2019 Mustang GT Auto Very Rough Idle/Engine Stall

[SHxTGUN]

Hey guys. I recently bought a used 2019 Mustang GT 10-speed (~32k miles) about a month ago and I’ve been having issues when idling and coming to a stop at red lights. Sometimes when the car is stopped at a red light for a few seconds, the RPM starts fluctuating and slightly going up and down and the car shakes pretty roughly as this is happening. This happens quite a lot. Now, sometimes during these fluctuations, the RPM will drop very low and then the engine will just shut off (stall). This happened to me 2 or 3 times all within a week until I decided to go to the dealership (6/8/20).


They initially diagnosed the issue as an Intake Manifold Runner Control problem and they replaced the faulty part. Just a few days after that repair, the same thing happened where the RPM went up and down at idle and the car stalled again at a red light. Went back into the dealership, and they said this time it was the throttle body that was bad and they replaced it (that was today 6/29/20). Decided to take the car out tonight for a test drive and lo and behold, the idle is still very rough at red lights although it didn’t stall out in my short test drive. Although it feels the exact same before with the RPM fluctuations and I feel like it’s a matter of time before I get the stall to occur. I haven’t called the dealership again yet because this was at night, but I am getting a little worried.


Does anyone have any ideas of what could be causing the rough idling and stalling of the car? I’m not sure what they’ll say after I call them tomorrow for the 3rd visit/repair, but I wanted some opinions.


--Also not sure if it’s related but sometimes when I shift from D to R, the car will suddenly increase the RPM quite a bit and it makes the car jump back initially if I don’t have my foot on the brakes. I’ve learned to keep my foot on the brake for an extra few seconds when shifting into reverse, but does that seem normal? I have to be extra careful in my garage when I’m backing it in, cause the car will lurch suddenly after changing the gear from D to R.

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[SHxTGUN]

Anyone have any ideas on the rough idle/occasional stalls?

 

[Bluemustang]

MAF or O2 sensor would be my guess.

 

[Cobra Jet]

It could be a variety of things without knowing if there are any DTC fault codes stored.

Check the pdf in this linked thread; any time there is major engine work or trans work performed on the S550 5.0, the Tech is supposed to do a crank relearn procedure. Does your work order indicate the Tech did it?
https://www.mustang6g.com/forums/th...s-weird-hesitation.125119/page-2#post-2679084


The problem also could be related to the VCT system. There is a common VCT issue that affects the 2018-2019 S550 5.0; see this thread with that info:
https://www.mustang6g.com/forums/threads/p0018-code.123932/page-2#post-2646415

Hope the above is helpful.

 

[Roger Blose]

The A10 jump forward / lurch from park to drive is corrected by having the transmission software reflasked with the latest version from Ford at your dealer. After the reflash, a detailed relearned test drive must be performed to set it up correctly. Your rough idle is a head scratcher that needs an advanced scan tool at the dealer. I would disconnect the battery for 30 minutes and then go drive the car to reset the fuel trims to see if that changes anything.

 

[Matti777]

Obviously their troubleshooting skills are poor. I would have a chat with them about the money you have spent to date which obviously was totally wasted.

 

[SHxTGUN]

It could be a variety of things without knowing if there are any DTC fault codes stored.

Check the pdf in this linked thread; any time there is major engine work or trans work performed on the S550 5.0, the Tech is supposed to do a crank relearn procedure. Does your work order indicate the Tech did it?
https://www.mustang6g.com/forums/th...s-weird-hesitation.125119/page-2#post-2679084


The problem also could be related to the VCT system. There is a common VCT issue that affects the 2018-2019 S550 5.0; see this thread with that info:
https://www.mustang6g.com/forums/threads/p0018-code.123932/page-2#post-2646415

Hope the above is helpful.

Unfortunately they didn't give me any papers for the second repair (the throttle body repair), but I checked over the sheet from the first IMRC repair and in the notes it said "Replaced Intake Manifold per WSM and reset KAM. Reset misfire monitor and cleared DTC." Not sure if that indicates the crank relearn procedure. They did reset the KAM though which I saw listed in the link you sent. Is the crank relearn separate from the KAM reset?

And yeah, I gave them a list of things to check for from some research on my part and the VCT solenoids were the first thing I listed. I dropped the car off for the third diagnosis on July 1st but still haven't received a call back on anything. But it was the holiday weekend, so hopefully I'll get a call by tomorrow.

Thank you for the reply.

