rocketman4321

I've had an on-and-off rough idle when I first fire up my 2.7T in the morning. When it's bad, the car idles unsteadily in neutral, and may hesitate or stumble in gear. Less than a minute later, all is well, and it runs fine after that.It seems somewhat worse after sitting for several days (usually garaged, but one notable tantrum was at the airport). May be worse cold, but today was just cool (55F) and damp. Some days it's fine. No codes.
Per suggestions here I replaced the coolant temp sensor, even though the old one measured in-spec. I've also cleaned the throttle body on the theory that if it was gummed up (and it wasn't too bad) that'd be worse/more sticky when cold.
I had checked the SAI system a little while back, but this didn't really start at the same time. Can't find any hoses disconnected or cracked, but of course it just takes one.
What's next? What operates for 30-60 sec at startup that might be affected?
Thanks for the help!

Prospeeder

The fuel system relies on the intake air temp sensor for cold enrichment as well as the coolant temp sensor. its located right after the throttle body under the intake. Iv seen plenty of cars get hard starts or real poor rough starts from intake temp sensors.

rocketman4321

Thanks--I'll take a look at that tonight. Is it by the coolant bleed bolt? I did damage the retention clip on a connector in there a while back. Secured it with a zip-tie, and it's been fine, but definitely a suspect.

rocketman4321

The connector by the bleed bolt appears to be just an in-line connector on a small bracket. Bentley says this sensor is under the manifold, and "the manifold must be removed" to replace it. Well, now we know that the genius who located the aux coolant pump somehow kept his job at least a few more days...
Bentley also shows a pigtail for test--I didn't find it tonight--will try in daylight.

redbullgotwings

i think you are looking at the wrong sensor in the bentley manual
i know the sensor prospeeder is talking about when you cleaned the TB you should have seen it. it looks like the picture below it bolts in with one bolt from the top of the malifold right behind the throttle body.
you can clean it by putting it in a ziplock bag with some alcohol in it and letting it soak for an hour or so. then shake the bag lightly. then take it out and wipe off any excess residue with a q-tip. be GENTLE!

rocketman4321

I found the correct pigtail--it's to the right(passenger's side) of the throttle body--had to remove one of the big black valves to get to it; almost to even see it. All seemed well--good contact, proper locking, and the sensor side read consistent with the Bentley--don't recall the exact number but it was ~ 2k ohms at garage temp last weekend. I'd earlier had a pigtail on the driver's side of the TB--that has only a single contact and is something else.
Redbullgotwings--it sure appears the sensor is under the intake on mine. Is yours a 2.7T? I'll take another look through the TB and know for sure.

But, unless it's gunked up, this is looking like it's not the cause. Hasn't stumbled for a few days. I'll be letting it at the airport a couple days next week, which will be a more serious test.

redbullgotwings

ya i have the 2.7t i have mine apart in the garage right now about to do the VCG and i will look. i tired to look the other day and didnt see it. but i swear when i took the TB off to clean it a few weeks ago it was sticking strait down from the top of the intake manifold.
then again my memory is horrible so i could be thinking of a different car.
i will look and get back to you.

redbullgotwings

okay so i just looked and it appears that the sensor we are talking about is actually in the rubber Y shaped pipe that is attached to the front of the TB.

rocketman4321

Ah! I removed that Y-pipe assembly as a whole when I cleaned the throttle body. I see the sensor, and I'll trace the wiring tonight. But there are 3 wires going into the connector--the IAT sensor should have 2 per Bentley. And the pigtail connector I found was definitely 2-wire.

Jim Will

I have had this problem more than once. It is more than likely the coolant temp sensor.

It will start as rough running when cold, then it will progress to hard starting when cold, then it will get worse and result in almost no start when cold.

Had my car 13 years and 245k, I've replaced 2 of these sensors.