samhain

ok i just bought a 99 xj 4.0. started doing the basic maintenance. replaced spark plugs with factory copper champions and wires but now when i rev the motor it takes a while for the idle to come down. any thought? its not throwing a check engine light either. another thing, the O2 sensor after the cat, just before the muffler is cut off. what would that do? shouldnt that be throwing code?

Roler

Was the plugs the only thing you changed, and was the idle normal before you changed them?

The high idle is regulated by the ECU.An broken/faulty/cut off O2 sensor prevents part of the 'feedback' system to send info to the ECU, which calculates hoow to manage the air-fuel balance. High idle could be from a couple of things, but that 02 sensor needs to be replaced for the above reasons. Start there and gives us the result so we can guide the advice from there.

Once again, it could be other causes (the IAC not functioning properly is one other common on, but since I reckon you didnt change anything else, that might be for later), but this will aid in the diagnostic part of things.

Roler

(following is copied from recent post on other subject by respected member TJWalker and might be helpfull in the thought process).
Once again, a possible cause.


• Poor idle control: The TPS (throttle position sensor, mounts on the throttle body at the level of the throttle valve) is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.

samhain

Yeah the idle was fine before I did the plugs and wires. And now the rpm is slow to come down when I rev it. I'm trying to fix a misfire at idle that is not throwing a check engine light. The needle doesn't jump around but you can definitely feel the shake

Roler

In addition to the above mentioned: a simple thing:
if something is playing up AFTER you changed something, go back and check over everything you have done, because it could be well caused by something you did.

Check the plugs being tight enough, make sure the wire cables are snug on tight on both ends (one loose one might very well cause the misfire issue) etc etc. Every step you've done. Check if you didnt nock around any sensors/harness, stuff like that. Before your intervention is was fine (regarding the misfire), so you'll have to rule out you changed something for the worse.

On the side: Do you have any issues when you drive it?

samhain

Yeah I well double check the wires and plugs. I haven't driven it yet as I just changed the plugs and wires a few hours ago. as for the misfire, I can only feel it during idleand it doesn't matter if i'm in gear or not. Its just weird that its not throwing a check engine light. The bulb for it does work as I do see it when I turn the key

Lewark

Not to make you sound stupid but you did check all the gaping before you put the plugs in? Always check every new plug, just because one is gaped right doesn't mean they all are and never assume they are gaped right from the box

samhain

Yeah I did check them they were already pre gapped. I listen to the motor and it sounds like a mini train. Kinda like when a motor spots out a plug. Should I pull them out and re do them? A for the rear o2 sensor that has been cut, where dots it plug into?

samhain

Ok so I rechecked the rear o2 sensor and this is what I see. The cat converter does have a o2 sensor directly bolted to it. There is also another o2 sensor between the cat converter and the muffler that had the wires cut off. Does this mean that someone before me installed an aftermarket cat converter and just cut off the factory oxygen sensor ?