2020 Mustang GT ESS G3 w/ intermittent rough idle/stalling, drives perfectly – extensive diagnostics already performed

[RexlessTazz]

Struggles of getting an already modified and boosted GT... I am making this post due to trying everything I know of to try. I'm at the point of believing it needs a new tune. Which is $1,300 with Wengerd Performance. I am trying to cross every t and dot every i prior to spending that money.

I'm looking for some help diagnosing a strange intermittent idle issue on my 2020 Mustang GT. I've spent quite a bit of time troubleshooting this already and would really appreciate input from anyone experienced with Gen 3 Coyotes, especially boosted/ESS cars.

Car/setup:

• 2020 Mustang GT, Gen 3 Coyote, 10R80
• ESS G3 supercharger, 120mm pulley
• ESS G4 intercooler
• Tial BOV
• FIC 1000cc injectors
• Fuel pump booster
• Long-tube headers/X-pipe
• 93 octane
• Unknown tune/tuner (car was purchased already modified)

The problem:

The car will intermittently idle extremely poorly. It can shake/misfire badly enough that occasionally the engine will actually stall.

The weird part is that it isn't consistently bad.

Sometimes it'll sit around ~800 RPM and idle reasonably well. Other times it'll start struggling/misfiring badly. Cold starts around ~1,000 RPM are generally fine, and if I hold the engine around 2,000 RPM it runs smoothly.

It also drives great. I don't have an obvious problem under load, acceleration feels normal, and after an idle stall it'll start right back up.

Important history regarding fuel trims/injectors:

Earlier in the diagnosis, the car DID have a more significant lean condition. STFTs could reach roughly +12–16%, particularly when it was running poorly.

During that diagnosis, I swapped injectors 7 & 8 and subsequently discovered an issue with the lower injector O-rings. The injectors were cleaned and the lower O-rings were replaced.

That appears to have corrected the previous lean condition.

The current idle problem remained after that repair.

Currently, when the idle is behaving reasonably well, STFTs are generally low. When the rough idle/misfiring starts, the trims may increase somewhat, but I'm generally only seeing roughly +5–9% STFT now, NOT the +12% or higher numbers I was seeing before repairing the injector O-rings.

So I don't want the old +12–16% trim readings confused with the condition the car has now.

Fuel rail pressure also appears normal. I've observed roughly 79 psi desired vs ~81 psi actual.

Things already checked/tested:

• Compression checked and good
• Spark/plugs checked
• Injectors investigated
• Previously swapped injectors 7 & 8 during diagnosis
• Lower injector O-rings replaced after discovering the previous issue
• Previous significant lean condition improved after the injector/O-ring repair
• Throttle body cleaned/checked
• Desired vs actual throttle position appears to follow correctly
• MAF unplug test made it run worse
• Intake/filter removed/tested with no improvement
• EVAP purge valve disconnected during testing with no meaningful change
• Purge valve appears closed when it should be
• VCT data checked and appeared normal
• AFR/fueling otherwise appears normal
• Multiple rounds of live-data monitoring with an Autel scanner

Vacuum/intake leak testing:

I've also spent a lot of time smoke testing it.

I have an automotive smoke machine and have tested the intake/manifold through different locations, including the brake booster connection.

I've capped the throttle body/intake opening appropriately during testing and isolated/reconfigured the PCV connections while trying to locate a leak.

The smoke machine's leak/flow gauge continues to indicate some amount of leakage (generally around the ~0.5–1.0mm range on its gauge depending on configuration), but I cannot actually find smoke escaping anywhere.

One thing I DID previously find was a leaking air/oil separator. That separator has since been replaced and that leak is fixed, but the intermittent idle problem remains.

The car's PCV system is modified because of the ESS setup:

• Passenger-side PCV runs through an air/oil separator
• Driver-side PCV is routed into the ESS supercharger intake

I've smoke tested/isolated these connections in different configurations without finding an obvious external leak.

I've also verified smoke can reach other manifold-connected circuits—for example, smoke will come through the BOV vacuum reference line when smoke is introduced through the brake booster/manifold.

Despite all of this, I still haven't located a definite intake/vacuum leak that explains the intermittent idle.

Current behavior in a nutshell:

• Cold/high idle: generally good
• ~800 RPM warm idle: intermittently terrible
• ~2,000 RPM stationary: smooth
• Driving/under load: runs great
• Can misfire/shake badly at idle
• Occasionally stalls at idle
• Immediately restarts
• Current STFT during the problem is generally only around +5–9%
• Previous +12–16% trims were BEFORE repairing the injector O-rings
• Problem can disappear again seemingly at random

At this point I'm trying to avoid blindly replacing parts or spending another large amount of money without actually identifying the cause.

I'm especially interested in anything specific to a boosted Gen 3 Coyote/ESS setup that could cause an intermittent severe rough idle/stalling condition while allowing the engine to run perfectly fine with additional RPM or load.

If anyone sees something in this diagnostic process that I've overlooked—or knows additional PIDs/data I should record on the Autel while it's actively having the problem—I'd greatly appreciate the help.

I'm completely willing to collect additional live data, perform another specific test, take pictures of the PCV/vacuum routing, or upload recordings/logs if someone knows what they'd like to see.

Thank you for your time!

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[Swtbabybilly]

What kind of fuel are you running is it on e85?

 

[Swtbabybilly]

And how do the spark plugs look? If your on pump gas and running octane booster they foul out plugs pretty fast

 

[RexlessTazz]

93 only. Doesn't have an upgraded fuel pump to my knowledge. Still haven't checked under the fuel hat yet to know.

Spark plugs have recently been changed.
NGK Iridium plugs with a gap of 0.035. Torqued to 11ft lbs.

