I bought this jeep a couple months ago, its a 99 4.0. cherokee classic, all stock. At first the oil pressure was sitting around 40-60. It still runs that on occasion however starting a few weeks ago, the oil will randomly rise to 80 on the gauge, and im pretty sure its going over 80 and maxing the gauge. I probably should have done an oil change as soon as I got it but I waited till last night to do it. It had a fram filter on and a nice rich black... I replace with mobil 1 full syn, 10w-30 and a mobil 1 filter. At first it was doing just fine, 40-60. And I just got back from a drive that had it running up in the 80's again. The only other things it does is 1) have a check engine light(it was the thermostat and o2 sensors, replaced all of those and never cleared the light) and 2) It occasionally likes to bog down a little or shift from overdrive into 3rd when I'm cruising on the highway at about 55-60 for a couple seconds.

Now that Ive got all that out, who has an idea for me??

 

From what I understand, high fuel pressure is nothing to worry about. When it reads high, it's more than likely a problem with the oil pressure sending unit. Disconnect it and inspect it. It'll probably be nice and caked with black gunk. Clean it and reconnect it and that should fix your problem.

This is what I'll be doing soon because mine also reads high and I'm hoping it'll fix it

 

1. If a problem is truly gone, the check engine light will go OUT, but the code remains stored for a period of time unless you clear it. If you still have a light, you still have a problem. Read it again and post the EXACT codes here for comment. Don't describe the codes, post the exact code numbers. With OBD, the devil is definitely in the details and interpretation is very often NOT straight forward.

2. Most high oil pressure issues are not truly an oil pressure issue at all. They are the fault of the oil pressure sending unit, located directly above your oil filter. If you buy it, buy it directly from Jeep for the best reliability/accuracy/mating with your factory gauge. Well worth the extra $20....

3. For your bogging symptom, first review tuneup hardware status. Fresh Champion copper plugs gapped to .035, new plug wires, distributor cap and rotor. If symptom continues, clean both the throttle body and idle air control and test your "throttle position sensor". More on that below.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS 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.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted acordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

 

sportxj, Ill try that out next weekend, hopefully that'll solve the problem.

Walker, I appreciate the post, if my buddy has the means to do this then ill get back to you next weekend. Otherwise its gonna be an after christmas thing.

I think after the oil annoyance, I'll go for a nice tune up and then look into the tps

 

If the gauge is right there is blockage on the system. That kind of pressurize will open the relief valve and oil will loop back to the pan. If the gauge is right you need to flush the engines.

 

well its not replicable, just does it randomly. not even depending on a long uphill climb or anything. First time it did it was down a mountain, even when i just put it in neutral. And if i do end up replacing that sensor, where would I buy it straight from jeep?

 

Is go to rockauto but that's me. Its a pretty common sender.

 

Yeah you can get it from pretty much anywhere. Regular auto part stores like autozone and napa carry it. I'd probably also go rockauto