toasterknight

I have had my xj for about 9 months now and I have fixed nearly all of its problems but one or two. I am most concerned about the intermittent power loss I am getting.

Occasionally when you give it a bit of throttle it bogs down pretty bad. Today the engine just cut out and then caught itself and started to go again. These issues typically only happen when the jeep is first started, first or second throttle application.

I have replaced both o2 sensors as well as the exhaust from the header back. Have also replaced the plugs, cap and rotor, as well as wires.

The only CEL I have gotten were a misfire in cylinder 6 months ago, o2 sensors grounding out on the driveshaft before that, and the time I cracked the oil pressure sensor during an oil change.

My thought on the issue are CPA, vacuum leak, or cracked header. I am looking for any advice anyone may have as a novice mechanic and first time jeep owner.

cruiser54

Ever cleaned your throttle body?

Courtesy of TJWalker:
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is retracted, the air control passage flows more air which raises the idle speed. When the valve plunger is extended, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea. This should be part of a normal tune-up procedure and whenever idle/stalling issues are present.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a Torx driver (2 bolts; one can be kind of hard to get to). On 91 and later, it may be easier to just remove the whole throttle body. Be sure to use a new throttle body to manifold gasket when reinstalling.

“Gently” wiggle out the IAC from the throttle body. Gasket/O-ring on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and oxygen sensors. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body bore itself, the butterfly valve inside of the throttle body and it’s edges, and all associated linkage as long as you have things disassembled

Revised 10-6-2014

toasterknight

Yeah I cleaned about 2 months ago.

cruiser54

Have you ever changed plugs, wires, cap and rotor?

EZEARL

Might not be causing your problem but the IAC could be not up to snuff. Here's the test specs.


contact order in IAC A B C D (A to D and B to C should test 40 to 80 ohms)
(B to A and C to D should read open)

toasterknight

I have changed them. It's paragraph 2 or 3 in the original post.

tjwalker

Is the check engine light on OR has it been on? If so, you will have "stored codes" and you must retrieve them with a code reader or a scan tool.

The "throttle position sensor" is in play here. Here is more on this very important sensor.
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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 accordingly 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 denied acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable 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.

toasterknight

Sorry for the delay on updating the thread checked the tps and didn't find anything wrong with it. When I started it without the sensor plugged in it threw a code and ran pig rich like you would expect.

I think the exhaust manifold is cracked because I can hear a noise that isn't the injectors when its running. Could that be part of the issue?

I also noticed that the exhaust tip is shooting up pretty quickly which I assume is another sign it is running rich.

sycoglitch

Cool. So you check the TPS' voltage range. Did any part of the range jump or stick at a voltage reading?

toasterknight

I was using a digital voltmeter but it seemed smooth to me.

sycoglitch

Got. Wanted to make sure there wasn't a spot where the same reading came up

toasterknight

The original problem was only intermittent so I could have been testing it on a good day and that's what worries me.

toasterknight

So I tested the tps again today and got some really weird results. Just testing the connector I had the ground on the left 4.6v in the middle and 5.2v on the right. Those were when it was not plugged into the tps. I then plugged it back in and and back probed the center wire and things got even stranger. Closed throttle was -.2 to around -1.2 volts. And it didn't change voltage at all during the sweep. I thing I may have screwed up the test but I tried 5 or 6 times to back probe it and I stayed negative the entire time.

cruiser54

Ever do Tip 5?

toasterknight

Do you know where the ground for the tps runs to in a 99. I will check them at work tomorrow as its too dark to see.