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

[Gojira94]

On 10/3/2026 at 11:57 AM, Big_Mark said:

1. Pull injectors get part no

You can probably get a part number from the side of the injector body. Bosch EV6 (skinny style) will start with 0 280 on one side and possibly an OEM (like Ford) part number on the opposite. Same with the EV1 (fat style). Hopefully zooming in with a smartphone camera will do the trick without having to pull any injectors.

[Big_Mark]

I was able to get to this today.
CPS is the non-high altitude version
 

Injectors have a bunch of numbers,

Side A: 0280155703

I think the first digit is a Zero, but I'm not 100% sure

 

Side B: 5277739

 

 

I have another set of injectors on hand that I suspect are Chinese knock off Volvo injectors to try if the ones I have are incompatible. I bought them off Ebay or Amazon a few years ago, installed them and they didn't work, but I was having other issues at the time so I'm not counting them out if I need to swap in a different set of injectors.

 

 

[Gojira94]

Those 703s are the Dodge Neon injectors that were initially popular some years ago. They really are too fat for Renix or even HO outside of race classes. The orange ones are 0280155700 or 746 knock-offs. Sometimes they work out ok-ish but actual flow and matching between sets will be off and inconsistent. If you’re not opposed to spending $120 - $140 on a properly refurbished set from Ksuspensionfab or another reputable and trusted seller you can have a whole

lot better.

 

The 0280155700 part number would be ideal but they are very scarce these days. A set of 0280155746 would be acceptable but they’re just a tad fat. But the Renix computer can adjust the long term fuel trim to compensate. I’m running a set of 0280155702 and they’re great. They’re in between the 700 and 746 and easily managed by my 429 Renix ECU.

[Big_Mark]

Cool, the Neon injectors were installed by the PO, I'll see if I can find some 0280155702's and see if that stops the Pop Pop Pop, thanks for the insight!

 

[Gojira94]

I got mine from here

 

https://www.ebay.com/itm/283645389229

 

Dimar Motorsports. Unfortunately they’re out of stock at the moment. Injector-Rehab has some in the 190-200cc @ 3 bar for our jeeps but they want $175 a set (ouch). They say they’d be yellow, black or blue depending on stock. I’m not familiar with what Bosch number the blue ones would be but I’m curious.

 

https://injector-rehab.com/product/87-98-jeep-4-0-upgrade-19lb-set-6-copy/