Inconsistent/rough idle

[A4 Quattro]

Hey all, it's been a minute since I've had to make a thread around here. But this one has me puzzled. Sorry in advance for the probably long post. For reference, the car is an '05 1.8T @ 174k miles.

I've been arguing with a rough idle that occasionally spikes a couple hundred rpm. First things first, I checked all my vacuum lines, and quickly found a broken check valve. I replaced it, test drove the car, all was well.

A day or two later, I started the car, and was greeted with a low, rough idle, and some random misfires. After arguing with various vacuum lines, pulling and cleaning the MAF, and clearing the the misfire codes, it seemed to go back to normal. I had my mechanic give it the vagcom treatment, which it passed flawlessly. No codes, fuel trims look normal, idle is normal, etc. Smoke test found a vacuum hose not seated properly, but nothing else.

That brings us to today. I drove about 40 miles with no issues, parked and turned the car off for about 10 minutes. When I turned it back on, I was greeted again with random misfires, and a VERY inconsistent idle, starting at about 700, and gradually rising to about 1350 or so. I let it go for a bit to see what kind of codes it gave me. These two stood out among the various pending misfire codes:
P0171 - fuel trim, lean
P0507 - high idle

The fueling code either means too much air or not enough fuel. I feel fairly confident the vacuum leaks are all sealed up, which rules out the excess air option. This is the first thing my brain put together: "Leaky injectors, low fuel, idle drops too low, engine misfires. ECU increases idle to add more fuel, registers high idle code."

I'd like to avoid blindly throwing parts at it in hopes of fixing it. But right now, I'm leaning towards changing injectors, and/or cleaning and realigning the throttle body. Any thoughts, or similar experiences would be helpful.

[Ted222]

Hey all, it's been a minute since I've had to make a thread around here. But this one has me puzzled. Sorry in advance for the probably long post. For reference, the car is an '05 1.8T @ 174k miles.

I've been arguing with a rough idle that occasionally spikes a couple hundred rpm. First things first, I checked all my vacuum lines, and quickly found a broken check valve. I replaced it, test drove the car, all was well.

A day or two later, I started the car, and was greeted with a low, rough idle, and some random misfires. After arguing with various vacuum lines, pulling and cleaning the MAF, and clearing the the misfire codes, it seemed to go back to normal. I had my mechanic give it the vagcom treatment, which it passed flawlessly. No codes, fuel trims look normal, idle is normal, etc. Smoke test found a vacuum hose not seated properly, but nothing else.

That brings us to today. I drove about 40 miles with no issues, parked and turned the car off for about 10 minutes. When I turned it back on, I was greeted again with random misfires, and a VERY inconsistent idle, starting at about 700, and gradually rising to about 1350 or so. I let it go for a bit to see what kind of codes it gave me. These two stood out among the various pending misfire codes:
P0171 - fuel trim, lean
P0507 - high idle

The fueling code either means too much air or not enough fuel. I feel fairly confident the vacuum leaks are all sealed up, which rules out the excess air option. This is the first thing my brain put together: "Leaky injectors, low fuel, idle drops too low, engine misfires. ECU increases idle to add more fuel, registers high idle code."

I'd like to avoid blindly throwing parts at it in hopes of fixing it. But right now, I'm leaning towards changing injectors, and/or cleaning and realigning the throttle body. Any thoughts, or similar experiences would be helpful.

I want to hear how you finally fix this problem.....
Why dont you just start the engine and start spraying brake kleen all around the suspecting areas and see if the engine RPM changes..
Waiting on your rely

[A4-Mike]

Hey all, it's been a minute since I've had to make a thread around here. But this one has me puzzled. Sorry in advance for the probably long post. For reference, the car is an '05 1.8T @ 174k miles.

I've been arguing with a rough idle that occasionally spikes a couple hundred rpm. First things first, I checked all my vacuum lines, and quickly found a broken check valve. I replaced it, test drove the car, all was well.

A day or two later, I started the car, and was greeted with a low, rough idle, and some random misfires. After arguing with various vacuum lines, pulling and cleaning the MAF, and clearing the the misfire codes, it seemed to go back to normal. I had my mechanic give it the vagcom treatment, which it passed flawlessly. No codes, fuel trims look normal, idle is normal, etc. Smoke test found a vacuum hose not seated properly, but nothing else.

