After my 60k mile 2003 996tt sits for a while and I start it up. The car starts easily the cold idle jumps up but after about 2 seconds the idle will quickly drop, then jump back up to it's high idle position. The idle then acts normally dropping down after a few minutes of running. Has anyone else had this happen? What are some potential causes?

TIA,
Emerson

 

Yep, air intake leak. It could be one of the small hoses that has popped off. Do a pressure test.

 

No check engine light?

 

No CEL or error messages. I did a boost leak test at the turbo inlet and the only slow leak I could hear after pressurizing the system to 15psi was through the intake and from the oil tank, which I think is normal for air to slowly leak past an open valve, etc. I will examine the vacuum tubes that go to the intake plenum as I believe this was bypassed the way I pressurized the turbos for the boost leak.

 

When pressuring from the turbos, the only things that can leak back into the pre turbo intake are the diverter valves or the vent line from the top of the plenum, but there's a check valve in that line so it should hold pressure. It ties into the drivers side intake piping, just before the turbo. You can pop it off and check for air.

 

lower the motor about 3-4" and pressure test to 20-25 psi. this is were the leaks show up. use soapy water in a spray bottle. its amazing the leaks you find that you never hear but easily see when the bubbles start popping up!

 

No other symptoms?

I think it normal.

My 2003 Turbo does this, has done it as long as I can recall.

I think it is when the DME switches the VarioCam Plus to low lift. The drop in RPMs is very brief and accompanied by a very brief dip in oil pressure which would otherwise remain quite high even with an RPM drop as the oil is cold and thick.

The low lift is engaged by oil pressure which drives locking pins -- one each intake lifter bucket -- out of engagement so the outer portion of the lifter bucket -- the portion that is worked by the high lift lobe portion of the camshaft can move up and down independently of the center portion which is driven by the low lift lobe portion of the camshaft.

But check for any air leaks just to be sure.

 

No other symptoms.

My initial pressure test revealed a leaking turbo wastegate diaphragm. The proceeding tests after the wastegate was replaced the only air leak I could hear after removing the air source was in the oil tank and through the intake plenum. I need to make or purchase a better device to pressure test. The Lowes version of the testing device with rubber couplings and pvc plugs won't hold past 15 psi. The pvc plug shoots out of the rubber coupling.

Thanks for the replies. I am going to purchase some semi solid engine mounts and a get a better pressure testing tool and drop the engine a couple inches as suggested and pressure test again.

Macster, The symptom is exactly like as you describe. The way the rpm's drop it seems very computer or electronically controlled rather than mechanically caused. I would add that it doesn't seem to happen after the car has been running. Only when its been sitting for hours. Not sure if it's because of the cold motor condition or something else that would be caused by the motor sitting for a day or so? It could just be the nature of how the system works as you eloquently described in detail.

 

Wrap some electrical tape around the end of your test plug until it's snug in the turbo hose and then push it in far enough so the clamp keeps it in there.