The A10 jump forward / lurch from park to drive is corrected by having the transmission software reflasked with the latest version from Ford at your dealer. After the reflash, a detailed relearned test drive must be performed to set it up correctly. Your rough idle is a head scratcher that needs an advanced scan tool at the dealer. I would disconnect the battery for 30 minutes and then go drive the car to reset the fuel trims to see if that changes anything.

Thank you for the info. I actually asked them when I came in for the second repair to check for any transmission software updates because of the issue, and they told me they couldn't find any. I didn't think about having it reflashed with the latest update, but I will see if I can ask my service advisor when he calls me back with the diagnosis on this third visit.

Obviously their troubleshooting skills are poor. I would have a chat with them about the money you have spent to date which obviously was totally wasted.

Well fortunately the car is still under 36k/3 years so I am under the warranty still so everything up to this point has been charge free. Although that warranty is running out (close to 33k miles), so the longer this takes to diagnose, the more worried I am getting. This is going on the third diagnosis, so I really hope they're able to get it this time or I may have to try a different dealership.

 

[Dana Pants]

If you figure it out, let us know. I had a similar problem. cleaning the throttle body seems to have solved the stalling, but I still feel the idle is unacceptably rough sometimes.

 

[Bluemustang]

Clearing DTC may not necessarily mean performing a crank relearn. KAM reset is different also from a crank relearn. KAM reset is just resetting all the learned driving habits. It's like clearing the memory. Crank relearn is a separate procedure (which can also be possible with a flash/tuner device). If you had a tuning device you could see all the codes and research on your own to double check or confirm Ford's recent attempts.

 

[shogun32]

find a different/better mechanic. Look for vac leak.

 

[Cobra Jet]

Here ya go, if you enter Engineering Mode from your dash cluster, you can see if there are any stored DTC’s - I had posted this a while back in another thread:

——-
If you have a 2015-2020 with the ANALOG gauge cluster, you should be able to enter the “Engineer Test Mode” and will be able to see stored DTC’s.

Follow this link:
https://www.mustang6g.com/forums/threads/engineering-test-mode.27581/

If you you have a 2018-2020 with the DIGITAL cluster, you should be able to access the same test mode found here:
https://www.mustang6g.com/forums/th...n-18-digital-dash-old-way-doesnt-work.115231/
https://www.mustang6g.com/forums/threads/can-i-adjust-mpg-on-18-digital-dash-old-way-doesnt-work.115231/

 

[SHxTGUN]

Didn't receive a call back yesterday so I decided to call them just now and they said they're looking at an issue with the VCT. They said it appears to be working correctly and then all of a sudden goes from one extreme to another. But they couldn't find any consistency so they're still trying to figure out what's going on. He didn't have an ETA on it either. Even though it's been a week since they've had my car, I can be patient as long as this really is the issue and hopefully they're able to diagnose and fix it. Just a bit weird to me how they seemed so confident in the first two repairs being the culprit, but here we are on the 3rd go around. So I'm just a bit worried still.

I appreciate everyone's input on this and I'll keep the thread updated if this was it and if it fixed this issue. I will also relay the crank relearn as well as the transmission software reflash once my advisor gives me the call before the repair.

 

[Jaay_d5.0]

The exact same thing happening to my 2019 10r80 bought it at 15780 miles now on 21 k miles happens sometimes cleaned my maf sensor and it worked fine for two months and it judt came back yesterday

 

[SHxTGUN]

Update: Yesterday (after 8 days they had my car), they said they fixed the issue and all it was, was..... an oil change. They said after the oil change they didn't see the issue with the VCT anymore and that was the only thing they did to the car. Since I bought the car used on 5/23/20, I haven't changed the oil yet (oil life monitor had it at like 40-50% at the time) so I couldn't call them out on it cause I wasn't sure. I was super skeptical though that a simple oil change would've been the issue for something this bad, but I took the car home yesterday.

Stalled out on me today at a red light and RPMs were up and down at several stops. Issue is still as bad as ever. Not going back to that dealership again, but I am getting really frustrated at this point.

Oh and I also talked to the advisor about the crank relearn and he told me there was nothing they could do about that? I don't know if he was telling me that wasn't a thing or what, but... Just gonna try a different dealership and hope for something better. Or I could buy an SCT/nGauge and do it myself I guess.

 

[SHxTGUN]

Also additional info about the problem: When I'm at a light and the car starts to get into the rough idle, the Air/Fuel Ratio gauge starts acting up and will go up and down while the car shakes. I've seen it go 16+ while at a red light and it'll drop into the 12s or maybe 11s as well.

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