97 XJ 4.0 - perfect idle but choking after pressing gas pedal

[vaco]

Hi!
I had some issues that I managed to solve, however there is still one problem with my XJ. It starts perfectly (except for few times, I will describe it), idles perfectly, drives perfectly (almost always), but sometimes when it is idling and I press gas pedal then it chokes for half a second and then most often it works normally till I get to idle and try to rev it again, rarely engine stops, and when it stops it needs a bit more time to start again than usual. The more rapidly I press the gas the stronger the effect is, but it is always very short (less than a second). It also sometimes jerk a bit during driving, but the effect is very subtle.
What I checked:
- TPS output on oscilloscope looks good, there are no 'holes' or 'peaks' on the graph when the effect occurs. TPS is a cheap replacement, I ordered Mopar TPS already, however I do not expect any change
- MAP - I have two MAP sensors, cheap replacement and the one that was in my XJ when I bought it (might be Mopar), there is no difference when I change them. Output also looks good on oscilloscope (I do not know what should I exactly get, but voltage rises when I open the throttle and there are also no peaks and holes in the graph)
- IAC - I also have two IACs, Meat&Doria and WVE, it does not make any difference which one is installed
- CPS was replaced with new NTK to solve another problem, so probably it is not the problem source

I will be grateful for any help.

 

[Anak]

Have you taken off the throttle body and cleaned the IAC inlet/port?

 

[Saudade]

Did you measure the fuel pressure using a gauge?

 

[vaco]

Have you taken off the throttle body and cleaned the IAC inlet/port?

Yes, I did it twice.

you measure the fuel pressure using a gauge?

Yes, I did it few times, it is 46-48 PSI. I do not think that fuel pump/filter could be the reason, because problem occurs just in the moment when I press the pedal when engine is on idle, just after that I can rev it to any RPMs or drive up the hill without any problem.

 

[Tim_MN]

The TPS is a highly likely suspect. Output voltage testing of the TPS should display an approximate range of from .25 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle. You probably should test the Coolant Temperature Sensor also.

TPS failure may have one or more of these Physical Symptoms:

1) The engine loses power and is stalling.
2) The engine will idle, but may die as soon as you press the gas pedal. When driving, it seems as if all power is gone.
3) Sometimes it feels as if the transmission is failed or isn't shifting properly, if at all. If you quickly jump on the gas you might be able to get the transmission to shift, but it won’t shift properly by itself. Shifting manually, the transmission goes through all the gears.

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

TPS TESTING

The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the
position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. It is best 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. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.5 volts at WOT wide open throttle.

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:

• 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 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 excessive misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.


Cheap crappy Chinese "Lifetime Warranty" parts are often out of specification or even failed right out of the box. Many times they have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep sensors.

 

[vaco]

I am 99% sure that TPS is ok, I checked it with oscilloscope and I cannot see anything worrying. From the described symptoms I have only choking/dying after pressing pedal and very rarely hasitation while driving (it happened two or three times). I will have new Mopar TPS and also MAP sensor in a week or two, I am going to install them and see what happens.
I observed coolant temperature with OBD app and it also looked ok, however my app for OBD interface has about 1 Hz refresh rate, so I could miss short distortions in the signal, I will also check it with oscilloscope tomorrow.

 

[vaco]

Ok, so TPS was not ok, however I have no idea what was wrong with it, as measurements seemed to be as they should. I replaced it and engine works perfectly...

 

[Ecomike]

Ok, so TPS was not ok, however I have no idea what was wrong with it, as measurements seemed to be as they should. I replaced it and engine works perfectly...

MAP sensor or vacuum line leak in the Vac lines in the MAP sensor lines when rapidly accelerating could have been the issue, not just the TPS.

 

[Ecomike]

Three things tell the ECU it needs more fuel, and right now. The TPS, the MAP sensor and a split second later the O2 sensor