Fulton_Hogan

TJ Enthusiast
Supporting Member
Joined
Nov 11, 2020
Messages
470
Location
Florida
In December I experienced a flashing check engine light while driving. I replaced all 4 O2 sensors and those seemed to smooth everything out.

That thread: https://wranglertjforum.com/threads/check-engine-light-flashed-while-driving.67087/

Since then I've experienced two instances of hesitation during acceleration; the first one was driving at about 55 mph and I began to accelerate climbing a hill. It was like I had a governor on the vehicle. I did a code check with the key and ignition; no stored codes. I decided to buy a Bosch fuel pump/module because it seemed like it was a fuel-related issue. The pump arrived and I continued to drive, running the fuel to almost empty in preparation for the pump change.

Before I was able to change the pump, I experienced the second hesitation. This time it was while accelerating through a flat turn. I also got the flashing CEL. I could replicate the hesitation this time, by accelerating over 3,000 rpm and each time it felt like it was starving/choking out. When I checked the code (both key/ignition and with a scanner) I now had code P0431.

The next day did some diagnostics with my OBD scanner. I checked the voltages of all the O2 sensors, thinking maybe one was bad. All 4 were working but both sensor 2 sensors reporting wildly fluctuating voltages. My thought process was, if the catalytic converters were functioning as intended, the post-cat sensor voltage should be like .5 V or something close. My readings were all over the map, swinging from numbers like .015 then .70, then .40, then .30, etc. and they changed rapidly. I turned the engine off, let it cool, and tightened up the bolts where the pre-cats connect to the manifold. If those were loose then air being sucked in could skew those numbers. I ran the scanner on the sensors again, no change. Now I started thinking, this is probably going to be expensive.

While letting the vehicle idle I kept hearing this intermittent, deep rattle. I thought it was the shift fork so I pressed in the clutch. Nope. The rattle was still there. It sounded like it was either under the engine or low on the left side of the engine so I broke out the stethoscope and prodded around. All the engine's moving bits sounded like they were moving as they should. Then I checked the pre-cats. No mistaking that (or the exhaust) was the area of the noise.

I checked for similar stories here and saw a post from @DropTopDon. His video is very similar to the rattle/rumble I'm hearing. https://wranglertjforum.com/threads/is-my-catalytic-converter-about-to-shit-itself.63293/

So it seems like what I've been experiencing is clogged/failing catalytic converters and the vehicle is choking out or the computer is unable to adjust fuel trim to keep things running smooth during acceleration. Or some combination or both.

Initially I was just going to do the fuel pump and then check the sensor performance; however, the rattling tells me I should probably start with the catalytic converters and then do the fuel pump.

Does that sound like I'm on the right track?
 
Sounds to me like you just need new cats. The solution to mine was three new cats and all new 02 sensors. If you are hearing any rattling noise in the cats then they need replacing
 
Last edited:
  • Like
Reactions: Fulton_Hogan
The flashing CEL indicates a misfire so bad that the cat will be damaged in short order. Basically, fuel was being dumped into the cat and melted it. The real question is why was that occurring? Don’t know how long it an in that condition but I would replace the cat like DropTopDon suggested and I’d hold off on the fuel pump. Just make sure you monitor for misfires or the new cat will be trashed shortly. Also, you can also tap on the cat with a rubber mallet to listen to it rattle and the muffler to see if any cat chunks are stuck in it
 
Sounds to me like you just need new cats. The solution to mine was three new cats and all new 02 sensors. If you are hearing any rattling noise in the cats than they need replacing

It may help to know that we've replaced many. The mini cats did not rattle at all since the element broke up completely, exited the shell entirely and wound up clogging the main cat.
 
The flashing CEL indicates a misfire so bad that the cat will be damaged in short order. Basically, fuel was being dumped into the cat and melted it. The real question is why was that occurring? Don’t know how long it an in that condition but I would replace the cat like DropTopDon suggested and I’d hold off on the fuel pump. Just make sure you monitor for misfires or the new cat will be trashed shortly. Also, you can also tap on the cat with a rubber mallet to listen to it rattle and the muffler to see if any cat chunks are stuck in it

This is one of my concerns. I thought it might be that the vehicle is not getting enough fuel, running lean due to the pump's age or crud in the fuel pump. (All this started when I was just above 1/4 tank.) Because it's running lean there's insuffient gas to fully combust (the weak acceleration) and then any unspent fuel (from poor combustion) enters into the cats and overheats them.

