Rough idle and running rich

[AaronC.]

Hey fellas, I need some help here....'97 TJ 2.5L manual, 98,xxx miles. I'm having running issues that I cannot figure out. The idle bounces around in the 750-850 range almost rythmically. It runs super rich (7mpg). Driving down the road it just bogs until I hit 3,200 RPM, then comes alive. I replaced both O2's, IAC, TPS, MAP, plugs, wires, distributor, cap and rotor, fuel pump. I haven't found any vacuum leaks whatsoever. I've read many posts on this topic, but have yet to find a solution. What am I missing here??

 

[RMETeeJay]

Did you use the recommended O2s (there is a thread here with the ones to use)?

Hold on to the Mopar/OEM sensors that you replaced unless you're sure they're bad; in many cases, they are preferred.

What does your OBDII show for coolant temp once fully warmed up?

 

[AaronC.]

The original O2's are long gone. I bought denso, but now the upstream is a bosch cause I replaced it again about 6 months after initially changing it. I did not pay close attention to the coolant temp on the scan tool, but the gauge reads just under half when at op temp.

 

[RMETeeJay]

The original O2's are long gone. I bought denso, but now the upstream is a bosch cause I replaced it again about 6 months after initially changing it. I did not pay close attention to the coolant temp on the scan tool, but the gauge reads just under half when at op temp.

Sorry, I should have said that the 02s don't need to be kept; they are a wear item.

Check the thread on O2s, but I believe that Bosch are a no-go for TJs.

If that doesn't do it, then you can verify your coolant temp with a reader (I use a cheap bluetooth one with my phone).

 

[AaronC.]

It seems NTK is the preferred brand for O2 sensors so I ordered one. I'll find out in a couple of days if that's the ticket, but in the meantime I'm open to other suggestions.

 

[Jerry Bransford]

The original O2's are long gone. I bought denso, but now the upstream is a bosch cause I replaced it again about 6 months after initially changing it. I did not pay close attention to the coolant temp on the scan tool, but the gauge reads just under half when at op temp.

Bosch O2 sensors have earned a terrible reputation for not working properly in our Jeeps. The only brand that should be used is NTK or (rarer) NGK. No need to replace any O2 sensor but the upstream (top) sensor which is the one that the computer uses to control the air/fuel mixture. The bottom O2 sensor on the catalytic converter does nothing for the engine, its sole job is to monitor the catalytic converter for proper operation.

And if your engine runs and idles well when it's cold but the problem starts once the engine begins to warm up, that's a good indication of a defective upstream O2 sensor.

 

[AaronC.]

It idles at 1,000 rpm it first (and smoothly) but after a couple mins it drops to the 800 range and starts running rough. If I just jump in it and go while it's cold it runs ok, but not for long.

 

[AaronC.]

Just changed the upstream O2 out for a NTK and wouldn't you know it....all better. I'll have a mopar TPS here in a couple days to change out the autozone one as well. Thanks for the info and advice. One last thing..... is it effective to clean fouled spark plugs or do they just have to be replaced once they're fuel fouled? Only a week old.

 

[RMETeeJay]

Just changed the upstream O2 out for a NTK and wouldn't you know it....all better. I'll have a mopar TPS here in a couple days to change out the autozone one as well. Thanks for the info and advice. One last thing..... is it effective to clean fouled spark plugs or do they just have to be replaced once they're fuel fouled? Only a week old.

Did you verify coolant temperature with a reader yet? It’s important for it to reach and stay at about 210 as quickly as possible to avoid running too rich.

Once you know coolant temp is good, install the TPS and clean the plugs (only if they’re the recommended type of plugs):

https://wranglertjforum.com/threads/what-are-the-best-spark-plugs-for-my-jeep-wrangler-tj-4-0.4196/

Then I would recommend resetting the ECM to force it to relearn trims with your new sensors:

https://wranglertjforum.com/threads/any-benefit-to-resetting-pcm.15762/post-254238

 

[AaronC.]

Ok. Just to get ahead of the potential issue....what brand coolant temp sensor and what brand thermostat are recommend? Just in case I have to go down that road.

 

[jrmcafee]

@AaronC. I’m working through a very similar issue on my 98 2.5. Mine has no throttle after cold crank. It dumps completely. After about a minute of idle it does alright. I had a muffler shop to replace the cat, both O2’s, fuel pressure regulator, and coolant temp sensor. I’d already changed plugs and cleaned the IAC and replaced the TPS. I bought an NGK upstream O2 and took it to them but they ended up putting in a Denso and a Walker because they were cheaper. I made sure they noted that on the invoice so they wouldn’t charge me for labor when I have to take it back.

Following this thread. Do you have any issues at cold crank with throttle or is yours backfiring?

 

[AaronC.]

No I didn't have those identical issues, but I will say they're not kidding when they say to use mopar sensors. I wish I had known before buying hundreds of dollars worth of trash . Anyway, I suggest changing your upstream O2 and TPS (at the very least) out for Mopar. That is what fixed my issues that I had been chasing for over a month.

 

[AaronC.]

I finally got everything straightened out. Changed the thermostat and it is no longer running rich and wasting fuel. There is barely a difference on the temp gauge, but it did the trick. Thanks for all of the advice and good luck to anyone chasing a similar problem. Hope this helps!

 

[Jerry Bransford]

I finally got everything straightened out. Changed the thermostat and it is no longer running rich and wasting fuel. There is barely a difference on the temp gauge, but it did the trick.

You meant the coolant temperature sensor, not the thermostat right?

 

[AaronC.]

No, the thermostat. It wasn't getting all the way up to operating temp.