Hello all
Here are the basics. I have a 94 Jeep Cherokee with manual transmission. It has a straight 6 in it. It has 153,000 miles on it. It has recently had the TPS and CPS replaced.
I have been having a serious issue with it – even when those things were replaced – and I am hoping (HOPING) someone can understand me.
The issue – Keep in mind if it is in neutral and you are going along or even rev the engine up – it is normal. I am driving along – in 1st, 2nd, 3rd and 4th – and now slightly 5th – it will buck like a crazed lunatic anytime the gas it barely touched – like if I press down with a toes strength, and it also happens when I lift off of the gas pedal. It also happens in gear not touching the gas at all. It just goes nuts. It literally throws you – it also does this in reverse. The cold and heat do not matter and it has gotten worse over time. Now, to top it off, while I am driving along at a “held” speed you can feel it hesitating as we are going along – little skips, sometimes larger than others. Noticeable. I have no clue anymore – is it my slave cylinder? My clutch? My engine mounts are fine – but what is up with it doing this bucking thing when I tap and lift off the gas as lightly as I do? Why does it throw me to and fro when just switching gears – it is nuts. Any help, as you can obviously see, would be greatly appreciated. Thanks everyone.

 

Have you verified all cylinders are reliably firing? Test spark on all cylinders. Check fuel pressure.

This is all assuming that the engine RPM changes with the bucking. If Engine continues to sounds steady and the car is bucking then drivetrain is suspect.

 

Hey everyone - thanks so much for all the help this far. Here is an update. Now, to add insult to injury, just yesterday - after my original post, I left work to find another issue. I started her up, and had her in neutral - now, if I ever so lightly press the gas when it is in neutral it goes to almost stall, and then kicks back into normal mode - but it does it every time with just a light touch to the gas pedal. Ugh. I am so sick of this - I thought the clutch too, but maybe gas - too many things to know, but I was hoping someone out ther may have had experienced similar. Keep the help coming - I appreciate it soooo much!

 

Any check engine light?

 

The fact that tipping in the throttle creates symptoms leaves fuel pressure and electronic controls for fuel high on the primary suspect list.

1. Did it have these exact same symptoms before and after your TPS replacement? Where did you buy your throttle position sensor at? I'd test it or consider replacing it with one from the Jeep dealer. Plenty of crap TPS sensors available today.

2. You MUST verify fuel pressure with a fuel pressure gauge. Many parts stores will rent you this tool inexpensively. For a 94, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure regulator and the pressure should jump to approximately 39 psi. Be sure that vacuum line is pulling good vacuum.

3. Check for codes, even if you don't have a check engine light. You can have "stored codes". How to easily do this shown below
-----------------------------------------------
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.
-------------------------------------------------------------

OBD1 Models: 1991-1995
1) Start engine (if possible). Move transmission shift lever
through all positions, ending in Park. Turn A/C switch on and then off
(if equipped).
2) Turn engine off. Without starting engine again, turn
ignition on, off, on, off and on within 5 seconds. Record 2-digit
fault codes as displayed by flashing MIL.
3) For example, fault code 23 is displayed as flash, flash,
4-second pause, flash, flash, flash. After a slightly longer pause,
other codes stored are displayed in numerical order. When MIL begins
to flash fault codes, it cannot be stopped. Start over if count is
lost. Code 55 indicates end of fault code display.

 

Did it do this before you replaced the TPS? How old are the spark plug wires and plugs?