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Thread: 99 TJ 4.0 cranks but wont fire.

  1. #1
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    99 TJ 4.0 cranks but wont fire.

    Im about to toss in the towel. Ive done various testing and it has led to part replacements IE: new fuel pump/regulator, new spark plugs, new crank sensor.
    One morning I went to start my TJ and it kept cranking but not fully turning over and firing up. no matter what Ive done, the Jeep is consistently making the same sounds and doing the same thing as that first morning.
    Help please!


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  2. #2
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    Kept cranking but not fully turning over? Does this mean it was turning over normal but not firing up? As opposed to cranking but turning over slowly? Take the battery and starter to autozone or the like and have them tested. If good come back to check spark..new plugs gapped properly and fuel pressure at the rail? Fuel pump running? Listen for it to prime up it normally just runs like 2 second when you turn the key on have someone turn it on you put your ear up to the fuel filler. If you have spark and fuel the next thing is compression or bad fuel. One of those missing maybe relay, fuse wiring or worse pcm. Any codes itís not running so check OBD or dash codes , that rule out the chance that you got bad parts. I had a fuel pump from Autozone that was dead took them 2 tries to get me a good one...donít give up you will learn from it and know your Jeep better good luck


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  3. #3
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    *that...should be donít rule out


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  4. #4
    Meme King WJCO's Avatar
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    Spark, fuel, compression.

    Has to be one of those 3. After replacing plugs, did you verify if you have spark at the plugs?
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  5. #5
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    Quote Originally Posted by Shadowfire View Post
    Kept cranking but not fully turning over? Does this mean it was turning over normal but not firing up? As opposed to cranking but turning over slowly? Take the battery and starter to autozone or the like and have them tested. If good come back to check spark..new plugs gapped properly and fuel pressure at the rail? Fuel pump running? Listen for it to prime up it normally just runs like 2 second when you turn the key on have someone turn it on you put your ear up to the fuel filler. If you have spark and fuel the next thing is compression or bad fuel. One of those missing maybe relay, fuse wiring or worse pcm. Any codes itís not running so check OBD or dash codes , that rule out the chance that you got bad parts. I had a fuel pump from Autozone that was dead took them 2 tries to get me a good one...donít give up you will learn from it and know your Jeep better good luck


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    NO CEL OR CODES.
    Initially I suspected it was fuel related so I tested the pressure at the fuel rail as well as a leak down test. Priming the fuel pump only brought it to 20 psi, and turning the key to try and start it brought it up to 40 psi but a leak down test revealed that it was losing pressure instantaneously and returned to zero within a couple minutes. So I replaced the whole entire assembly for the fuel pump as mentioned before. Upon replacing the pump/ regulator, I tested it again and my reading went to 45 upon priming and turning the key, and also there was no more leak down. (I didnt check the injectors because Ive had them in only for a few months and have periodically checked them from other builds while I had them off the engine and fuel rail. Theyre 12 hole bosch 24lb)
    So I moved on to spark. I pulled the plugs and to my surprise, number six had a broken prong so I suspect Ive been running on 5 cylinders for a while. The rest of the spark plugs were definitely fouled so I replaced them all and got a set of pregapped .035, and I double checked the gap before installing. Still, the same result as before, the starter was strong but it still wasnt getting the engine to fire up. (Ive been doing all this trouble shooting while jumping my Jeep because without successful starts it has been killing my battery)
    After that I just started poking around the internet. Moved on to the crank sensor, replaced it.. same thing. Did more internet poking (today), replaced the camshaft sensor, same result.
    So far nothing I have done has made ANY difference in the way the Jeep sounds, feels, or does. My results are the same as the morning I tried to start her up but couldnt.


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  6. #6
    Meme King WJCO's Avatar
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    Quote Originally Posted by stevenmuck View Post
    NO CEL OR CODES.
    Initially I suspected it was fuel related so I tested the pressure at the fuel rail as well as a leak down test. Priming the fuel pump only brought it to 20 psi, and turning the key to try and start it brought it up to 40 psi but a leak down test revealed that it was losing pressure instantaneously and returned to zero within a couple minutes. So I replaced the whole entire assembly for the fuel pump as mentioned before. Upon replacing the pump/ regulator, I tested it again and my reading went to 45 upon priming and turning the key, and also there was no more leak down. (I didnt check the injectors because Ive had them in only for a few months and have periodically checked them from other builds while I had them off the engine and fuel rail. Theyre 12 hole bosch 24lb)
    So I moved on to spark. I pulled the plugs and to my surprise, number six had a broken prong so I suspect Ive been running on 5 cylinders for a while. The rest of the spark plugs were definitely fouled so I replaced them all and got a set of pregapped .035, and I double checked the gap before installing. Still, the same result as before, the starter was strong but it still wasnt getting the engine to fire up. (Ive been doing all this trouble shooting while jumping my Jeep because without successful starts it has been killing my battery)
    After that I just started poking around the internet. Moved on to the crank sensor, replaced it.. same thing. Did more internet poking (today), replaced the camshaft sensor, same result.
    So far nothing I have done has made ANY difference in the way the Jeep sounds, feels, or does. My results are the same as the morning I tried to start her up but couldnt.


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    Check the wiring harness everywhere. Look for corrosion, rodent damage, etc. Sounds like pcm or wiring issue. Did this just randomly not start one day or did it die on you?
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  7. #7
    Nothing but a Thing QuicksilverJK's Avatar
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    You said that you replaced the plugs, but did you verify that they are actually sparking?


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    Quote Originally Posted by WJCO View Post
    Check the wiring harness everywhere. Look for corrosion, rodent damage, etc. Sounds like pcm or wiring issue. Did this just randomly not start one day or did it die on you?
    Yes just randomly one morning is when this nightmare began.


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    Quote Originally Posted by QuicksilverJK View Post
    You said that you replaced the plugs, but did you verify that they are actually sparking?


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    I havent checked for spark yet. Ill do that tomorrow.


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  10. #10
    Fresh Catch mntj91's Avatar
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    Iíd also suggest a good strong battery. Have you checked the coil? Engine grounds?

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