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996tt rough when cold

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Old 03-16-2017, 06:50 AM   #1
777mech
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Default 996tt rough when cold

The 2001 996tt I just got runs very smoothly, but for about the 1st minute or so during warmup it shakes alittle and you can hear it misfire occasionally through the exhaust, is this normal or is something out of adjustment?
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Old 03-16-2017, 07:06 AM   #2
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This has been discussed quite a bit. One of the consensus reasons is the O2 sensors are not warmed up and providing closed loop feedback, therefore, the ECM is giving a preset mixture ratio, which is probably richer than what normally would be achieved with the proper feedback from the O2 sensors. Hence, the rough running. Most feel its ok and not something to worry about.
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Old 03-16-2017, 08:26 AM   #3
NoogaSparrow
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This is a new to you car correct?
I think you've also mentioned it has a tune?

Could very well be the tune, especially if it's had the issue since you've had it.
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Old 03-16-2017, 03:59 PM   #4
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Quote:
Originally Posted by 777mech View Post
The 2001 996tt I just got runs very smoothly, but for about the 1st minute or so during warmup it shakes alittle and you can hear it misfire occasionally through the exhaust, is this normal or is something out of adjustment?
Absent a CEL or untoward engine behavior at other times probably normal.

My 2003 Turbo engine runs a bit rough -- compared to say my 2002 Boxster engine -- at cold start. The VarioCam Plus low valve lift is activated to reduce engine friction at cold start and extra fuel is fed to the engine to be burned in the exhaust system to help warm up the converters.

In an enclosed space I can hear faint exhaust backfires for a while as the cold engine idles. Exhaust backfires are a sign the engine's running rich.

But as the engine idles it gets smoother and by the time the RPMs drop (which is concurrent with the secondary air shutting off and the DME stolps adding extra fuel) the engine idles pretty smooth. Still not as smooth as my Boxster but much smoother than right after a cold start.

Your car is I take it "new" to you? Be sure all service items are up to date. Plugs and possibly coils. I've changed the plugs in my Turbo several times without noticing any improvement. But when I had the plugs (which didn't have that many miles on them) and the coils (which had around 140K miles on them) replaced together the engine perked up some afterwards. I attribute this to the coils being replaced.

Similarly when I had the O2 sensors replaced (even before the plugs and coils at around 132K miles -- one was throwing an error code (P0035 IIRC and another (pending code) I don't recall) -- the engine ran better afterwards.

The fuel filter is kind of small and should be changed on schedule.

Gasoline is important. Buy a name brand/top tier gasoline of the proper octane grade (93 if you can get it, 91 is "ok" if that is all there is: California) and from a busy station to ensure fresh gasoline.

You might try a bottle of Techron. I get the Techron in a bottle that "treats" 20 gallons and dump the whole bottle in the tank even though the Turbo gas tank holds just 16+ gallons. Then fill the tank up with your chosen premium grade of gasoline and drive the car.

When the fuel tank level gets low, at or below 1/4 tank, consider repeating the Techron treatment, using a 2nd bottle. After the fuel tank with the 2nd bottle of Techron added is down to 1/4 tank or thereabouts it is recommended to change the oil/filter.
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Old 03-16-2017, 06:54 PM   #5
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Yes tune parameters include warmup mixture and rpm.
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Old 03-17-2017, 12:35 AM   #6
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Lazy tuners will not spend much time tweaking the warm-up maps. These maps are also very time consuming to perfect, since the engine warms up quite fast leaving little time to get much datalogging at specific points for each logging session. IMO it is much easier to tune the regular maps than the warm-up maps.

If you don't have a tune, rough running in the morning can also be a sign that the plugs/coils need to be replaced. The engine has to misfire pretty badly and for a certain length of time before OBDII actually throws a code and lights the CEL.

Dan
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Old 03-17-2017, 01:03 AM   #7
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Reflash the stock file into the ECU and then figure out your issues.. See if they follow over to the stock file.. AND, disconnect your MAF and see what your idle does (you will need to clear codes)

The single most idle and run issue on this engine is Intake leaks and cracked control lines. Pump up your intake system (both sides, pre turbo and post turbo).

Exhaust leaks between turbo and exhaust can cause cold start issues..

Replacing the wideband O2's will clean up sloppy idle and transition to from cold start.

A plugged fuel filter AND splits in the front fuel pump "internal nylon and rubber" will result in lower fuel rail pressure which impact the idle. So, measure rail fuel pressure..

A fouled throttle body (clean it) and sluggish throttle/plate (internal electric) will not allow a perfect adaptation procedure.. I'd also perform the throttle body adaptation "process"

Fouled and/or leaking fuel injector can also impact start up.. We are starting to see carbon build up in the injector block/intake...

Lastly, you can see what your short term and long term fueling is doing....
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