Random cranking RPM

Bxnanaz · June 25, 2017

I've noticed recently that sometimes during starting, the truck will suddenly jump significantly in cranking RPMs (double probably). Almost like it starting cranking in V4. Truck fires up and sounds good, no CELs. Anyone ever have this, or is there any reason for concern?

Justin101714 · June 25, 2017

Does the truck sound different when it cranks, like can you hear the difference in RPM, because if you can't, I can only imagine this is voltage spiking during cranking from the alternator. Why it chose the IPC to send the voltage to? I don't know but it should be fine.

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Bxnanaz · June 25, 2017

Does the truck sound different when it cranks, like can you hear the difference in RPM, because if you can't, I can only imagine this is voltage spiking during cranking from the alternator. Why it chose the IPC to send the voltage to? I don't know but it should be fine.

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I can clearly hear the difference in RPM. Not sure what the cause could be. To my knowledge a starter only turns up to a certain torque. So the only way to make it turn quicker is to lower compression, right?

Justin101714 · June 25, 2017

Well lower compression of course it would turn faster, it would have less force to turn. No MIL on, I doubt you have a compression problem. Now, if you have pre igniting fuel.....that would do it too.

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Bxnanaz · June 25, 2017

Well lower compression of course it would turn faster, it would have less force to turn. No MIL on, I doubt you have a compression problem. Now, if you have pre igniting fuel.....that would do it too.

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Yeah, I know I don't have lower compression because it's random. That's why I wonder if it has something to do with AFM/V4. Pre-igniting fuel could definitely do it... but I have always ran 89 or 93 octane since the day it was new. Only has 5500 miles

Justin101714 · June 25, 2017

Yeah, I know I don't have lower compression because it's random. That's why I wonder if it has something to do with AFM/V4. Pre-igniting fuel could definitely do it... but I have always ran 89 or 93 octane since the day it was new. Only has 5500 miles

The engine cranks as if AFM doesn't exit though, so it's hard to imagine that that would have something to do with it. You'd almost have to monitor data while it does it.

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Bxnanaz · June 25, 2017

The engine cranks as if AFM doesn't exit though, so it's hard to imagine that that would have something to do with it. You'd almost have to monitor data while it does it.

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Yeah, unless the lifters had a leakdown issue maybe?

Thanks for the help. Was really just curious if anyone has had the same "issue", or if this was the beginning to an AFM issue

Justin101714 · June 26, 2017

If they leaked down they would also rattle from aeration at start up until the oil pressure built up to them.

Of all the concerns I've had working on the 5.3L since the introduction of AFM or DOD when t first came out. This is the first complaint I've ever heard of an RPM change while cranking.

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