RPM's fluctuating at idle?

[merlin803]

When I started the Mustang this morning, RPM"s consistently fluctuate while in park between about 1,000 and 2,000 RPM. It was like I was giving it throttle and then letting go, over and over. You could hear it from in my house after I remote started it.

I went out and turned it off and back on but it continued.

Giving more throttle does not have an effect on the RPM's or fluctuating at all. Almost like the gas peddle wasn't connected.

However, I had to move the car to get my other vehicle out and driving in reverse and then drive seemed like the power/throttle may be normal.

I didn't have much time to mess with it because I had to get to work but any ideas what could cause this?

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[inectarios]

When I started the Mustang this morning, RPM"s consistently fluctuate while in park between about 1,000 and 2,000 RPM. It was like I was giving it throttle and then letting go, over and over. You could hear it from in my house after I remote started it.

I went out and turned it off and back on but it continued.

Giving more throttle does not have an effect on the RPM's or fluctuating at all. Almost like the gas peddle wasn't connected.

However, I had to move the car to get my other vehicle out and driving in reverse and then drive seemed like the power/throttle may be normal.

I didn't have much time to mess with it because I had to get to work but any ideas what could cause this?

It could be a number of things... but my first suspicions would be throttle body and/or fuel pump. I had similar bounching RPM issues (for years) but these occurred at a lower RPM range (I'd say between 800 - 1200 rpm). These happened on a few occassions, specially on cold starts. I noticed it happened more often when I would gently blip the throttle while the car was still trying to figure out the idle RPM to keep while the engine was warming up. After a blip it would start bouncing for 30 seconds to 1 minute so what I would usually do is just blip the throttle again to a higher RPM and that usually got the bouncing to stop. I had this since the car was ~6 months old /w 4K miles on the clock.

It turned out the factory fuel pump was faulty all along. It completely died a couple of months before the car's 6th birthday. I simply replaced it with the same OEM one. I then discovered that what I was experiencing with the bouncing RPM (and other "chocking" downshift issues) for 5.5 years - especially during cold weather - was all related to the old faulty fuel pump.

I do not recall experiencing the bouncing RPM issue nor any chocking since the new fuel pump went in about a year ago. Mine's a 2016 GT. From your profile photo, I gather yours is a Mach1? The Mach1's have the Gen3 Coyote engine which has dual fuel injection (Direct & Port as opposed to only Port on Gen2's) but I don't think the fuel pump or the throttle body are any different. Also worth noting in my case the car would not throw any DCTs when I was diagnosing it since according to the diagnostics the fuel pump was "working". I guess there is no sensor or software algorithm looking for an "erratic" fuel pump in these cars. In any case, it might be worth running a quick OBD2 diagnostic first to see if anything shows up.

I hope this helps.

 

[merlin803]

Thanks for the insight. I tried blipping the throttle but it did not have any affect to the constant RPM fluctuation when in Park.

No "check engine" light but when I get home from work, I'll check Forscan to see if any codes are being thrown.

 

[Need4SpeedMotors]

Most likely going to be a vacuum leak. Have the car smoke tested, should be easy to find

 

[Pistol_91]

Vacuum leak

 

[merlin803]

I got home and started it and it idled/ran fine.

I ran Forscan and it showed the following:

===PCM DTC P050A:00-64===
Code: P050A - Cold Start Idle Control System Performance

Status (-64):
- DTC Maturing - Intermittent at Time of Request
- Malfunction Indicator Lamp is Off for this DTC
- Test not complete

Seems crazy that I stopped/started it a few times this morning and let it warm up and it was acting so crazy and then I get home and it starts/runs with no issue.

I thought maybe if it was much colder this morning that it may explain it but it is only about 5 degrees warmer now than it was then.

I have an appointment in the morning to drop it off at the dealership. I don't have much confidence that they will actually fix anything, especially if it is running ok at the time but maybe I can get my 1st oil change over with at least.

 

[inectarios]

I got home and started it and it idled/ran fine.

I ran Forscan and it showed the following:

===PCM DTC P050A:00-64===
Code: P050A - Cold Start Idle Control System Performance

Status (-64):
- DTC Maturing - Intermittent at Time of Request
- Malfunction Indicator Lamp is Off for this DTC
- Test not complete

Seems crazy that I stopped/started it a few times this morning and let it warm up and it was acting so crazy and then I get home and it starts/runs with no issue.

I thought maybe if it was much colder this morning that it may explain it but it is only about 5 degrees warmer now than it was then.

I have an appointment in the morning to drop it off at the dealership. I don't have much confidence that they will actually fix anything, especially if it is running ok at the time but maybe I can get my 1st oil change over with at least.

I have a feeling that DTC is related to your issues. A quick Google search prompted me with this:

https://www.obd-codes.com/p050a

Causes for this code may include:
- Defective or dirty IAC valve
- Vacuum leak
- Open or shorted circuits or connectors
- Defective electronic coolant temperature (ECT) sensor

There's also an older post from in this forum from 2018 but that involved a Whipple-d car so the cause might have been different. Also, no mention of a fix , the car seemed to have "fixed itself".

https://www.mustang6g.com/forums/th...rt-idle-air-control-system-performance.97529/

 

[KingKona]

Dead battery.

 

[merlin803]

The battery being the issue would seem to be odd, I had no problem starting/driving it Saturday (after being out of town and the car not being started for 8 days). Yesterday it started right up but had the weird pulsing RPM's, so I just left it at home and drove my wife's car.

This morning I had an appt to drop it off at the dealership and it started and ran with no issues.

Of course they have not been able to replicate any issue today, so they are going to keep it overnight and try to start it in the morning again.

I won't hold my beath that they will fix anything and will probably be lucky if I just get it back in the same condition that I left it.

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