One additional thing I noticed, when I started all the diagnostics, I believe the system started in closed loop; which was odd. I disconnected the O2s to force an open loop condition, just to test the scanner. It ran fine without them, and all 4 sensors threw codes. I let it go a while and reconnected everything, then cleared.

Edit: Vehicle does start in open loop, then switches to closed loop, as would be expected. I just went to double check this. Disregard the "started in closed loop" comment in the previous paragraph.
 
Last edited:
It may help to know that we've replaced many. The mini cats did not rattle at all since the element broke up completely, exited the shell entirely and wound up clogging the main cat.

It does help. Thanks.

However, I'm just a dude with an average garage and some tools; hoping I don't misdiagnose, get in over my head, and end up dorking up my Jeep.

(Side question, @mrblaine : I know you're limited to what brands/models you can use for CARB standards, but do you have experience with Magnaflow kits? If so, thoughts?)
 
Last edited:
Check your freeze frame data to hopefully identify the cylinders in which the misfire was occurring. If that’s not an option, do a power balance test to isolate them. Either way, gotta determine what‘s causing the misfire. Fuel, ignition, or compression.
 
  • Like
Reactions: Fulton_Hogan
I replaced my cats twice due to having a misfire issue that wasn't corrected prior to the first cat replacement. The first replacement cat failed again in less than 2 years. My misfire was cured by a Standard Motor Products coil rail and then I installed a Pacesetter 324236 3-cat setup. It's been working well for the last 6 or 7 years. I'm also running a Magnaflow cat-back system.
 
Check your freeze frame data to hopefully identify the cylinders in which the misfire was occurring. If that’s not an option, do a power balance test to isolate them. Either way, gotta determine what‘s causing the misfire. Fuel, ignition, or compression.

I wish my scanner was that complex.
Freeze frame is good at showing all the things that happened in the moment, but I don't know if it gets to the cylinder level of detail.
As for tests, some seem like they work, others, not so much.
 
I replaced my cats twice due to having a misfire issue that wasn't corrected prior to the first cat replacement. The first replacement cat failed again in less than 2 years. My misfire was cured by a Standard Motor Products coil rail and then I installed a Pacesetter 324236 3-cat setup. It's been working well for the last 6 or 7 years. I'm also running a Magnaflow cat-back system.

Thanks for this. I know its all (air, fuel, spark, compression) interconnected, and that's what has me concerned. I do not want to flub it.
 
Power balance test can be done without any computer tools. Is your engine running rough right now? If so, start disconnecting injectors. If the engine runs worse then that cylinder was more than likely fine. If you disconnect an injector and there’s no change to the way it idles then that’s your bad cylinder.
 
  • Like
Reactions: Fulton_Hogan
I'll be honest, I had to look up how to do that one. Seems easy enough to do. I will add it to the list of things to check.

As far as mine goes, it doesn't really have a rough idle. The only time I'm noticing any loss of power, or 'off' performance, is when I'm already at speed and need to increase speed. It's typically been when I'm in 3rd gear and hit over 3,000 rpm.
 
Sounds like a misfire under load. Generally that’ll be your spark plugs or coil. Take it for a spin, get it to misfire, then see which cylinder is misfiring via the code. Then you can play the “swap a plug and injector” game to determine the faulty component. Either the injector, plug, coil, or bad compression.
 
  • Like
Reactions: Fulton_Hogan
Power balance test can be done without any computer tools. Is your engine running rough right now? If so, start disconnecting injectors. If the engine runs worse then that cylinder was more than likely fine. If you disconnect an injector and there’s no change to the way it idles then that’s your bad cylinder.

Ok, power balance test went as well as I could do it. Each time I disconnected a connector, the engine would lug/rough idle a bit. It would return to normal once reconnected.

I couldn't undo the #3 injector, it's almost upside down under the bracket by the intake. That one is for a later date.
 
Sounds like a misfire under load. Generally that’ll be your spark plugs or coil. Take it for a spin, get it to misfire, then see which cylinder is misfiring via the code. Then you can play the “swap a plug and injector” game to determine the faulty component. Either the injector, plug, coil, or bad compression.

I've never had it give a code for any of the cylinders, plugs, or anything like that while under operation and it acts up.

The only time I get those specific codes is while it's in the garage and I disconnect something for a test. Then it's an immediate CEL and corresponding code.

The only code I've had associated with the hesitation is the one I mentioned in my first post, P0431.
 
I also gave things a stethoscope listen again after I tested the injectors.

The rattle noise is definitely more present in the second (bank 2) pre-cat. Which jives with the P0431, "Warm Up Catalyst Efficiency Below Threshold (Bank 2)" code.