Bog down at full throttle after spark plug change

MATTHEW STEPHEN STENGER

MEMBER
2004 SUBARU FORESTER
2.5L
4 CYL
4WD
AUTOMATIC
207,000 MILES

Hi, I recently changed spark plugs with new iridium plugs. Engine runs great throughout the rpm range, slightly more power than before, as expected. However now when I open the throttle/gas pedal above 3/4 (full throttle), regardless of rpm the engine makes a loud humming noise rather than adding power. It seems as if it is choked for air or fuel. I have since also changed the plug wires, ignition coil, air filter and fuel filter, and the fuel pump is relatively new, but the issue persists. Could this be the TPS? MAP? A bad cat? Trying to decide what to do next.

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,724 POSTS

Hi,

What led you to replace the plugs? Was there a prior issue?

Also, the noise you hear could be a vacuum leak. Make sure the air box and intake tube are fully connected and tight. If either is loose, the humming sound could be what is heard. Also, the sound would get louder the more throttle you give it.

Is it loud enough to record and upload it for me to hear?

If you look at the attached pic, it shows what I am referring to. Please make sure none of the connections are loose, cracked, or disconnected.

Let me know.

Joe

Image (Click to make bigger)