intermittent weird running

[decampos]

There's a strange thing that my car keeps doing: When starting from cold the engine runs and sounds great and remains so until some point down the line (20 minutes or longer sometimes), where the engine note (or exhaust note) changes to a more rhythmic beat (think of like a machine gun sound) and the car begins to run slightly sluggishly.

Any idea what this could be? I'm stumped.

Any pointers very much appreciated.

 

[61porsche]

It's running lean after warm up. Pull the plugs and check the color.

 

[decampos]

Thanks for your reply 61porsche. If the state of the plugs suggest it is running lean would you suggest trial and error tinkering with the fuel volume knob on the ECU or should I begin somewhere else?
Many thanks
Ben

It's running lean after warm up. Pull the plugs and check the color.

 

[61porsche]

If the lean condition is indicated by the plugs, first check for restrictions in the fuel, dirt, rust, etc. Check your fuel pressure regulator to see if it's the pump or regulator. ( Slowly dieing perhaps.) If the pump is suspect, amp it, clean the connections, etc.