Ben_C

Hi all,

A quick question: when I start my car cold, it settles into an idle then within a few seconds "stumbles" and then is noticeably noisier as it re-settles into idle. It almost sounds as though an exhaust bypass has been activated (pretty sure I don't have one but not 100%!). Is this normal? I thnk it's running pretty rich during this "noisy" phase (based on smell).

Car is 2003 996tt with Tubi exhaust. The car runs great and this query is out of curiosity more than anything else.

Regards, Ben

"02996ttx50

they all do that. however, rough idle AFTER starting running for a few secs is *usually* a sign of a dirty or failing MAF. from my limited experience. the rough idle stutter after starting is nothing to be concerned about.

Macster

I think what you are hearing comes from the switchover of the intake valves from their normal high lift to special low lift operation.

At some times, and shortly after cold start is one such time -- as best I can tell from the info I have -- the intake valve zero-lash lifters are switched so the intake valves are connected to lower lift lobes.

Among other things this helps the engine obtain better combustion efficiency while cold to cut down on emissions during this time.

This switchover doesn't take place before engine start because the default is high lift is selected. Once the engine obtains sufficient rpms then the switchover occurs.

The switchover is accomplished by oil (under pressure) being routed to special ports that intersect holes in the intake valve lifter buckets and these holes then direct this oil to push small (but hard/tough) pins into a position that allows the low lift lobes to drive the valves.

Sincerely,

Macster.

Kevin

My take is that you are hearing the LEAN burn "air quality" feature of these engine being kicked in on a cold start. Yes, you are correct the engine is running rich similar to a Choke being turned on to keep the engine running while cold. The electric air pump activates on cold start, you hear the gurgle. The air is injected in the exhaust port. The leans out the RICH fuel mixture.

My recommendation for a 10 year old car is to replace the wideband O2's, and MAF. Newer plugs and coilpacks. This will smooth out these early RPM stumbles.. There have been numerous folks that have done this and have noted the better running conditions.