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Thread: Scanner RPM has lost it's mind!

  1. #1
    Tuner in Training
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    Scanner RPM has lost it's mind!

    I have a new build with about 25 miles on it so far. Was having an erratic stalling issue that I tracked down to a bad coolant sensor. After replacing that sensor I started the engine and monitored the warmup cycle. Everything was looking fine and then all of a sudden the Engine RPM in VCM Scanner started seeing these wild swings in RPM. Engine is sitting there purring at 800RPM (factory tachometer works fine in the truck), but Scanner is saying 2500, then 9000, then 10,000, then 3000. It's absolutely all over the place, even though the engine is just sitting there idling smoothly.

    Has anyone seen this before? I tried the Crank Relearn Procedure, but it did nothing to fix the issue. Crank is new, but cam is same as I've had for years. The PCM must have an accurate RPM signal since the engine is running just fine, but for some reason the Scanner is reporting a crazy number.

    Also tried disconnecting VCM, disconnecting/reconnecting MVPI, rebooting laptop, starting the process all over again. Reloaded the tune. Did Crank Relearn Procedure. Let the truck sit overnight, let it sit for a week, etc. No matter what I do now when I connect and start the engine the engine RPM is completely inaccurate.

    This was a build that was working fine for years with same OS (2bar OS) and a similar tune until I pulled the engine to rebuild it after a catastrophic valvetrain failure. Most sensors are the same as what was in the truck before I pulled engine and it was working fine. Hopefully someone has had this issue before and it's something simple.
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  2. #2
    Tuner in Training
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    Must be an isolated incident since no one has responded. Was able to connect my buddy's MPVI2 to the vehicle and it reads everything perfectly fine. So issue is either with the software version I'm using (5.0.4?) or my MPVI. I have an open ticket with HPTuners and will hopefully get this resolved soon.

  3. #3
    Senior Tuner
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    Quote Originally Posted by tt460 View Post
    Must be an isolated incident since no one has responded. Was able to connect my buddy's MPVI2 to the vehicle and it reads everything perfectly fine. So issue is either with the software version I'm using (5.0.4?) or my MPVI. I have an open ticket with HPTuners and will hopefully get this resolved soon.
    Do you have something plugged into the DLC like a AEM Can WB? or maybe running boards controller or something like that?

    When this connection is loose it will do odd things like that. Sometimes even on the car dash.
    Tuner at PCMofnc.com
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  4. #4
    Senior Tuner TheMechanic's Avatar
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    Any time I have erratic data my first check is powers and grounds. Next if it is isolated to RPM I check to make sure the shielding is good on the CKP sensor wire and no other "aftermarket" wiring is running parallel. Next is connector and sensor issues. Last but not least is a failing ECM/PCM. See if the module is temp sensitive. Throw some ice on it and then some heat from either a choke tester or an old school incandescent drop light. After that you will have to significantly widen your search/diagnostic.

  5. #5
    Tuner in Training
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    After going back and forth with HPT, checking harness for wiring issues, ground issues, driving the vehicle to put it through heat cycles, etc and not coming up with anything; I was able to test my buddy?s MVPI2 on it and it worked flawlessly. So both HPT support and I conclude that it is interface related.

    Since mine is an older MVPI Pro, I decided to swap it for a newer MVPI3. Just waiting for my new one to arrive.

    Thank you for those who replied.