What is normal idle speed and how do I read it?

[driggs]

At startup, TPS readout flickers about once every 5 seconds or so. Gradually till fully warmed the TPS flickering increases to about once or twice per second. Fully warmed engine is when the low idle is happening. Also wanted to mention that I see the flickering at all positions of the TPS range not just at idle. Example of TPS: Idle = 0 but flicker reading is 18, Cruising 40MPH = 16 but flicker reading is 27. There is a larger difference at idle than off idle.

I'm seeing the exact same behavior in my 2001 XJ, and am also using a ScanGauge II to help debug it.

Once my engine is up to operating temperature, I idle between 650 - 675 RPM. When idling, I note the TPS reading at 0, but "flickering" to 16 about once per second. Ease on the throttle and the "flicker" frequency is roughly the same, approximately once or twice per second, and the spacing is similar but decreases with throttle; 0->16, 1->17, 2->18, 3->19, 4,5->20, 6,7,8->21.

When I purchased the XJ earlier this year, it had an irregular idle, which I fixed by replacing the spark plugs with the OEM specified plugs, NGK ZFR5N. Several months later idle RPM dropped lower and lower until I had to feather the throttle to keep from stalling at idle; I replaced the IAC and arrived at the present state.

I most recently replaced the air filter, adjusted the throttle valve cable, and disconnected the battery for 20 minutes to reset the PCM; no change to my idle was noted.

 

[Greenz]

I have been asked about this through PM a couple of times now so I'm updating what I know to help anyone else having like issues.

No, I have not fixed the problem yet but I am more comfortable knowing it's not really a serious issue. As you have read what I have done to find the problem and your own troubleshooting it's not a clear solution. I have completely eliminated any possibility of an electrical, fuel, or engine mechanical. My rig is running awesome, as if it were new. Lots of power, starts easy and gets normal fuel mileage for this model. But the vibration persists at low idle. The slight miss is odd but normal. This has been reported by many 4.0l owners and I don't think now that it's the cause of vibration.

Prior the vibration issue, I had driven over a curb between two parking lots. One parking lot was about three feet higher than the other and it bottomed out making a horrible thunk. Chunked out about a six inch section of concrete too, oops. I couldn't see any obvious damage other than scratches on the cross member at that time. well, over the past summer I was underneath changing the TC oil and noticed my cross member bent slightly. Upon further inspection, I noticed two of the four bolts that secure the Transmission mount to the cross member had been sheared off. ....Clue?

I am now thinking that the vibration is coming from the broken tranny mount. The vibration is more noticeable during colder weather. Might be why I didn't notice the vibration issue right away. It could also be worn motor mounts or a combination of the two. I am waiting for a few extra bucks to replace all the mounts. The problem has not got any worse than when I first noticed it so I have slacked off on the "worried about it" part. One more thing to note is, Jeep considers the motor mounts to be vibration dampers as well as mounts. That is why I am leaning this direction now.

The idle RPM's are normal at ~650 even though there is no documentation that specifies exactly 650RPM at idle. My 02 Dodge GC idles at 650RPM same as the XJ and it too now has the little vibration at idle. Another clue! So I really do think it's the motor mounts, tranny mount. Even though they look normal, the rubber can harden over time. I don't know for sure but it kind of makes since and I won't know for sure until they get them replaced.

Scangauge still shows the slight flicker in TPS updates and I now believe that is normal communication condition between the ECU and Scangauge.

Hope this helps...

 

[sbxj]

I replaced my plugs recently with fresh NGK ZFR5N's which the Dealer had previously installed about 30k miles ago.
For the first time I torqued them to the correct spec rather than guessing and noticed a definte improvement in my idle. It still has the 4.0...not as smooth as almost any other engines idle but it's better.
just wanted to pass this along

 

[joe_peters]

I replaced my plugs recently with fresh NGK ZFR5N's which the Dealer had previously installed about 30k miles ago.
For the first time I torqued them to the correct spec rather than guessing and noticed a definte improvement in my idle. It still has the 4.0...not as smooth as almost any other engines idle but it's better.
just wanted to pass this along

Good. Too many over torque their plugs and that effects the gap, and can lead to a rough idle.

 

[sbxj]

Thanks Joe,
Is it true you can have problems from under-torquing as well?

 

[joe_peters]

Yep.

http://www.youtube.com/watch?v=U9OQ9_u5Tng

 

[Greenz]

Thanks guys for the tip on spark plug torque. Many will benefit from this information.

However none of the current symptoms I'm experiencing are coming from over/under torqued spark plugs. Like mentioned above, the ole girl is running great. Idle is normal at 650RPM no A/C on when fully warmed, for this model year. It's not a near stall condition causing a the vibration.

I did read back through the thread and noticed I had not mentioned changing the spark plugs again to NGK ZFR5N. Between that and everything else I had done, as mentioned above, idle no longer dips to 500RPM. 650RPM is as low as it gets, but the slight vibration persists.

I need to get the mounts replaced next in order to eliminate the vibration at idle. I suspect many XJ owners have the same issue but many overlook it as abnormal. After all, It's just a jeep and not new anymore. I am/was detail oriented enough to worry about the slight vibration but no longer see it as a big deal.

I would like to get joe_peters setup with the brown dog motor, daystar poly transmission mounts. That setup alone could cause a slight vibration at idle negating any positive confirmation of the stock mounts being the root cause of the vibe.

In summary:
- idle speed for 01 XJ is normal at 650RPM no A/C and fully warmed.
- My factory tach is spot on accurate compared to Scangauge II.
- original idle dipping to 500 RPM has been fixed, I think by replacing Autolites with NGK ZFR5N spark plugs. I overlooked that as one of the original symptoms because the slight vibe still exists.
- Slight occasional miss is still present but reported by many 4.0l owners as normal.
- Slight vibe at idle is still present, more noticeable on colder days.

 

[cherokey]

I have the exact same problems. Idle at 500rpm (or what I interpret as 500). Tach is half way between 1k and 0. I have yet to do anything.

Off-idle TPS: 0.9v
WOT TPS: just under 4v

I have cleaned the IAC and TB. A long time ago I replaced the plugs hoping to fix the slightly rough idle with no success. Don't remember what's in there now, but they're cheap platinum plugs.

Edit: I have an 01 Cherokee 4.0