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Thread: Cam and Crank Sensor Testing

  1. #1
    Join Date
    Aug 2009
    Posts
    5

    Default Cam and Crank Sensor Testing

    I just had the motor rebuilt for my 1997 GMC K1500 (350 Vortec). After installing it into the truck - it will not start! My timing light shows that I am getting spark. If I shoot some starting fluid into the air cleaner housing, it will attempt to run.

    There is nothing more disappointing or frustrating than a new engine that will not start!

    I made a noid light out of an old taillight bulb, and I get nothing from every pin pair in the connector. I checked the cam position sensor (CMP) and the crank position sensor (CKP) using the procedure in your manual (24065, pages 6-15 and 6-16) and I get similar behavior from both. If I follow the book procedure to backprobe the signal (I used a paperclip) with the positive lead of my meter and connect the negative to a ground - I never get past 0.25 VAC on the CMP and 0.6 VAC on the CKP. If I backprobe the signal output and and attach my meter's negative lead to the signal return on the connectors - I see about a lot of variety with a peak of 3.5 VAC from the CMP, and a pretty steady 2.5 VAC from the CKP. Both connectors have a good 12 volt supply between the pink wire and ground.

    Does probing the connectors produce a valid result? Does the fact that I don't get a decent reading between the connector and ground indicate a wiring problem?

    Your advice will be greatly appreciated.

    Thanks.

  2. #2
    Join Date
    Jan 2007
    Posts
    4,831

    Default

    Hi again,

    Before you go any further, do two things:

    1) Check to see if any trouble codes are stored in the PCM

    2) Check for fuel pressure

    I'm not so sure your homemade 'noid light is a valid test. An actual 'noid light doesn't use a bulb, it uses an LED.

    Since you're getting similar performance from the CMP and CKP sensors, I doubt if either of those are your problem. As far as the values you're getting, keep in mind that the manual states that those values are "approximate."

  3. #3
    Join Date
    Aug 2009
    Posts
    5

    Default Fuel Injection Pain

    Thanks for the response.

    I am using Actron AutoScanner Plus (CP9180), and there are no codes.

    I have a gauge on the port right now. When I turn the key on, the gauge peaks at 60-62 psi and then falls back to 56 when the pump goes off. I don't have the return blocked.

    I tested each injector by keying the ignition on to pressurize the fuel system and then turning the key off. I then connected 12 volts to a pair of injector pins to see if the fuel pressure would drop. I repeated this procedure for each injector. Seven out of 8 injectors do drop the fuel pressure when energized, so I know I have 1 bad one.

    I made the noid light with a bulb because the dealer I spoke with said that they use the 194 bulbs out of the marker light. I will make one out of one of my marker lights and try it again.

    As mentioned in the prior post, if I shoot some starting fluid into the air cleaner box - it shows immediate signs of life.

  4. #4
    Join Date
    Aug 2009
    Posts
    5

    Default It's alive!

    I re-did the TDC and distributor installation, poured a little gas down the intake, and it fired right up (on 7 cylinders).

    I decided to try running some SeaFoam through the fuel system before replacing the injector - and it worked! The engine is running on all 8.

    I had codes for a bad O2 sensor and throttle position switch (TPS). The TPS had definitely had something leak on behind the O-ring. Replaced the TPS and O2 sensor and all is well. No codes!!

  5. #5
    Join Date
    Jan 2007
    Posts
    4,831

    Default

    Good job! It's always good to check the basics before getting in too deep.

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