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Thread: Jeep stalls and backfires

  1. #1
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    Jeep stalls and backfires

    1997 Cherokee Sport 4.0 L6 Automatic 166k

    Gets worse the longer I drive....Cuts out while accelerating, backfires, the more I push the pedal the more it backfires and RPMs drop...After it drops, it surges back up and immediately drops back down...Gets so bad that all I can do is idle until I shut it off and turn it back on, then it runs fine at first, then starts all over again...It's getting worse and worse.....Check engine light is on for a bad O2 sensor, downstream, thats new...The light was on before this started happening...I took the whole top end apart tonight and it's getting a valve job and I'm cleaning the intake....I don't have high hopes those will fix it but I'm shooting in the dark now......

    I have REPLACED
    Coolant temp Sensor
    Fuel Pressure Regulator/Filter
    Air intake temp sensor
    Catalytic Converter
    Both O2 sensors
    MAP sensor
    Idle Air Control Valve
    Throttle Position Sensor
    Plugs
    Wires
    Cap
    Rotor
    Distributor Pickup
    Coil
    COMPUTER
    Fuel Pump (and relay)
    ASD Relay
    Crankshaft Position Sensor

    I have CLEANED
    Grounds
    QD Electronics Cleaner on All sensor and computer plugs
    Throttle Body
    Ignition switch contacts
    Battery Terminals

    I have POURED
    Two bottles of seafoam down the brake booster vacuum hose into the intake
    Fuel Injector cleaner in gas tank
    108 Octane Boost in the gas tank
    PREMIUM fuel last three tankfulls

    I have CHECKED
    Fuel pressure- 49 PSI at idle and under load accelerating (The ONLY time the motor didn't fall on it's face was when the fuel pressure gauge was hooked up..Weird, I thought....But that was when the problem was more intermittant than it is now so I considered it a coincidence.)

    I have SMOKED
    Three packs of cigarettes....Thats alot for someone who doesn't smoke.


    I am at wits end as this has turned out to be the biggest piece of junk vehicle I have ever owned....I still love it though and want it fixed by the end of deer season so I can get out and hunt some birds!

    Any advice appreciated. If it leads to a correct diagnosis, you win free beer.
    Last edited by kc0ded; 11-22-2010 at 08:52 PM. Reason: Added replaced parts
    1997 XJ 4.0 Auto

  2. #2
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    sounz like it could be a vaccuum leak on the intake. as the engine warms up, the "leak hole" could swell and make things worse. also something to look at could be timing or spark plug gaps... good luck.
    "Failure is the opportunity to begin again, more intelligently." - Henry Jeep...Er, I mean Ford.
    http://i297.photobucket.com/albums/m...11091132-1.jpghttp://i297.photobucket.com/albums/m...10814341-1.jpg
    I've got your "Jeep Thing"... and now it burns when I pee. '91 XJ Sport w/ high output. 4". 31's. Rails. D30 and D35 lockrights. Trail-modified bumpers, undercarriage and side panels. Oh, yeah... And my sweet li'l wheelin' dog.

  3. #3
    Club Member spunebil's Avatar
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    I see you listed that you replaced fuel pump but I didnt see a new fuel filter listed.

  4. #4
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    fuel pressure regulator?
    4x4LAND.com 785 862 8008 Mike

    Knowledge is priceless

  5. #5
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    Fuel pressure regulator and fuel filter are the same piece on 1997 models. The fuel pressure never dropped below 49 PSI while testing....As for the vacuum leak, I have checked ALL vacuum connections, and replaced any that were even questionable. Sprayed ether at them, wiggled them, listened with my stethoscope, no dice......

    Excellent suggestions though.....I waited three weeks before I admitted defeat and posted on here.....I wanted to run out all the ground balls before posting...I'm stumped.....

