Check Engine Light After Changing Spark Plugs
 

Hello, All
A new member on on this forum, but have owned several Volvos in the past. Currently, have a 2008 S40 that my daughters drives to school. The car has 139K miles and bought it for her three years years ago. It has been running fine. I just changed the five spark plugs with Volvo part # 30751806-8. After changing it, it started right up and ran fine. Didn't test drive it. The next day, she drove it less than two miles and the Check Engine light came on. I went through all the plugs again ad made sure the connections are seated properly. I might add, I took each one off and replaced it one at a time. I took it to an Auto Part store and they checked and said it was "running lean". More over, I hear a hissing noise, indicating an air leak somewhere, but I don't see any. Can anyone help? Thanks in advance for your assistance.

Sounds like a vacuum leak - you've got to find that to figure this out. I like to use a piece of tubing as a stethoscope, and just poke around under the hood until you REALLY hear the hiss (you'll only hear it when you're very close to the source).

Also, did you gap the plugs before installing them?

Don't you have to take the upper part of the intake manifold off to get to the coils/plugs? (you did not list your car as a turbo) And there are 5 orings that you replace when you reinstall the upper part of the intake manifold? Could it be leaking from those orings or the two or three vacuum lines you had to disconnect?

An easy way to find air leaks is with a can of carb cleaner - closely spray areas that might be leaking - if there is a leak there will be a momentary (or longer) change in the engine speed.

Thanks for chiming in. No, I did not gap the spark plugs. The were pre-gapped, so I was told. Your idea of of listening using a piece of tubing is sound. I have to enlist my daughter or my wife's listening skills as I wear a hearing aid on one ear and a Cochlear implant on the other.
Regards,

Yes, there are five what I would call rubber gaskets, one for each intake manifold. I didn't replace them and they seemed in good shape. I made sure they were properly placed. Mine is not turbo. I also did not disconnect any of the vacuum lines. I left them in place and gently tied the cover upright using a bungy cord, folding where the hoses are. I had previously made an appointment with my local Indy mechanic to have the vehicle checked tomorrow. So, that is rather timely and hope they can figure this out. Thanks for your input.

My bet is the O rings for the intake , always replace with new , even when they look ok as they will seat in a different position and the rubber compound hardens