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Unread 12-06-2010, 04:02 PM   #1
thundering89
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idle problems// backfire// no power on accelaration

hi, i am new to this jeep forum.
I have been having a big problem with my 92 jeep cherokee. sometimes it will idle fine but sometimes it will start sputtering. it has started developing a popping in the intake, such as backfire.

when i put it in gear it begins to pop in the intake manifold really bad and the jeep doesnt go anywhere. i try slamming the gas pedal to the flood but it stays around idle RPMs and continues to sputter.

i have been chasing this problem for a while now. ive read many different threads and ive replaced the basics. plugs, wires, cap, rotor, fuel pump, filter, CPS,
i have a new ECU and new injectors in the mail. they should be here today

can someone give me an idea to what else it could be?

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Unread 12-06-2010, 04:38 PM   #2
hookuspookus
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Quote:
Originally Posted by thundering89 View Post
hi, i am new to this jeep forum.
I have been having a big problem with my 92 jeep cherokee. sometimes it will idle fine but sometimes it will start sputtering. it has started developing a popping in the intake, such as backfire.

when i put it in gear it begins to pop in the intake manifold really bad and the jeep doesnt go anywhere. i try slamming the gas pedal to the flood but it stays around idle RPMs and continues to sputter.

i have been chasing this problem for a while now. ive read many different threads and ive replaced the basics. plugs, wires, cap, rotor, fuel pump, filter, CPS,
i have a new ECU and new injectors in the mail. they should be here today

can someone give me an idea to what else it could be?
timing chain
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Unread 12-06-2010, 04:59 PM   #3
tjwalker
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I'd get a fuel pressure gauge on the schrader valve and verify fuel pressure, even though you have replaced the fuel pump. A backfire in the intake can indicate a lean fuel/air mixture. Regulator is still in play.

Your fuel pressure might be okay, but 5 minutes with a fuel pressure gauge will tell you if it is or not. It needs to be ruled in or out early in the troubleshooting process.

Also testing of the MAP sensor would be a good idea as that contributes to fuel/air mixture.

Checking your computer for any stored codes another good idea. Yours is old enough where you can use the key-on, key-off technique to check for those codes.
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Unread 12-06-2010, 06:37 PM   #4
rafterman45
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I'm now suffeing from a similar problem. It will idle "ok" once warm, but no power, intake backfire, stall and die
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Unread 12-06-2010, 06:57 PM   #5
thundering89
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Quote:
Originally Posted by tjwalker View Post
I'd get a fuel pressure gauge on the schrader valve and verify fuel pressure, even though you have replaced the fuel pump. A backfire in the intake can indicate a lean fuel/air mixture. Regulator is still in play.

Your fuel pressure might be okay, but 5 minutes with a fuel pressure gauge will tell you if it is or not. It needs to be ruled in or out early in the troubleshooting process.

Also testing of the MAP sensor would be a good idea as that contributes to fuel/air mixture.

Checking your computer for any stored codes another good idea. Yours is old enough where you can use the key-on, key-off technique to check for those codes.
i replaced the fuel pressure regulator and the MAP sensor also. i did a compression test on the cylinders to rule out worn valves. each cylinder was between 100 and 110 psi.
i couldnt find my fuel pressure gauge so ill have to run to the shop tomorrow and pick one up.
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Unread 12-06-2010, 11:10 PM   #6
thundering89
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i put the new ECU in and nothing. the jeep still runs horrible.
the next thing is checking the fuel pressure and timing chain.
is there any way to adjust the timing on a 92 cherokee?
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Unread 12-07-2010, 02:51 AM   #7
CJ7-Tim
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Test the Throttle Position Sensor and the O2 sensors.

For the HO years, 91-02, the O2 sensor has 4 wires. One wire is the 12-14 volt power that comes from the O2 sensor heater relay on the passenger side firewall (unless they moved them in later years to the relay & fuse boxes?), and that powers the internal heater in the sensor so that the sensor can work at idle, and almost immedietly after start up. Loss of that power will hurt gas mileage even with a good O2 sensor.

