RPM fluctuation, vibrations

Hi, first post, yay! This forum is a real treasure trove, so thank you all for keeping it alive.

I have a problem with my 1997 Jeep Cherokee, 4WD, 4.0, Auto, NV242. I did post it on two separate forums, but no luck. My local mechanic also has no idea.

The problems:

1. Slight fluctuation of RPMs in idle, barely visible on the gauge.
2. Shudder/vibration while in idle.
3. Lack of power in the low range (but it might be my imagination).
4. 10-12 MPG highway/city. Conservative driving.
5. New one, last 3 days: weird start when it got cold outside.

I have a video of a start and idle here. The problems are not that visible, but it might help. The RPM problem starts at 0:55. I did try to catch the shudder but no success.

http://www.youtube.com/watch?v=7k5AIIEMBF8

What was done:

1. New plugs, wires, distribution cap and rotor.
2. New manifold gasket.
3. Checked for vacuum leaks, but no idea if done right.
4. New IAC.
5. Cleaned connectors for O2, TPS etc.

Thank you very much for any help rendered .

Miles?

The O2 sensor, or wiring, circuit... may be having a problem. The TPS may be worn and giving part of the problem. Could be both!

http://www.naxja.org/forum/showthread.php?t=1012701 is all about Jeep O2 sensor testing.

TPS testing for wear requires an old style analog volt meter, needle gauge style to test the resistance, power off, for a smoth transition from idle to Wide open throttle.

Have you checked to see if one cylinder is missing, like a bad plug, low compression, bad injector on one cylinder?

Thanks, I will try to diagnose the O2 sensor. If I am correct Mopar = NTK? Thank you for the write up.

Auto is running fine, shifting a bit high though, TPS might be next.

The Jeep has 130 000 miles on it.

I will post results for future generations once done.

It took a while but I was able to connect an OBDII scanner. It provides no codes. Would a faulty O2 sensor show lean/rich codes? Or is it possible it won't show anything?

I will try to get a voltmeter hooked up tomorrow.

Thanks!

irontom said:

It took a while but I was able to connect an OBDII scanner. It provides no codes. Would a faulty O2 sensor show lean/rich codes? Or is it possible it won't show anything?

I will try to get a voltmeter hooked up tomorrow.

Thanks!

Click to expand...

I had an O2 sensor go bad and did not give a code. The mileage dropped from 20 MPG's +/- to 12 MPG. The other tell tale sign was the rich gas smell at the exhaust and a sooty black tip.

My 4.0L always seem to have a skip at idle but not enough to try to fix. When it skips, the RPM would fluctuate about 100 RPM and bearly descernable on the RPM guage but the trained ear could hear it.

I bought an OBDII->USB cable. It works fine, but won't show info for the O2 sensors (voltage). Looks like I will need to hook up the voltmeter after all. Or I just need to stop being so cheap and buy new sensors :>.

So, this happened:



But it runs better now. I had to drop the electric fan to get back home (it did impact the RPMs whenever it turned on now when I think about it), the radiator was punctured.

What gives?