P0138 and P0300 and all misfiring

[TJ_Everywhere]

Hey everyone, I have read about the p0138 code along with some other codes in other forums, but I'm a little confused with what I have going on. 1997 TJ, 4.0 manual, 230k miles. I was driving home, CEL came on and threw p0138, p0300, and p0301 thru p0305.
I just replaced the crankshaft position sensor, which solved a previous CEL, drove it for a day now this. Also did a tune up.

I ordered NTK o2 sensors for the upstream and downstream. My question is, can a p0138 cause an engine misfire? I thought it was just for emmisions. Has anyone had this? Could it be a timing issue? Vacuum leak? If it's not the o2 sensors I don't exatly know where else to look.

 

[CharlesHS]

Whatever is causing the misfire can cause the P0138, but I doubt the P0138 would cause a misfire unless the engine was super rich running. The P0138 comes on when the ECM detects an very high O2 sensor voltage (0.90 V) from the rear sensor sensor which means the engine is running very rich which can be caused by a few problems.
Cracked exhaust manifold, bad upstream O2 sensor, bad MAP sensor, even a bad catalytic converter are to name a few.

 

[TJ_Everywhere]

Whatever is causing the misfire can cause the P0138, but I doubt the P0138 would cause a misfire unless the engine was super rich running. The P0138 comes on when the ECM detects an very high O2 sensor voltage (0.90 V) from the rear sensor sensor which means the engine is running very rich which can be caused by a few problems.
Cracked exhaust manifold, bad upstream O2 sensor, bad MAP sensor, even a bad catalytic converter are to name a few.

Thanks for the reply. It is running super rich right now, and given the p0138 I'm inclined to say its a faulty upstream o2 sensor causing the issues. Will update when the o2 sensors are replaced (wednesdayish).

 

[Jerry Bransford]

Go with just placing the upstream O2 sensor, the downstream has nothing to do with any of that. It only monitors the catalytic converter for proper operation. You can return the downstream for a refund.

 

[TJ_Everywhere]

Go with just placing the upstream O2 sensor, the downstream has nothing to do with any of that. It only monitors the catalytic converter for proper operation. You can return the downstream for a refund.

Didn't even think of that. Everywhere I've been reading has said always replace both, but I guess you don't have to. I'll test out replacing just the upstream first. Thanks!

 

[CharlesHS]

Didn't even think of that. Everywhere I've been reading has said always replace both, but I guess you don't have to. I'll test out replacing just the upstream first. Thanks!

Before changing out any O2 sensors; I would connect a scan tool that can do Live Data readings and see what the upstream O2 sensor voltage is providing the ECM. There may be another reason why the ECM is pulsing the fuel injectors at such a high rate; as I stated in my original post.....

The P0138 comes on when the ECM detects an very high O2 sensor voltage (0.90 V) from the rear sensor sensor which means the engine is running very rich which can be caused by a few problems.
Cracked exhaust manifold, bad upstream O2 sensor, bad MAP sensor, even a bad catalytic converter are to name a few.

 

[TJ_Everywhere]

Alright, not sure if anyone is still interested, but I went ahead and finally got the o2 sensors replaced (already had them on order), and for the first time, was able to clear the misfires and p0138. Pretty sure the upstream sensor was biased rich, and now it isn't misfiring. Hopefully this is the fix.