Two black wires are grounds for the heater power and O2 signal to the ECU. A poor ground will limit current to the O2 sensors heater or cause an error in the O2 sensor output voltage read by the PCM (ECU) causing poor mileage even with a good O2 sensor. Test the 2 ground wires with power off. Test between the wire end at the sensor and the battery negative ground post. It should read less than 1 ohm.

The last wire, #4, is a signal feed wire, 0-1 volts, from the O2 sensor to the PCM (ECU) sensor. The O2 sensor is an O2 concentration sensitive variable voltage generator. At optimal O2 concentration the O2 sensor puts out 0.45 volts.

At idle that voltage should read 0.1-0.9 volts oscillating quickly back and forth roughly once every second. At 2000 rpm it should run between 0.4 and 0.5 volts max (in park). A digital meter can NOT be used for reading the O2 sensor voltage, but it can be used to test the grounds and the 12-14 volts to the heater. You must use an old style analog meter with the needle gauge on the display to see the voltage swing back and forth.

If the O2 sensor readings are not right, say they read .1 volts or .8 volts steady, you have a problem. BUT before you blame the O2 sensor make sure it has good wiring, and make sure the proper voltage is feeding it, by turning power on, engine off to read the engine off voltage feeds (12-14 on one, and ensure the ground wires (power off) reads less than 1 ohm to the battery negative post.

If any one of the grounds or the O2 sensor to ECU/PCM wires reads a high voltage chances are the wiring harness is damaged and the 12-14 volts for the O2 sensor heater is getting through a bad spot in the wires insulation and contacting a ground wire or the O2 sensor wire feed to the ECU/PCM. This is not the O2 sensors fault, but a wiring problem.


A leaky exhaust system or leaky fuel injector(s), or bad compression or leaky valves, bad plugs, wires, cap, rotor, HV coil, and so on, or combination of these, can also cause a lean or rich condition that gives you high or low O2 sensor readings that are not the O2 sensors fault, so try and verify those other items also before buying parts like an O2 sensor to fix your problem.
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Unread 12-08-2010, 02:02 AM   #8
thundering89
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checked everything. timing looks good. no worn cam lobes. hooked up the car to the modis scan tool and all sensors working good. still waiting for the injectors to get her. it idles fine and runs good before 1500 rpm, once you get to 1500 rpm or more it starts popping real bad in the intake.
This problem is boggling my mind.
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Unread 12-08-2010, 04:30 AM   #9
cruiser54
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Have you checked the fuel pressure yet?
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Unread 12-08-2010, 10:41 AM   #10
thundering89
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Originally Posted by cruiser54 View Post
Have you checked the fuel pressure yet?
fuel pressure at 15 psi. the range for my jeep is 12 to 15, so it is good
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Unread 12-08-2010, 05:01 PM   #11
cruiser54
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Quote:
Originally Posted by thundering89 View Post
fuel pressure at 15 psi. the range for my jeep is 12 to 15, so it is good
Where in the world did you come up with that spec for your fuel pressure?

It should be in the high 30s.
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Unread 12-08-2010, 05:27 PM   #12
Topgun514
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I had the same problem, my TPS had been installed by some idiot (Myself) and the sensor was not underneath the throttle jam itself. I would check your TPS first.
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Unread 12-08-2010, 05:38 PM   #13
cruiser54
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Quote:
Originally Posted by Topgun514 View Post
I had the same problem, my TPS had been installed by some idiot (Myself) and the sensor was not underneath the throttle jam itself. I would check your TPS first.
You would check the TPS first when he has only 15 PSI of fuel pressure and 31 to 39 is required?
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Unread 12-08-2010, 05:42 PM   #14
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by cruiser54 View Post
Where in the world did you come up with that spec for your fuel pressure?

It should be in the high 30s.
Cruiser is right on.

I believe that the fuel pressure at idle should be approx. 31 psi.

If you don't have these numbers, STOP and resolve this!
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Unread 12-08-2010, 06:05 PM   #15
thundering89
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Quote:
Originally Posted by cruiser54 View Post
Where in the world did you come up with that spec for your fuel pressure?

It should be in the high 30s.
woops. i was looking at the wrong section
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