Loss of power after spark plug replacement

[RichardChen]

Hey guys,

I own a 2017 Mustang V6 Manual (MT) with roughly 100,000 km on the clock. I've been fighting a strange power loss issue and could really use some diagnostic thoughts/ideas.

Initial Plug Change: Replaced the original plugs with NGK 6905s. Right after, I noticed immediate power loss (sluggish performance, hesitation during acceleration). No CEL or stored codes. Checked the new NGK plugs and found they weren't gapped correctly. Put the old plugs back in, and full power returned immediately.

The Issue Returns: Drove fine with the old plugs for about 3 days. I drove to the office in the morning and it still behaved normally. On a super hot afternoon in Toronto, right as I was about to drive home, I noticed the power loss returned out of nowhere. It feels like it's down to ~200 HP. Still no CEL or codes.

Recent Rebuild/Maintenance: Assuming it was an ignition issue, this week I replaced:

• 6x Motorcraft Spark Plugs (SP589 - gapped correctly)
• 6x Motorcraft Ignition Coils (DG563)
• Intake Manifold Gasket
• PCV Valve

The Current Problem: Even with all new Motorcraft plugs, coils, and gaskets installed, the car is still severely lacking power and feels sluggish—just like when the incorrectly gapped plugs were in. There are still no CELs or pending/stored diagnostic codes.

Has anyone run into something similar, or have suggestions on what to check next (MAF sensor, fuel pressure, vacuum leak, clogged cat, etc.)? Any thoughts or guidance would be greatly appreciated!


Update 2026/08/05: I start driving it daily for like 2 days and I found that the engine brake also reduced as the torque reduced. Does that means cylinder loss compression?

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[robvas]

Put the car on a scanner so you can check the sensors and log some data. Don't just throw parts at it

 

[ORRadtech]

Put the car on a scanner so you can check the sensors and log some data. Don't just throw parts at it

Agreed. Put it on a good bidirectional scan tool. Check fuel trims, misfires, 02 sensor function, timing, injector function, fuel pressure, etc, etc.
The parts cannon approach is expensive and you have to get lucky for it to be effective.

 

[horsepower addiction]

Check for knock if it shows knock weather it’s real or false it is pulling timing, check intake air temps if it’s too high ether real or bad sensor it’s pulling timing. Check fuel trims maybe you got bad gas. Check cylinder contribution. Check oil pressure. Check cam advance. Check throttle body angle

 

[Craig Brown]

When you pulled the coils of did any of the rubber boots separate from the coils? If they did there is a spring inside the boot that has to be seated in the boot and if it is not, you could be getting weak spark. You say that you have the proper plug gap, but you don't say what that is. I don't run the gap at the .042 - .045 I set my gap at .030.

 

[allfivefifty]

There has been a lot of reports on counterfeit spark plugs. Amazon and ebay from what I recall. They make the electrodes out of cheap materials and can wreck the engine. I recommend you research how to identify counterfeit ones if you didn't get yours from a Ford dealer or reputable source.

 

[BullittTheBlueSky]

Check to be sure you re-connected everything when you finished the job. I missed a cam position sensor when I did my plugs, and boy did it run terrible. Plugged in that sensor, and it ran just fine.

 

[WItoTX]

Yeah, I'm with something got bumped during install.

Regardless, what you replaced is exactly what I'd do, and probably good maintenance.

 

[RichardChen]

Agreed. Put it on a good bidirectional scan tool. Check fuel trims, misfires, 02 sensor function, timing, injector function, fuel pressure, etc, etc.
The parts cannon approach is expensive and you have to get lucky for it to be effective.

I agree. Before replacing the whole ignition system, I scanned the car using a Vgate vLinker MC with FORScan, but it didn't show any DTCs or pending codes.

A friend of mine had a similar issue with his Fiesta a few years ago—rough idle, misfires, and trouble starting, also with no codes. He initially replaced just the spark plugs, which didn't solve it. Eventually, he discovered that a bad ignition coil had damaged the new spark plugs, and the damaged plugs in turn ruined the replacement coils. He ended up having to replace all the spark plugs and coils at the same time to break the cycle. That's why I decided to replace the entire system here—I thought I might be dealing with the same scenario.

 

[RichardChen]

There has been a lot of reports on counterfeit spark plugs. Amazon and ebay from what I recall. They make the electrodes out of cheap materials and can wreck the engine. I recommend you research how to identify counterfeit ones if you didn't get yours from a Ford dealer or reputable source.

I got my plugs from the dealer and the coils from RockAuto, so I think I should be on the safe side.

 

[RichardChen]

Check to be sure you re-connected everything when you finished the job. I missed a cam position sensor when I did my plugs, and boy did it run terrible. Plugged in that sensor, and it ran just fine.

I did check all connctions that need to removed during the plugs change. Don't see anything loose. I checked my vacuum gauge. Usually 500 mmHg ish without AC on at 650 RPM. Does a smoke test necessary?

 

[RichardChen]

Yeah, I'm with something got bumped during install.

Regardless, what you replaced is exactly what I'd do, and probably good maintenance.

By the time the power loss returned, my first thought was that the weather was just too hot, and thermal expansion or vibration caused something to come loose. I just don't know why no codes and CEL at all. That is something unusual. Do you think the smoke test will help?

 

[RichardChen]

Check for knock if it shows knock weather it’s real or false it is pulling timing, check intake air temps if it’s too high ether real or bad sensor it’s pulling timing. Check fuel trims maybe you got bad gas. Check cylinder contribution. Check oil pressure. Check cam advance. Check throttle body angle

IAT depends on weather and traffic. Sometime I can hit ambient temp+8 c. Most case ambient temp+15 C. Sometime can go up to ambient temp+25 C in heavy traffic stop condition. However, no matter at what temperature condition, I don't see much power change. LTFT looks fine +/-1% while STFT +/-5% ish.

I just filled up with 91 gas and plan to drive it for few day to see how it react.

I don't see if there is any PID relate to fuel pressure. Let me know if any.

I attached 3 pics. The first one was logged during testing driving. I am on my 3rd gear on the highway from ~2k rpm to ~4k RPM. APP 1 and APP 2 is stand for accleration padle postion. TP is the Throttle body position. I don't know if I got the correct PID for the knock sensor. But that is the only one that I found about knock. Does it means there are some knocks there pulling timing? The other two pics are the test result for mode$06. I don't see anything unusual. If you can find something let me know. Thanks.

 

[RichardChen]

When you pulled the coils of did any of the rubber boots separate from the coils? If they did there is a spring inside the boot that has to be seated in the boot and if it is not, you could be getting weak spark. You say that you have the proper plug gap, but you don't say what that is. I don't run the gap at the .042 - .045 I set my gap at .030.

Rubber boots still there. Springs still there. According to Ford, the gap should be 0.049 to 0.053". Are you also own a V6 as well? Ecoboost usually run a narrower gap as far as I know.

 

[horsepower addiction]

The knock is all negative meaning it’s adding timing everything looks good

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