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Unread 04-01-2012, 11:44 PM   #1
KYJeepGirl1980
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Symptoms of bad fuel pump or fuel pressure regulator

I know there are a lot of posts on this topic, but I still cant quite figure my issue out. My jeep is running fine and no changes in fuel efficiency or performance. The only issue I am having is that I have to push the accelerator all the way down while turning the ignition to get it to start.

I am wondering if it's the fuel pump? Or could it possibly be the pressure regulator? An how long can I drive it with this issue?

I am living in a new town away from my regular jeep guy, and a local mechanic me it was the fuel pump without even looking at my jeep, so I dont really trust him. Would appreciate any advice on what it might be, dont really know the technical aspects of the fuel system...

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Unread 04-02-2012, 12:47 AM   #2
keepishop
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Sounds a bit like a TPS issue.
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Unread 04-02-2012, 01:18 AM   #3
robsjeepxj
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Could be either, I'd start with the TPS since its easy.
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Unread 04-02-2012, 01:19 AM   #4
DullesJeep
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Does the fuel pump prime up okay when you turn the key to the on position (without starting)?
You should hear a hum from the rear of the Jeep.

Could be like Keepishop said, a TPS issue, could be a bad fuel pump relay or ASD relay, but I bet you a new fuel pump, that it's not your fuel pump.

Start with the simple things first before listening to anyone who uses a jump to conclusions mat as a diagnostic tool for troubleshooting a Jeep.
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Unread 04-02-2012, 01:22 AM   #5
robsjeepxj
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Moving the gas pedal has no effect on fuel pressure, it just changes the TPS input signal that's why I'd say that sounds more likely.
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Unread 04-02-2012, 05:38 AM   #6
tjwalker
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I agree that the "throttle position sensor" is the primary suspect here. Having to tip in the throttle to start is a pretty common symptom of a faulty TPS.

You can test the TPS; here is more on this engine management 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 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.
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Unread 04-02-2012, 04:24 PM   #7
buildin1XJ
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I'd say TPS I've had the same happen and no codes show up, Just remember no cheap parts
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Unread 04-02-2012, 11:43 PM   #8
KYJeepGirl1980
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Thanks! How long would it take to replace the TPS? I am away from my trusted Jeep man, and I want to make sure someone doesn't say it's a two hour job when it would really take 15 minutes. Also, if anyone out there is in the Chicago area, please let me know if you have recommendations for a mechanic.
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Unread 04-02-2012, 11:58 PM   #9
robsjeepxj
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Its a 15 min. job. It mounts to the back of the throttle body with two small torx bolts. You just remove the hose from the air cleaner, remove the connector from the TPS and unbolt it. You have to twist the throttle a little so the new one will lock in and put all back together. Not hard at all.
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Unread 04-03-2012, 12:10 AM   #10
jeepjunkymike
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same symptoms for a bad temp sender,,,the one on the thermostat housing that talks to the ecu,,,it happened to me and few others,, there's a thread here somewhere,,if you google jeep xj hold gas to floor to start theres many forums that talk about this . change this change that and no fix, 17.00 part might wanna look into that 1st
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Unread 06-12-2013, 03:40 PM   #11
garymiceli
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Tps???

Could the TPS be the culprit if my '98, 4.0, 6 cyl. Cherokee (Sport? Bought used. Has Police Package, no insignia) starts, stalls, starts, stalls, runs, keeps running, stalling, running for a mile or so, then runs for a while longer then tries to stall again and after a few shaky miles, runs just fine the remaning 20 miles to my job. After sitting for a few hours the whole cycle repeats itself. Tried STP Gas Treatment and Sea Foam fuel system cleaner. Computer check shows no codes. Check engine light not on. This started after two straight weeks of rain, if that's a clue.
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Unread 06-12-2013, 04:31 PM   #12
FullMoonMadness
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Quote:
Originally Posted by garymiceli View Post
Could the TPS be the culprit if my '98, 4.0, 6 cyl. Cherokee (Sport? Bought used. Has Police Package, no insignia) starts, stalls, starts, stalls, runs, keeps running, stalling, running for a mile or so, then runs for a while longer then tries to stall again and after a few shaky miles, runs just fine the remaning 20 miles to my job. After sitting for a few hours the whole cycle repeats itself. Tried STP Gas Treatment and Sea Foam fuel system cleaner. Computer check shows no codes. Check engine light not on. This started after two straight weeks of rain, if that's a clue.
My TPS went out on me (97 Cherokee Sport). It showed symptoms of being bad 3 times, two of which occurred during wet conditions.

Look at my thread for more background (posts one and seven, in particular), maybe it will help.

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