That brings us to today. I drove about 40 miles with no issues, parked and turned the car off for about 10 minutes. When I turned it back on, I was greeted again with random misfires, and a VERY inconsistent idle, starting at about 700, and gradually rising to about 1350 or so. I let it go for a bit to see what kind of codes it gave me. These two stood out among the various pending misfire codes:
P0171 - fuel trim, lean
P0507 - high idle

The fueling code either means too much air or not enough fuel. I feel fairly confident the vacuum leaks are all sealed up, which rules out the excess air option. This is the first thing my brain put together: "Leaky injectors, low fuel, idle drops too low, engine misfires. ECU increases idle to add more fuel, registers high idle code."

I'd like to avoid blindly throwing parts at it in hopes of fixing it. But right now, I'm leaning towards changing injectors, and/or cleaning and realigning the throttle body. Any thoughts, or similar experiences would be helpful.

Had a similar issue that turned out to be a burnt out MAF sensor

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[rollerton]

Definitely check out the PCV valve and suction jet pump. I had similar issues and #1 code I had consistently was Idle, Higher than expected. Occasionally I'd get the fuel trim code. Replaced the PCV valve and it's fixed- but like you it came back 40 miles later. THE BRAND NEW PCV VALVE DIED IN 40 MILES.
While it's idling try and remove the oil cap. If it's STUCK on there very good you have a bad PCV valve. I actually fixed it by replacing the PCV valve with a one-way vacuum valve. It's not right but the car has run better than ever. I have the new PCV valve to put on but it runs so good I feel like leaving it.

[Cybersombosis]

Have you changed your coolant temp sensor? This is a relatively cheap part for under $10 from Rock Auto or a little more from the dealership. I had idle issues because of it. Low idle at start up, fluctuating up and down.

Doug

[Grenalis420]

Yes chech your suction pump valve. The older versions had a tendency to pop apart at the top, causing rough idle, and even stalls. Its near the back center, just to the right. Part number 058133753D is the newest rev.

https://www.ecstuning.com/b-hamburg-...8aAoCyEALw_wcB

[Audishiit]

What stands out to me is you said fuel trim levels were normal,
What were the LTFT and STFT
Because I recently dealt with P0172 running too rich.

And from my understanding is if the fuel trims #s are too far out of range you’ll throw a code and too throw a too lean code you’re fuel trim levels will have to be out of spec.

If you truly want to avoid throwing parts at it
You should do a couple tests.
1. Test the fuel pressure at the rail(you’ll have to get the specs online because I don’t know them off hand)
2. Get your friend with vag-com to measure MaF readings, Idle should be between 2-4g/s. Peak airflow for a chipped car is 150-170g/s. Stock tune should be in the 130ish g/s range.
You can test peak airflow by going WOT in 2nd , 3rd gear.
If you are getting 2 g/s and lower at idle you may want to first clean the MAF sensor before going any further. Another possibility for a low g/s accompanied with a rough idle could be a result of a very small vacuum leak. Check your STFT and LTFT. A high adjustment in the STFT at idle would indicate the the ECM is adding fuel to adjust for the false air condition where some of the air is entering the intake after the MAF sensor.
3. Like mentioned above you’re rear Coolant temp sensor is a $10 part and a good place to start
4. old guy suggested this to me for checking injectors. Pull the fuel rail, and get 4 clear cups , measure the cups and then have a friend turn over the car and shoot the fuel into the cups while visualizing the spray pattern and then weigh the cups to verify there all spraying same amount of fuel.


I’m assuming you don’t have a boost gauge to measure you’re vacuum under idle?

[Audishiit]

have you done a boost leak test yet?

[EuroxS4]

Sounds like a failing pcv valve or sjp. Very common once they get up in mileage. If its setting the fault at idle you should be able to hear it sucking up air. I would check all the pcv related hoses as well as sjp and physically check the pcv valve and the elbow that always breaks.