    With the timing, It's computer controlled.....Replaced the computer, same exact problem.....In came the sensor ideas, and I started replacing them even if they weren't throwing a check engine code....No timing chain noises....And it runs perfect sometimes.....It runs REALLY rich when it decides to run crappy... The sensors telling the computer to dump massive amounts of raw fuel through the injectors? Why?
    Last edited by kc0ded; 11-16-2010 at 11:46 PM.
    1997 XJ 4.0 Auto

  6. #6
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    Sometimes overfueling is caused by the ecm being given an incorrect engine temperature reading. There is usually two temp sensors, one for the gauge and one for the ecm. I beleive the one for the ecm has two wires and is located right in front in the thermostat housing. the one for the gauge is located in the rear corner of the cylinder head, drivers side. The one in the thermo housing feeds the ecm and could be telling the ecm it's really cold and needs alot of fuel. Have seen them fail before causing overfueling.
    Last edited by 4x4 LAND; 11-17-2010 at 01:40 AM.
    4x4LAND.com 785 862 8008 Mike

    Knowledge is priceless

  7. #7
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    have ya checked that thing on the firewall (can never 'member th' name of it) that is comparable to a mass air flow sensor?
    btw, good call on the temp. sensor, Mike.
    "Failure is the opportunity to begin again, more intelligently." - Henry Jeep...Er, I mean Ford.
    http://i297.photobucket.com/albums/m...11091132-1.jpghttp://i297.photobucket.com/albums/m...10814341-1.jpg
    I've got your "Jeep Thing"... and now it burns when I pee. '91 XJ Sport w/ high output. 4". 31's. Rails. D30 and D35 lockrights. Trail-modified bumpers, undercarriage and side panels. Oh, yeah... And my sweet li'l wheelin' dog.

  8. #8
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    I will check the temp sensors when I get the motor put back together...Still waiting on the guy for the valve job.....I just went downstairs and looked and there isn't one on the head itself, but I see the one on the thermostat housing.... Wouldn't the computer produce a code if it didn't read normal after warm up?

    I think you're talking about the MAP (Manifold Absolute Pressure) sensor....On the previous models they were mounted on the firewall....On mine it's on the throttle body, and it's one that I changed on my sensor changing binge.........
    Last edited by kc0ded; 11-17-2010 at 05:47 PM.
    1997 XJ 4.0 Auto

  9. #9
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    the temp sensor will not set a code or light.
    4x4LAND.com 785 862 8008 Mike

    Knowledge is priceless

  10. #10
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    Update......

    Temp sensor didn't work.

    Re-rented the fuel pressure tester kit. Before the schrader valve on the tester broke I was able to find out when it starts the backfiring and loss of power, the fuel pressure was dropping down to 20 PSI, sometimes 10 PSI. Also, when I shut off the key the pressure didn't hold like the book says (not supposed to drop below 30 PSI within 5 minutes, it immediately dropped to 10 or 20.....So I......

    Replaced the ridiculous $116.00 fuel pressure regulator/filter...Blew through the fuel line to the tank and the return line, no obstructions....Installed brand new (warranty) fuel pump and pickup sock. I even bought a new gas cap.

    While the head was off, all carbon was removed from head, intake, valves, valve seats, and I replaced the valve seals since I was in there anyway. There was no indication of any leak between the cylinders through the head gasket. The areas of the cylinder walls I could see were nice and smooth, no pitting or damage. It doesn't smoke or use any oil. New intake/exhaust gasket, no leaks.

    It seems like once the throttle goes beyond a certain point, the problem starts and gets worse and worse.

    Should I cut the muffler off? It feels and sounds like it's flowing like it should...I know it will sound like a big dirt bike but I'm dumbfounded. I miss my Jeep. I feel like Dr. House trying to figure out what nobody else can. Any chief Jeep diagnosticians in the house?

    Vehicle Speed sensor??????
    Last edited by kc0ded; 11-22-2010 at 08:54 PM.
    1997 XJ 4.0 Auto

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