Rough idle (RPM constant dropping from 700 to 300) when the AC is on [help]

[Gaz142]

Car runs super smooth and drives super nice however, it idles really bad. RPM drop from 700 to 300 and goes back to 700 on random intervals. It happens on a stop and when the AC is on and rarely if ever happens when ac is off but it doesn't idle good either. I really have no idea how to fix this, below is things I tried so far.

- Cleaned throttle body
- Cleaned IAC valve
- Cleaned MAF sensor
- Added fuel additive to clean fuel system and fuel injectors

**There are no codes and nothing stored in the memory**

Nothing change and no improvement what so ever. I read somewhere in this forum that this commonly happen when the VCT solenoid gets debris or need cleaning however dealership has to clean it on request or just change oil and it will flush it too. Any suggestions on what to do next ? I might do oil change however I only drove 1,500KM since last oil change which was around month/2months ago. I'm also wondering what is the connection between AC load on engine and VCT solenoid ? Things to note that weather here is 48 to 50 Degrees Celsius.

Car is:
- 10r80
- has double X-pipe
- HP catless tune
- Catalytic converter delete
- Car has 96,333KM and it is 2018 GT

any support is welcomed

-----------------------

UPDATE: Symptoms only occur when car AC is running. Today car started to have a humming sound (hum, hum, hummm) then few seconds and repeat and it seem in rhythm when AC is on and the sound is only audible if you listen closely inside the cabin. All idling issue happen when AC is on and I drove for 45min and idling for 15mins with AC off and no issues what so ever and smooth idle, the moment I started AC idling became rough. Any thoughts of what could cause it ? bad compressor ? potential AC leak ? low freon ?

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[njweatherman]

The problem you are having is somewhat similar to what I and others have experienced on this forum. When my car has been driven for about 30 minutes to an hour and I stop at a red light, my car begins to shake and my idle drops from 600 to about 500. Thereafter, the idle fluctuates quickly from 500 to 600 and the car continues to shake. It does not stall or trigger any codes. Once the light turns green, it may show up again at the next red light or not at all. One thing that temporarily helped me was cleaning the mass air sensor and throttle body. However, my problem has now returned again. I doubt mine is dirty VCT solenoids because I am currently at 14,000 miles and I have had 4 oil changes already. A band-aid solution others on here talked about is having a tuner raise the idle.

 

[GazV8]

The problem you are having is somewhat similar to what I and others have experienced on this forum. When my car has been driven for about 30 minutes to an hour and I stop at a red light, my car begins to shake and my idle drops from 600 to about 500. Thereafter, the idle fluctuates quickly from 500 to 600 and the car continues to shake. It does not stall or trigger any codes. Once the light turns green, it may show up again at the next red light or not at all. One thing that temporarily helped me was cleaning the mass air sensor and throttle body. However, my problem has now returned again. I doubt mine is dirty VCT solenoids because I am currently at 14,000 miles and I have had 4 oil changes already. A band-aid solution others on here talked about is having a tuner raise the idle.

Well... this issue is common on F-150s too it seems which makes coytee gen3 engine the culrpit. They seem to also fix it by changing VCT solenoids. I will drop mine to dealership tomorrow and request full vct change and see how it goes. This issue is driving me crazy and killed the joy to drive

 

[njweatherman]

Well... this issue is common on F-150s too it seems which makes coytee gen3 engine the culrpit. They seem to also fix it by changing VCT solenoids. I will drop mine to dealership tomorrow and request full vct change and see how it goes. This issue is driving me crazy and killed the joy to drive

Yes, please keep me posted. BTW - the dealership may first offer to clean the VCT solenoids before changing all of them. If I recall correctly, someone posted about this procedure in one of the forums. I think they cycle them like ten times to get ride of any debris. If they do change all four of them for you, please let me know how long it takes?
Thanks!

 

[GazV8]

Yes, please keep me posted. BTW - the dealership may first offer to clean the VCT solenoids before changing all of them. If I recall correctly, someone posted about this procedure in one of the forums. I think they cycle them like ten times to get ride of any debris. If they do change all four of them for you, please let me know how long it takes?
Thanks!

Went to dealership and they said they gonna change it "as per client request" which will not gurantee anything and total cost including labor is around 5200 SAR (1350$ U.S dollar). So i passed. Ill learn to live with it until car completly stall or a code show up

 

[njweatherman]

Went to dealership and they said they gonna change it "as per client request" which will not gurantee anything and total cost including labor is around 5200 SAR (1350$ U.S dollar). So i passed. Ill learn to live with it until car completly stall or a code show up

It's a shame that the dealership over there is the same as the ones here. If they do not see an error code from the computer, they assume everything is ok, and they don't look into things. When I brought mine in a few weeks ago for the idle issue, this is what the mechanic wrote:
"Test vehicle OK, check oasis for tsbs none, hook up ids and check for codes none, check mode 6 ok, visually inspect under hood for issues ok, unable to verify customer concern."

 

[Gaz142]

It's a shame that the dealership over there is the same as the ones here. If they do not see an error code from the computer, they assume everything is ok, and they don't look into things. When I brought mine in a few weeks ago for the idle issue, this is what the mechanic wrote:
"Test vehicle OK, check oasis for tsbs none, hook up ids and check for codes none, check mode 6 ok, visually inspect under hood for issues ok, unable to verify customer concern."

By the way, when the idle RPM fluctuate, do you hear a loss in exhaust sound ? It is hard to explain but easy to hear, it is like the volume of the exhaust go lower then louder and again louder etc. and when it goes low it is like a warning that rpm will drop if that make sense.. I don't know why this too happens.. I called my tuner to check the car tomorrow maybe he need to update ambient temperature since it is summer now.. he will take a look at the car physically too since he worked with so many mustangs before and kinda familiar with common issues. I will keep you posted if he found the exact issue.

