Question about 1999 Jeep Grand Cherokee

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Missfireing, compression good, only code on obd11 is po500 [vehicle speed sensor] new plugs , spark from all coils,black sooty build-up inside throttle-body and on old plugs, suspect getting fuel in oil pan

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  • thesprink Jun 02, 2010

    hard to do any tests while car is running, it idled rough, but smoothed-out well at about 2000 rpms.ran it for aprox. 5 to 10 minutes, and its back to missing so badly can barely keep it going. forgot to add it has a 4.7
    v-8. no steam from tail-pipe either, just alittle black exhaust.

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  • Jeep Master
  • 6,826 Answers

Have the fuel pressure regulator checked,if it is not allowing fuel to bypass back to the fuel tank,then the injectors would spray to much fuel,and of course cause a miss fire,also,you may have fuel contamination,try a fuel dryer,and injection cleaner,like lucas for the fuel treatment,and heet for the water in fuel,this maybe all that it is,

Posted on Jun 02, 2010

  • thesprink Jun 02, 2010

    tried the dryer, will try the cleaner. How do I test the regulator?

  • Ronny Bennett Sr.
    Ronny Bennett Sr. Jun 02, 2010

    You need to test it with a fuel pressure tester,hook it up,and put a hand held vacuum pump on the fuel pressure regulator,and pump the vacuum up to about 20 psi,and turn the key on ,the fuel pressure ,what ever it reads will increase as you release the vacuum on the pump,if it does not,the fuel pressure regulator is not working,also check from the return line to the tank,remove it from the fuel rail,and remove the fuel cap from the fuel tank,and blow compressed air threw the return line to the tank,it should flow easy,with no restriction at all,if it does not,then the line is restricted.Also,make sure the egr valve is not stuck open,at all,not even a little bit,remove it,and clean it,and make it work smooth before replacing it back on the engine.

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  • Master
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Have you had anyone do a running compression test or a power balance test

the running compression test can show floating valves
the power balance test will show leaking injectors and weak spark

the running compression test you do with a regular compression test

the power balance test you need a scan tool


Posted on Jun 02, 2010

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