2003 2.4 backfires on acceleration

Sputter

Sputter

TJ Enthusiast
Supporting Member
Joined
Aug 13, 2020
Messages
326
Location
Jacksonville
I have a 2003 2.4 that I am rebuilding for my daughter. This thing was in rough shape when I got it but at least it was running. Since I had it, I have replaced the radiator, hoses, CPS, tps, battery, terminals, air box, fuses, coil, coolant sensor, map sensor, oil sensor and O2 sensors. I also had the pcm rebuilt because it failed and I rebuilt part of the engine wiring harness.

So I finally have this thing running again but I still have an issue with the fan relay and backfires after the engine is warmed up. Relay I am addressing in another thread. The backfires are what is concerning me now. When I start up it idles smoothly around 850. Once it reaches operating temperature though it starts to stutter. If I step on the gas it almost dies and backfires.

I have codes for the transmission (which I think it low fluid), p1491 for the fan relay (unrelated) and small and major leak detection.

I thought sure the stock air box might resolve the issue but no joy. Any suggestions on what to check next?

Also jeep has no catalytic converter.

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Took it for a test drive - if you can call it that. Jeep wouldn't let me accelerate and just idled and stuttered down the road, backfiring along the way. Barely got it back home. When I did, the dash gave me additional codes - P0320 and P0700. I am researching the codes but please let me know if you have any other thoughts on this.
 
Flying blind in respect to codes.
Sounds like a timing issue concerning the backfire. When engine is cold it should idle higher than when warm. When the engine warms up the computer switches from an open loop (no real computer function based on O2 sensors) to closed loop (computer dialing in fuel levels based on O2).
So when warm it should kick the idle down to it's normal rpm's. So I would verify your timing.
Secondly, you may have low fuel pressure causing the lack of acceleration when you step on the gas pedal. This is an easy check using a gauge at the Schrader valve on the fuel rail.
Now, I totally suck at all things computer. Go by what the codes tell you first. It could be as simple as one of the sensors you replaced either reading wrong or not plugged in right.
Hope this helps. Shafer tree mechanics aren't as useful as we used to be.😥
 
Goatman said:
Flying blind in respect to codes.
Sounds like a timing issue concerning the backfire. When engine is cold it should idle higher than when warm. When the engine warms up the computer switches from an open loop (no real computer function based on O2 sensors) to closed loop (computer dialing in fuel levels based on O2).
So when warm it should kick the idle down to it's normal rpm's. So I would verify your timing.
Secondly, you may have low fuel pressure causing the lack of acceleration when you step on the gas pedal. This is an easy check using a gauge at the Schrader valve on the fuel rail.
Now, I totally suck at all things computer. Go by what the codes tell you first. It could be as simple as one of the sensors you replaced either reading wrong or not plugged in right.
Hope this helps. Shafer tree mechanics aren't as useful as we used to be.😥
Click to expand...
Thanks! I going to run down the codes a bit more. Going to replace the CPS with a mopar one coming in on Friday. After that, I will need to get some tools for measuring fuel pressure. Will post back later this week.
 
Installed a new crank position sensor this afternoon and cleared the codes. Still having problems with backfires and stuttering when you add fuel. New symptom though. Hissing noise on throttle. Maybe vacuum leak?

 
What happened to your problem? I also have a backfire in the 2.4L exhaust system. There are no stored codes, the jeep has no power and overheats the mufflers. The fuel mixture does not burn in the engine, but in the exhaust system.
 
Still waiting on the PCM to get back before I do anything else to it. Will post once I have updates.
 
So SIA Electronics couldn't fix my PCM. They couldn't get the fan relay to engage the way it was supposed to. They recommended I replace the unit. So I ended up ordering a replacement PCM for $400. Should get it in about a week so we'll see how things go. In the meantime I have plenty to keep me busy. New PCV, plugs, wires, evap canister and rear driver brakes will take my attention in the interim. Will update once I have new details.
 
Sputter said:
I’m crying “uncle.” Going to send the pcm off to SIA before doing anything else.
Click to expand...
Before doing anything else what brand of O2 sensors are did you install? NTK? If so, good! Bosch? Bad, they don't work well in our Jeeps and are known to cause problems.

If they aren't NTK replace them with NTK. A bad/incorrect O2 sensor will definitely cause your exact symptom that starts only when the engine has started to warm up but doesn't happen when cold.
 
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