 

[cbrtrx]

Plugs should be at .028 at that boost level but I'd check the vct cam timing while it's running rough compared to when it's running smooth. It's probably erratic pointing to vct solenoids or phasers.

 

[robvas]

How long have you had it and did it ever run right?

 

[RexlessTazz]

Plugs should be at .028 at that boost level but I'd check the vct cam timing while it's running rough compared to when it's running smooth. It's probably erratic pointing to vct solenoids or phasers.

120mm pulley makes 8psi. The listing for the NGK's also recommended 0.035 with boosted applications. (I'm NOT arguing/denying) Just what I've seen. Would you happen to know why it would be recommended to make it lower given its 8psi on 93? Thank you very much for the info and response.

 

[RexlessTazz]

How long have you had it and did it ever run right?

A little over a month, and no. It has always done this since I've gotten it. I have gotten it and got it up to date on everything I can think of maintenance wise and been trying to solve this as well.

 

[cbrtrx]

120mm pulley makes 8psi. The listing for the NGK's also recommended 0.035 with boosted applications. (I'm NOT arguing/denying) Just what I've seen. Would you happen to know why it would be recommended to make it lower given its 8psi on 93? Thank you very much for the info and response.

Depending on your rev limit the 120mm pulley makes 10 psi, their 8 psi rating is at 7200 rpm with the smaller g2 blower, even then it's closer to 9 psi. At 10 psi I'd recommend no more then .028 from my personal experience. It's not causing your rough idle but just figured I'd point that out. I'm going to guess phasers is the issue it's actually pretty common.

 

[RexlessTazz]

Depending on your rev limit the 120mm pulley makes 10 psi, their 8 psi rating is at 7200 rpm with the smaller g2 blower, even then it's closer to 9 psi. At 10 psi I'd recommend no more then .028 from my personal experience. It's not causing your rough idle but just figured I'd point that out. I'm going to guess phasers is the issue it's actually pretty common.

Based on your response I just checked the VCT/cam timing data with my Autel while I was able to catch the idle acting up. I logged actual intake/exhaust cam position on both banks along with desired-minus-actual.

The exhaust cams stayed basically at 0° with only a few tenths of a degree of error. The intake cams were being commanded back and forth quite a bit during the rough idle, but both banks followed each other pretty closely and returned to the commanded positions. I did see brief desired-vs-actual differences of around 10–15° while the cams were actively moving, but once they reached the commanded position the error dropped back to roughly 0–2° on both banks. (It was perfect the very next "frame")

So from what I can tell, the phasers seem to actually be responding correctly, unless there's another way I should be testing them. Is there a specific phaser failure/test you're thinking of that wouldn't necessarily show up in desired vs actual cam position?

Seriously man, thank you so much for your help and time. I have spent so many hours looking at this and appreciate someone with more knowledge taking the time. Thank you.

 

[cbrtrx]

Based on your response I just checked the VCT/cam timing data with my Autel while I was able to catch the idle acting up. I logged actual intake/exhaust cam position on both banks along with desired-minus-actual.

The exhaust cams stayed basically at 0° with only a few tenths of a degree of error. The intake cams were being commanded back and forth quite a bit during the rough idle, but both banks followed each other pretty closely and returned to the commanded positions. I did see brief desired-vs-actual differences of around 10–15° while the cams were actively moving, but once they reached the commanded position the error dropped back to roughly 0–2° on both banks. (It was perfect the very next "frame")

So from what I can tell, the phasers seem to actually be responding correctly, unless there's another way I should be testing them. Is there a specific phaser failure/test you're thinking of that wouldn't necessarily show up in desired vs actual cam position?

Seriously man, thank you so much for your help and time. I have spent so many hours looking at this and appreciate someone with more knowledge taking the time. Thank you.

So when the intake cams weren't oscillating around and it was very close to commanded was the idle smooth? Was the bank to bank airfuel steady?

 

[RexlessTazz]

So when the intake cams weren't oscillating around and it was very close to commanded was the idle smooth? Was the bank to bank airfuel steady?

I did not have the bank to bank, like any air/lambada PIDs selected with that recording. I can go add those and record now. Thank you.

 

[RexlessTazz]

So when the intake cams weren't oscillating around and it was very close to commanded was the idle smooth? Was the bank to bank airfuel steady?

I logged desired/actual RPM, both intake cam actual positions and desired-minus-actual, B1/B2 AFR, and B1/B2 STFT.

At one point the idle dropped to 689 RPM with an 800 RPM target, but both intake cams were essentially stationary around 0 and very close to commanded. B1 was -2.0 with +2.0 desired-minus-actual and B2 was +0.87° with -0.87° error.

At that same point B1/B2 AFR were 13.706/13.765 and STFT were -8.59%/-7.03%, so both banks were moving together pretty closely.

A few seconds earlier I had around 11° desired-vs-actual error on both intake cams while they were transitioning, but RPM was still 825. Then once the cam error had mostly disappeared, RPM dropped to 689.

So it doesn't look like the bad idle only occurs when the cams are oscillating or failing to track commanded position. To my understanding they are tracking properly.

I should also clarify that the car has 33K miles on it. ESS kit was installed around 20K.

Let me know if there's anything else I can look at or other info I can provide. Thank you!

 

[Swtbabybilly]

I would find out what kind of fuel system the car has, see if your running a bap see if you got an upgraded hate and maybe 1 of the pumps is shitting out just to be safe, you dont want to mess with fuel and go lean then you will have bigger issues

 

[Swtbabybilly]

Also im sorry if you answered this but who is it tuned by? Could they maybe look at logs to check if they see something weird

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