Completed spark plug replacement now sputters

Welcome to 2CarPros.

If it ran okay before but lost power as it warmed up, that doesn't sound like a spark plug issue. Since now it runs worse, then something happened when he replaced the plugs. Did he properly gap each plug? Is the check engine light staying on when it is running? If it is, have you scanned the computer to determine what diagnostic trouble code is stored?

When an engine seems to lose power when at operating temp, my first suspect is a restriction in the exhaust system or a sensor issue. Let me know if the light is staying on, if he is sure he got the correct coil pack going to the correct cylinder, if the computer was scanned since the work was done, if wiring to each coil pack was replaced to the correct cylinder, strange sounds other than the sputtering. Anything you can think of.

Also, tell him to make sure he didn't by accident disconnect or damaged any vacuum hoses. Here is a link that shows how to check for vacuum leaks. It may help.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know,

Joe

A lot of things can cause a misfire, fuel spark, compression. Did you visually check for spark? Any applicable trouble codes? Sometimes I can use digital multi-meter on low voltage setting, engine idling, touch the coils, one at a time, Some makes, I've seen readings of.6 or.7 volts, not even a full volt, it's an inductive reading, you don't pierce anything or take anything loose, you just touch it. Say you get one or two where the reading is bouncing off zero, that's where you start testing. I don't know what the reading will show on your vehicle? Just a thought.