Coast2coast

Hi All, I am new to the forum & need some help & advice.

I am in the UK & have a 1998 Cherokee 4.0 Petrol Auto 4 x 4

The vehicle starts & runs OK on tickover but sometimes when you are driving it it starts to miss & feels like it is overfuelling because if you put the accelorator flat on the floor & rev the engine it clears.

I have changed the distributor cap & leads, spark plugs & rotor arm but no change.

I wondered as this is fuel injected if it could be the ECU / Brain or has anyone experienced this before & can advise.


Thanks Gordon

chill

found a problem like that on a YJ. ended up being the distributor. I wouldn't jump to the ECU yet. Lots of small stuff to check first. what plugs are you using? cam or crank sensor? any check engine light?

Donnie_K

under the rotor arm, that big black piece of plastic plate looking thing, pull it up and look between the magnets on the backside, clean? coveried in rust? what about the half cup looking thing beneath that that passes between those magnets when it's installed? That plastic piece with the magnets would be your camshaft position sensor and it's what controls the timing. Along with the crankshaft position sensor anyway.

Donnie_K

Also, I forgot to mention in my last post... pull the throttle body off, take the sensors out of the throttle body and give everything a good cleaning... this sounds like an IAC problem.

tjwalker

Test your way to a solution.

1. Test the "throttle position sensor". More on that below

2. Test your fuel pressure. For a 98, you should have 49 psi at idle, plus or minus 5 psi.
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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.

Coast2coast

Hi, Thank you all for your messages. I will have a look at the points mentioned over the next few days & will put a reply up as soon as I have solved the problem.

Thanks Gordon

Coast2coast

Hi All, Thank you for the advice. I have cleaned out the distributor & also the throttle body & it runs like a dream now.

In fact the best it has run in the 2 years ive owned it.

So a big than you for the advice & have a great xmas & a prosperous new year.

Thanks Gordon