 

[njweatherman]

When my idle drops low, the car shakes, and yes the exhaust gets quieter as well. The first sign for me it is going to happen is when I come to a complete stop, the car begins to shake. The other day when it happened, I counteracted it by putting the AC on max. Then, my idle increased and the car went back to normal. I'm going to test this theory out again when it happens.
Yes, please keep me posted on what the tuner says.
BTW - the only aftermarket part on my 2020 GT is the BORLA ATAK catback exhaust. Everything else is stock.

 

[GazV8]

When my idle drops low, the car shakes, and yes the exhaust gets quieter as well. The first sign for me it is going to happen is when I come to a complete stop, the car begins to shake. The other day when it happened, I counteracted it by putting the AC on max. Then, my idle increased and the car went back to normal. I'm going to test this theory out again when it happens.
Yes, please keep me posted on what the tuner says.
BTW - the only aftermarket part on my 2020 GT is the BORLA ATAK catback exhaust. Everything else is stock.

Met the tuner and increased idle, drive and reverse RPM to 800. Still issue occurs but at least it doest shake anymore but rpm does drop from 800 to -600. I read online some people managed to fix it by swapping to 5w50 oil but Im not sure if it gonna harm my engine since I dont track it or hard drive daily. Other than that it seems typical vct solenoid/ cam phasers issue.. good news is, as scary as it feels it does not harm the car since it just out phasing issue. What oil are you using ?

 

[njweatherman]

Wow, an 800 idle is pretty high. When my car is fully warmed up and just the radio is on, I idle around 600 or so. I wish mine idled higher; I bet the exhaust sounds awesome with the higher idle
The oil I use is full synthetic, 5W 20, which the dealer puts in. So far, I had 4 oil changes and I am at 14k miles.
When the idle drop occurs, the lowest my idle dropped so far has been about 450 or so. The last time it did this, I immediately put the AC on and it seemed to offset it and the shaking. I might have found a band-aid for it

 

[GazV8]

Wow, an 800 idle is pretty high. When my car is fully warmed up and just the radio is on, I idle around 600 or so. I wish mine idled higher; I bet the exhaust sounds awesome with the higher idle
The oil I use is full synthetic, 5W 20, which the dealer puts in. So far, I had 4 oil changes and I am at 14k miles.
When the idle drop occurs, the lowest my idle dropped so far has been about 450 or so. The last time it did this, I immediately put the AC on and it seemed to offset it and the shaking. I might have found a band-aid for it

Yea base tune I used to idle 650 which was too low tbh, I then increased it to 700 which was nice spot and feel but this issue made me hate.. with shaking and all, so now no shaking just rpm drop but yea exhaust sound so good on idle now im disappointed with ford.. this issue really ruined the fun for me and fixing for 1350 U.S. dollar is a no for me because its not gurantee to fix it. Did you notice your vacuume gauge/number drop when it happens ? Now im thinking it might ve vacuume leak ? I just wish CEL show up so I can read the code and put me off this misery. I just changed oil yasterday but I might early change next month and try 5w50 since too many reported 5w50 fixed this issue for them which is the cheapest un-harmful test

 

[njweatherman]

No, I have not watched the vacuum gauge when it happens. It just happened to me before. The car began to shake and I put the AC on, and it went away
It's a shame how these dealerships are. My rear rotor is grooved and marked. Yet, the other three are spotless. They refuse to change it for me because they said it is a cosmetic issue.
Only time will tell when and if it ever triggers a code. I'll keep you posted if I find anything. Please do the same. Happy driving

 

[Gaz142]

UPDATE: Symptoms only occur when car AC is running. Today car started to have a humming sound (hum, hum, hummm) then few seconds and repeat and it seem in rhythm when AC is on and the sound is only audible if you listen closely inside the cabin. All idling issue happen when AC is on and I drove for 45min and idling for 15mins with AC off and no issues what so ever and smooth idle, the moment I started AC idling became rough. Any thoughts of what could cause it ? bad compressor ? potential AC leak ? low freon ?

 

[GazV8]

No, I have not watched the vacuum gauge when it happens. It just happened to me before. The car began to shake and I put the AC on, and it went away
It's a shame how these dealerships are. My rear rotor is grooved and marked. Yet, the other three are spotless. They refuse to change it for me because they said it is a cosmetic issue.
Only time will tell when and if it ever triggers a code. I'll keep you posted if I find anything. Please do the same. Happy driving

UPDATE: I took it to my friend shop and he ivestigate it and found that all 8 spark plugs are burnt and there is small oil leak from spark plug gasket which makes the coils to burn faster than designed. However, he ddint recommend fixing it unless with chanhing VCT solenoid since can change both in one open. Anyways, battry gave low voltage during off state too ~ 11V. So they changed both and at the shop it did it after change but he told me give the car some thottle on ur way back and let it re learn its idle again. So ill keep u updated if this was the fix.

 

[njweatherman]

UPDATE: I took it to my friend shop and he ivestigate it and found that all 8 spark plugs are burnt and there is small oil leak from spark plug gasket which makes the coils to burn faster than designed. However, he ddint recommend fixing it unless with chanhing VCT solenoid since can change both in one open. Anyways, battry gave low voltage during off state too ~ 11V. So they changed both and at the shop it did it after change but he told me give the car some thottle on ur way back and let it re learn its idle again. So ill keep u updated if this was the fix.

When my AC is on, my idle is higher. It also offsets the idle drop.
Hmm, do you think the custom tune and no catalytic converters leaned out your air/fuel ratio and fried your spark plugs over time? No sense in changing the spark plugs if the tune is off.

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