sejone63

I have a 2001 A6 4.2 Quattro. it starts and runs fine right up until it warms up a little bit then when I am at a stop it seems to miss causing a rough idle. I don't get trouble codes. and it seems to go away after I start moving and get above 5-10mph. Anyone have any experience with this kind of issue?

twentysevenlitres

Check your vacuum hoses - even a slight leak can cause lumpy idle and hesitation.
Clean your throttlebody - properly with throttlebody cleaner, you'll need a second person to turn ignition on and depress the throttle to open the butterfly.
Perform a throttlebody adaption - either via VCDS or manually by turning ignition on but not starting the car for 1 or 2 minutes (I forget which) and you'll hear it whirr away for about 30 secs...

Failing that, give the MAF a clean.

SloopJohnB@mac.com

The coolant temperature sensor would tell the ECU the engine wrong things.
It seems to fail a lot.
IIRC CTS failure won't show up on VCDS.

sejone63

Ok so a code popped up yesterday, 1269 engine coolant temp sensor. Thank you both for time in helping with this problem. I am going to buy the part and change it out. Does anyone know if there is a good video showing how to change it out? I have some instructions and it seems pretty straight forward but it's nice to see something if possible.

sejone63

Sorry I think the code was 1249...

SloopJohnB@mac.com

The trick is to make sure the engine is cold and the cap is on the expansion tank. I don't know what the vacuum setting is on the cap, but it won't allow much fluid to drain out without the cap being open.

Get the new part (get a NEW C-clip too!!) ready with a dab of coolant around the O-ring and plastic bit before you pull the old C-clip and old CTS…make sure you get the old O-ring out!, then quickly stuff the new sensor in there and new C-clip.

the ebahn Bentley says it's in the coolant pipe behind the intake manifold.


But I know dang well there's also some type of coolant sensor in the lower radiator hose, I changed it on GP a few years ago on our 2K4.2A6Q when I had the alternator out and was working on the oil cooler coolant transfer pipe.
Here's a pic of the lower radiator hose, you can see where the sensor goes:
2000-2004 Audi A6 Quattro Radiator Hose - Cooling System - Genuine, Lower 00-04 A6 Quattro Radiator Hose - 7452-01061099 - PartsGeek

sejone63

Ok, so I pulled the engine coolant temp sensor and replaced it. I cleaned the MAS. I traced all noticable vacumn lines and even put my small tank of propane I use for soldering around the lines and no increase in rpms. I found the sensor in the lower radiator line, that is for the aux fan. I have not cleaned the throttle body yet because I have not had a second person, but I will soon.

I have started noticing the miss running down the highway at 70mph. Still no codes showing up. Any other adivce? and by the way thank you all for your suggestions.

johnkk

Since the missing occurs only after warm up, I suspect the MAF. You may just be getting intermittent misses that haven't reached the point of lighting the MIL. Your code reader might not be able to monitor issues without the MIL on, as can a VCDS(vag-com). Without a VCDS, I would try unplugging the MAF(MAS) and see if the missing stops. If so, I think you might have a MAF on the way out. You have to get lucky for cleaning the MAF to fix a MAF issue.

SloopJohnB@mac.com

It's unusual but not unheard of for a spark plug to give problems. Other issues are leaking cam cover seals..you may have a plug well with oil in it. VCDS will tell you which hole is misfiring..you can switch coils from the misfiring hole to an adjacent hole and see if the misfire moves. If the misfire moves, replace the bad coil. I've had two bad coils in 15 years on the old 2K4.2A6Q. I would change plugs every 30K miles per scheduled maintenance although the old plugs looked fine, no misfires, and there was no improvement in mileage with new plugs. I did have a misfire on my 02S6 and when I went to that plug I discovered it was loose!! Last time plugs were changed an audi dealership tech did the job when putting in the new engine. I went around and torqued all the plugs myself.

I don't recall if your original post indicated random misfires on all plugs.

sejone63

I have pulled a couple plugs just to verify they have been changed lately. I found a service reciept the previous owner kept and it said the coil packs and plugs were changed in Oct 2014. I am going to get it on a computer today at a shop so I will let you know what the scan results are.

thanks for your very helpful suggestions. I did purchase the VCDS attachment over the weekend.