1997 Grand Cherokee Assorted Problems

[hammmy]

4 main problems:

Rough idle - When cold, (first run of the day), the Jeep starts fine, but after 1-2 minutes the idle becomes rough, with erratic loping. After another 1-2 minutes, the idle revs suddenly, and smooths out immediately after.

Stalling - When cold, whenever I release the accelerator at low speeds or at a stop, the tach drops like a rock, and either stalls or recovers. After warming, this is not a problem, or at least it doesn't stall.

Brakes - Front discs have warped for the 2nd time in 30K miles. Can you suggest an aftermarket replacement to solve this?

Noise - The front left makes a whirring/whining noise when driving at 50-60mph. The noise increases in frequency in relation to speed, and vice-versa.

Troubleshooting - The TPS reads 600mV-3.85V, closed to wide-open throttle. Haynes specs are 200-900mV to 4.5V. Should I consider replacement? Fuel pressure reads 45psi, with Haynes specs 44.5-54.5. Is there a way to clear codes without an OBD-II scanner?

Update--Replaced Idle Air Control valve after I found erratic test readings. Stalling problem appears to have been alleviated, but rough idle problem still exists. I'm going to test the O2 sensors next; I suspect the upstream sensor is inaccurate until the engine warms up completely, which is causing the rough idle after the PCM runs the open-loop fuel mixture mode on start-up.

Update #2--Replaced upstream O2 sensor ($75 from Jeep dealer-includes connector pigtail). This seems to have fixed the rough idle problem.

Something I forgot--does anyone know how to take dried sap off of a car's paint? There's an evil tree that needs to die...

Also, just to save time, and in the faint hope someone posts about my brake problem, here's what I know. I'm currently undecided between buying slotted front rotors, or just plain-jane Raybestos. I've read somewhere that the issue causing the JGCs to warp rotors is that there are two different alloys in the stock rotor; one for the rotor surface, and one for the hub. This leads to unequal heating characteristics, which leads to warping. Assuming the Raybestos plain-jane variety are single alloy, that would be my first choice, lacking any further info, because it's a difference between $30 and $150 per rotor.

As for the front-end noise, I have no idea where to start. I guess I'll start with the wheel hub.

Jeez louise, I just remembered something else. My Quadra-Trac full-time 4WD has stopped showing an indicator on the dash, as well as on the onboard console computer when I use 4-low. I'm fairly certain that the system does actually engage in 4-low, as the steering and handling characteristics change substantially. Anyone know of a sensor that senses the 4WD position/mode? Haynes has no useful info on this.

Update #3--Replaced front rotors with plain vented Brembo OEM replacement rotors. Also replaced all pads with Satisfied Pro pads, all from tirerack.com for $160, including shipping. Broke in the pads and rotors by doing 12 consecutive progressively harder stops from 60-5 MPH.

Regarding the 4WD not showing a 4-low indication on the Vehicle Information Console (VIC): I've found a page that details a possible solution (http://www.thelylefamily.com/jeep/4wd/zj_4wd.htm).
I have an NV 249 transfer case, and I couldn't seem to find a connector that matches the one illustrated. I may just try a part swap to see if that is indeed the case.
Can anyone tell me a good way to verify that the transfer case is in low range? I had it up on jackstands today, but as I have the full-time Quadra-Trac it makes it difficult (for me) to be certain that the low range was engaged.

[raf1618]

Hammmy,

Iam having the same problems with my 1995 JGC, I pulled the Idle control sensor and cleaned it good- there was some black stuff-carbon? on the plunger and it seemed to help with the loping problem. I'am still having the rough idle and need to know which "upstream O2 sensor" you are talking about. And can you think of anything else that maybe causing the rough idle. The veh runs great on the road and I'am getting 19-20 mpg.

Randy

[hammmy]

There are two oxygen sensors. For the 5.2l V8, one of them is located before the catalytic converter, and is referred to as the "upstream (of the catalytic converter) sensor". The other is located after the catalytic converter, before the muffler (I believe), and is referred to as the "downstream sensor".

The upstream sensor is actively involved in the fuel/air mixture process, as the Powertrain Control Module (PCM--computer) monitors it constantly (or nearly so) in order to adjust the fuel/air mixture so as to produce the desired exhaust emissions.
If this O2 sensor produces erratic or erroneous readings, the PCM will make incorrect fuel/air mixture changes, which can result in a rough idle. However, the PCM is supposed to set a trouble code, and then switch to what is called an "open loop" fuel/air mixture program, which sets the mixture according to specifications stored in the PCM's memory. The PCM also engages this mode upon initial cold startup of the engine, in order to allow a smooth idle until the O2 sensor has reached it's 600F operating temperature.

The downstream sensor, as I understand it, is used primarily to log the final exhaust emissions composition. It is not actively involved in the fuel/air mixture process.

On California models (after a certain year--unsure which), there are 4 O2 sensors--2 upstream, and 2 downstream. This is also true for the later year (99ish) federal (49-state) models.

I'm deleting this bookmark--fair warning that I won't be around to elaborate further, although this should certainly be enough material to understand (basically) the O2 sensor operation. Good luck.