Engine intermittent popping noise

[Derpanieux]

Yesterday I was driving back down to Jersey after I spent thanksgiving with my family. While I was there, I also took the jeep to get emissions tested (required in CT until the vehicles 25 years old... my '99 is almost exempt...) but it ended up failing due to an evap leak and an o2 sensor. I figured I would deal with it later, neither are major issues. However, on my drive back the jeep started to make this weird sound. I've included a video of it. Video was taken immediately after a cold startup. The popping noise varies the rpm, definitely loss of power, and when my foot is on the gas pedal, the pedal flutters a little under my foot. Also during thanksgiving, I did an oil change. The level is still reading as being fine on the dipstick. I don't see how the emissions test or the oil change could have bunged up my jeep, but since I have no idea what the issue actually is, I figured I'd include the background info anyways. Its a 1999 TJ Sahara with the 4.0 and AX-15. Any help or insight anyone could offer would be appreciated. I suspect that this is a job I won't be able to complete parked on the street, so I might need to hand it off to a proper mechanic...

 

[CharlesHS]

That popping sound could be a collapsed lifter which would affect the operation and performance of the engine.
I would suggest you pull the spark plugs to see if any of them appear carboned up.
IF one has an excessive amount of carbon on the plug; you might consider pulling the valve cover to see if any of the rocker arms are really loose.

 

[JEEPCJTJ]

... Also during thanksgiving, I did an oil change.

I'd call the "popping noise" something else but whatever it's called:

How intermittent is it?

How long (time, miles) between the oil change and that happening? What viscosity of oil? What brand of filter?

If the oil is correct maybe you just have a filter with a defect (regardless of brand) or maybe something accidentally ended up in the oil pan and is now clogging the filter. I'd probably change it just to see if it helps.

 

[Derpanieux]

I'd call the "popping noise" something else but whatever it's called:

How intermittent is it?

How long (time, miles) between the oil change and that happening? What viscosity of oil? What brand of filter?

If the oil is correct maybe you just have a filter with a defect (regardless of brand) or maybe something accidentally ended up in the oil pan and is now clogging the filter. I'd probably change it just to see if it helps.

It happens quite a lot when I try and drive the jeep, so not very intermittent I would say. The noise started about two days after the oil change, and maybe 200 miles if even that, Most of those miles were cruising on the highway. The oil was kirkland synthetic from costco, 5w30 and the filter was a fram. I've been using the same filter/oil combo for the last two oil changes with no issues. Also, I chose to call it a popping sound because I have no idea what it is. No keywords I could think searching online led me to find anybody describing the same symptoms as my jeep right now. If there is a proper word for what is happening I would love to learn it.

I took the jeep to a mechanic to get his opinion on it, and he said he suspects there might be an issue with a connecting rod. He advised me not to drive it any more than necessary and I would be inclined to agree. One thing he commented on is that is gets worse when the throttle position changes suddenly, and he thinks that is the rod slapping around on the crankshaft when theres a sudden pressure change. Would like to hear some thoughts on that?

 

[AjRagno]

No codes?

The video is very short but I swear my engine makes the same noise when I’m spraying throttle body cleaner into the IAC, bogging it down.

If you have/get a scanner with live data you can see the fuel trim and O2 sensors readings.

 

[Goatman]

So, listening on a phone. But sorta sounds like either a backfire or an exhaust leak on 1 cylinder. Again, only hearing this on my phone.
Any chance you could have gotten a bad tank of gas? Also check your spark timing to see if it is still set right. Then weight till dark and open the hood and look for any spark jumping on the plug wires. This would be my first place to start without a code reader. Double check that the plug wires are in the correct position. If they got knocked off at the Sniffer Station they could be replaced out of order. Also, what do the plugs look like?

 

[Derpanieux]

No codes?

The video is very short but I swear my engine makes the same noise when I’m spraying throttle body cleaner into the IAC, bogging it down.

If you have/get a scanner with live data you can see the fuel trim and O2 sensors readings.

I do not have a code reader, but I do know that the O2 sensors are bad. From what I read about online, when the PCM knows that the O2 sensor is bad it doesn't use the data from it. My understanding is that the PCM will use hard coded data to determine fuel trim instead of sensor data.

So, listening on a phone. But sorta sounds like either a backfire or an exhaust leak on 1 cylinder. Again, only hearing this on my phone.
Any chance you could have gotten a bad tank of gas? Also check your spark timing to see if it is still set right. Then weight till dark and open the hood and look for any spark jumping on the plug wires. This would be my first place to start without a code reader. Double check that the plug wires are in the correct position. If they got knocked off at the Sniffer Station they could be replaced out of order. Also, what do the plugs look like?

I suppose it is possible that I got a bad tank of gas, but the jeep behaved normally for ~150 miles on that tank before suddenly having issues. The emissions test in CT doesnt get that deep, they didnt touch the spark plug wires. They mostly just scan the OBD port and then make sure you have a catalytic converter installed. I haven't pulled the plugs yet, so I am not sure what they look like, but that seems to be a common suggestion so I will probably try that next