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Thread: Random RPM & Spark

  1. #1

    Default Random RPM & Spark

    So I have a 2013 LS3 58x and I only get RPM or spark every once in a great while. I did search for similar problem and I saw a lot of people had bad crank sensors, so I tried a new GM sensor, but it didn't help. I back probed the crank sensor and I got 4.9V going in, but 13V coming out on the signal wire. I also back probed the cam sensor, it also has 4.9V going in but has 7.8V coming out on the signal wire. I'm not sure if those reading are good or not. Any help would be greatly appreciated, thanks.

  2. #2
    Join Date
    Dec 2009


    So the engine doesn't run, correct? I'd record a System Log to determine which sensor is at fault.
    Also, fully charge the battery and ensure the engine is cranking fast enough, at least 100+ RPM.
    Most LSx no start problems are caused by defective crank & cam sensors, or sensor air gap issues.
    Have you tried completely disconnecting the battery for 20 minutes? If so, I'd try
    reflashing the ECU firmware (LINK). These two tasks resolve a lot of strange issues. (Datalog & System Log Information)
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  3. #3


    Correct, the engine does not run. When I do get spark, it'll fire for about two seconds, then die because I lose spark. I wanted to do a System Log, but forgot my laptop. That's why I was back probing the sensor when I turn it over by hand. Both of them do read, but I will run a System Log next time I go to work on the car.

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