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A little bit of background on this: A few months ago I took the truck to the dealer to get an update done on the PCM as I was complaining about hard downshifting causing the whole vehicle to shudder. They had problems flashing the PCM but finally were able to after a couple visits. I'm not sure if this was related to what I'm experiencing.
Shortly after I took a couple hour highway trip and noticed that once the truck had warmed up and driven for a bit that it would have severe rough idling. It felt and sounded like it was puttering and about to stall. After I got home I pulled the negative lead on the battery for 10 minutes to see if that helped. It didn't end up helping a whole lot.
I was due for an oil change as well so I went and got one done. They said my oil level was low. After the oil change it seemed like most of the shuddering was gone except for the RPM fluctuations I'm experiencing still to this day a couple months later. No engine codes or hidden codes have popped up.
I've attached a gif of the tachometer to show what it's doing at idle. For info, I'm stopped with the engine having fully warmed up and in drive. You can see the RPM dip below 500 or so and overshoot before coming back to the original 500. The truck sounds rough and shudders when this occurs. It happens a couple times a minute.
What you see is the system Idle "hunting".
The idle speed is taken from the front Crankshaft sensor as a Tach signal and applied to the PCM for comparison to the 500 value in a Table.
If the difference is more than +/- 50 RPM, a correction signal is sent to the Throttle body to adjust air bypass until the correction is within the +/- limits the Crank Sensor signal agrees with.
Hunting is a minor fault in that loop or can be a VCT issue, a fuel issue or even a shift in the fuel tables for some reason causing the air-fuel ratio to be rich or lean, or a loose Crank plugup, or even the harness that usually is shielded from other system signal interference pickup.
Normally the correction is so smooth, the TACH so does not show the correction because it is so fast and tight. The dash update interval can also cover up the change.
A good Tech. needs to take a good look at the live data trying to detect a lead to follow and NOT always depend on a code set.
Some of these actions are set a bit loose so the Check Engines light does not light for every small variation. that is not critical to drivability.
Hope you understand how the Loop system works and some other effects that can be a cause.
Good luck.
What you see is the system Idle "hunting".
The idle speed is taken from the front Crankshaft sensor as a Tach signal and applied to the PCM for comparison to the 500 value in a Table.
If the difference is more than +/- 50 RPM, a correction signal is sent to the Throttle body to adjust air bypass until the correction is within the +/- limits the Crank Sensor signal agrees with.
Hunting is a minor fault in that loop or can be a VCT issue, a fuel issue or even a shift in the fuel tables for some reason causing the air-fuel ratio to be rich or lean, or a loose Crank plugup, or even the harness that usually is shielded from other system signal interference pickup.
Normally the correction is so smooth, the TACH so does not show the correction because it is so fast and tight. The dash update interval can also cover up the change.
A good Tech. needs to take a good look at the live data trying to detect a lead to follow and NOT always depend on a code set.
Some of these actions are set a bit loose so the Check Engines light does not light for every small variation. that is not critical to drivability.
Hope you understand how the Loop system works and some other effects that can be a cause.
Good luck.
Thank you very much for this. I appreciate the insight.
A little bit of background on this: A few months ago I took the truck to the dealer to get an update done on the PCM as I was complaining about hard downshifting causing the whole vehicle to shudder. They had problems flashing the PCM but finally were able to after a couple visits. I'm not sure if this was related to what I'm experiencing.
Shortly after I took a couple hour highway trip and noticed that once the truck had warmed up and driven for a bit that it would have severe rough idling. It felt and sounded like it was puttering and about to stall. After I got home I pulled the negative lead on the battery for 10 minutes to see if that helped. It didn't end up helping a whole lot.
I was due for an oil change as well so I went and got one done. They said my oil level was low. After the oil change it seemed like most of the shuddering was gone except for the RPM fluctuations I'm experiencing still to this day a couple months later. No engine codes or hidden codes have popped up.
I've attached a gif of the tachometer to show what it's doing at idle. For info, I'm stopped with the engine having fully warmed up and in drive. You can see the RPM dip below 500 or so and overshoot before coming back to the original 500. The truck sounds rough and shudders when this occurs. It happens a couple times a minute.
Anyone else experience anything like this?
Hi! I am experiencing the same issue with my 2019 5.0 F-150z were you ever able to find the root problem ? Thanks !
09-23-2022, 03:07 PM
2019 5.0 Weird RPM Fluctuations When Idle
