I have a new to me 08 JKU purchased in July this year. It 139kmiles on it. has always, since I've had it, slowly bleed off speed while trying to maintain cruising speed. I'll get to say 65mph, then after a few moments without moving the pedal I'll notice I have list 5mph and need to accelerate. This is on "flat" road or not going up a hill.

I've never had a vehicle behave like this.

It threw a P0430 and randomly throws P0420. I replaced spark plugs and wires the codes came back two weeks later.
I think this bleeding off speed thing could be the cause but I don't know what part is failing to cause the symptom.
I thought maybe TPS or APP sensor so looked up symptoms of failure for both and slowly bleeding speed off with the pedal stationary wasn't on the list.
When I accelerate, it's fine, shifts normally ect.

Any help would be appreciated.

 

Here's the possible causes for P0420, P0430 (cat efficiency) from the FSM:
A skilled tech can look at a graph of the signals (with an OBD2 scanner) from the four o2 sensors and usually narrow it down quickly. I'd bet on either both upstream o2 sensors, or both cats are failing. The bleed-off of speed may be related to how the engine is responding to the problem.

Stay in touch,

 

I did read that before. I've also successfully troubleshot a P0420 from a single bank exhaust vehicle also. I have a graphing scanner.
I've just never seen a bleeding off of speed be a symptom. Usually it's very poor gas milage as a first symptom besides the CEL being on.
Most times the catalytic code is caused by some other thing like an air fuel Imbalance.
I'm thinking fuel pressure needs to be tested to make sure the pump isn't failing. Both fuel filters are in the tank. Clogged filters? Failing pump? Not delivering proper fuel pressure to the injectors causing an imbalance?

There is no exhaust leak and the scanner says the A/F and 02 are working properly. No vacuum leaks to cause improper air input.

Again I've never seen an engine bleed off speed this way so that's why I'm stuck where to go next.

 

The way the pcm detects low cat efficiency is comparing the lean/rich oscillating of the upstream sensor (normal is about 0.1 to 0.7) to the downstream sensor (normal is ~0.75 and fairly steady).

Graph it at hot idle, and steady highway cruise, post a pic if you can. The classic cat low efficiency fail is when the downstream sensors are oscillating similar to the upstream. If the cat was acting like a straight through pipe with no catalytic action, the upstream and downstream would look the same. Actual A/F ratio problems will likely set other codes. Suggest concentrating on the cause of the DTC's. The speed bleed-off is probably related, but a DTC is simpler to nail down.

 

Here's a O2 sensor graph at idle of a normal engine with about 140K miles on the engine/cats and no DTC's.
Top is the upstream sensors (oscillating rich/lean). The lower chart is the downstream (steady and almost the same).

 

Yes. I'm familiar with this. I'll get a another graph.
In this case the only DTC is P0430. Twice stored to trigger the CEL and once later P0420.
Those codes aren't simple to nail as there are many possibilities for cause that don't trigger other codes.