use my hand to shift down, so I pushed in the clutch and let off the gas and coasted around a corner, sure enough the popping began.

 

I have two working theories;

1. The PO put in new injectors (4 holes)I wonder if the injectors are dumping to much gas into the engine at the lower speeds to burn it off, hence popping/backfiring in my exhaust?

2. The CPS isn't signaling the system about the reduction in speed so the ignition is dumping too much fuel.

Do any of you have other ideas?

Has this happened to you?

 

I've done all the grounding, troubleshooting steps in Cruisers guide, and otherwise the engine runs like a top, I'd just like to iron out this wrinkle, thanks in advance!
 

[89 MJ]

Do you have a regular CPS or a high altitude CPS?

 

Immediately after the engine swap in my truck, it started doing this and did so for probably 1500 miles or so. At that point, I switched to the high altitude CPS and have had it happen much less, it maybe happens a few times in the first hundred or two hundred miles of pulling the truck out of winter storage, then it gets itself sorted out. 

 

My other thought would be that it runs too rich with the current injectors.

 

Do you have a REM? I'd be curious to see what it says for O2 information on deceleration.

[pizzaman09]

You might have a small exhaust leak up stream of the cat.  Check that your header isn't cracked and that the conncections between exhaust sections aren't leaking.  I had this specific issue on a vintage car of mine.  

[Big_Mark]

Thanks guys, cracked header seems likely, as for the CPS I am not sure if I got the high altitude one or not, I'll see if I can figure out which one it is since I replaced it a couple of years ago. and no REM here, cept on the stereo! 

 

It's always something!

[89eliminator]

factory exhaust?  mine pops but thats just due to the lack of cat and flowmaster.

[Gojira94]

Our computers do have acceleration enrichment and deceleration enleanment routines. If the truck is running, the computer is working, and it hasn't changed. Foot off the gas, it's trying to reduce fuel and adding a little timing at the same time, as MAP is decreasing. Can you post the part number on one of the injectors? I can tell you if it's a good fit, or good and fat.

[JZLAJeep]

On 10/1/2026 at 7:59 AM, Big_Mark said:

...and no REM here, cept on the stereo! 

As a man that's a certain age to appreciate this, I think this is hillarious!

 

@89 MJ dumb question, but how does one know whether they have a high altitude or regular CPS. Living in Idaho and owning a MJ that was originally owned in Oregon, it's a coin toss on the original quipment.

[89 MJ]

1 hour ago, JZLAJeep said:

@89 MJ dumb question, but how does one know whether they have a high altitude or regular CPS. Living in Idaho and owning a MJ that was originally owned in Oregon, it's a coin toss on the original quipment.

Unfortunately, I’m not sure. I do know the high altitude CPS has 5* more timing than a regular. 

[Ωhm]

 

Bottom one is the High Altitude CPS. Note how its shifted off center of the mounting plate.

[fiatslug87]

And you can make your own:

 

[pizzaman09]

4 hours ago, fiatslug87 said:

And you can make your own:

 

I did this with my factory one when I had my transmission out!  Definitely easy to do if you have it out.

[Big_Mark]

Thanks for the input gang, looks like y'all have assigned me some tasks, 

1. Pull injectors get part no

2. Pull CPS see if it's high altitude/adjust per Renix tips (there always seems to be something new and relevant there) 

 

Once I have these handled, I'll post up thanks again all