[A4 Quattro]

Had a similar issue that turned out to be a burnt out MAF sensor

I drove out to the junk yard today and grabbed a MAF for $20. It threw a P0101 immediately. Dammit. I cleaned the old one I took out (again), put it back in, and the car seems to run much more normal. Let's see how long this lasts lol. I bet my MAF is going bad, or reporting wrong numbers to the ECM, and cleaning it gives me a few days of bliss.

I’m assuming you don’t have a boost gauge to measure you’re vacuum under idle?

I do have a boost gauge. Vacuum usually sits between -19 and -22 at idle when things are running normal, and about -23 to -25 under load. Boost measures up stock 8psi and holds. When it idles low, vacuum will drop down as low as -10 to -17 and misfire like all hell. When it's idling high, the vacuum is normal -19 to -22.

Sounds like a failing pcv valve or sjp. Very common once they get up in mileage. If its setting the fault at idle you should be able to hear it sucking up air. I would check all the pcv related hoses as well as sjp and physically check the pcv valve and the elbow that always breaks.

Definitely check out the PCV valve and suction jet pump. I had similar issues and #1 code I had consistently was Idle, Higher than expected. Occasionally I'd get the fuel trim code. Replaced the PCV valve and it's fixed- but like you it came back 40 miles later. THE BRAND NEW PCV VALVE DIED IN 40 MILES.
While it's idling try and remove the oil cap. If it's STUCK on there very good you have a bad PCV valve. I actually fixed it by replacing the PCV valve with a one-way vacuum valve. It's not right but the car has run better than ever. I have the new PCV valve to put on but it runs so good I feel like leaving it.

I put my ear up the to the SJP, and hear nothing, so for the time being, I'm going to say that is okay. I have some new injector cups coming in the mail to replace my existing ones to try and stop my leaking injectors (maybe related to this?), which means, I'll have the intake manifold out pretty soon. I feel like I can hear a "whooshing" sound coming from the area the PCV valve should be under the IM, so I'm excited to find some cracked valves/lines that I can't see from the topside. I pulled my oil cap off while idling, and it came right off, which I was kinda bummed about after reading your suggestion on that.


Also while I have the IM off, I'm going to be replacing the j-plug on the tstat housing that has been causing my driveway to be covered in coolant. I'm going to threadjack my own thread here. Has anyone done a tstat/j-plug job without taking the alternator off? All the DIY threads suggest it, but they also still all have their intake manifolds on. One way or another, I'll be finding out soon.

Thank you all for your inputs!

[Gin+]

Plan on replacing this too...https://www.ecstuning.com/b-uro-part...a103213am~uro/

[A4 Quattro]

Hello again everyone. I pulled the intake manifold today and found one or two things that might be causing my problem. Nearly all of this has/had the single use clips Audi loves to use, so I'm betting these parts have never been replaced. I noticed the T fitting was covered in lots (and I mean lots) of gunk, which means it's either leaking or being leaked on. There isn't anything above it that would leak, so I'm assuming it's leaking. I also noticed the end of it that connects to the elbow was falling apart (IE: the leak). The elbow itself, as I was warned, broke upon removal. Also upon removal, the panake valve at the top behind the vale cover broke. I'm going to swing by Audi tomorrow and see what all they stock locally. Otherwise I'll be making an ECS order and waiting a few days to get this all back together. I will update again hopefully soon. Let's hope I can put all this back together now lol.



Also while the intake manifold is out, I picked up some billet injector cups to preventatively solve the problem of the original plastic ones breaking. I might threadjack my own thread again and post some details on that.

PS: if anyone has been following along, I went ahead and did the j plug today with the alternator still on. In case anyone was curious if it was possible, It is.

[Audishiit]

Old guy suggested using Teflon tape for injector cups, if you search around this forum you’ll see his post,
Basically go around couple times counter clockwise.

They should have a Pancake valve instock locally, it would be a good idea to replace the hose that connects to the pancake valve, mine was really dried out and had developed a decent size crack/hole

Crankcase breather: given that it broke you should piece it back together to see if there’s any large plastic pieces missing that potentially fell in while removing it.

Onetime hose clamps are anoyying to remove haha, but now would be a good time to get a count on how many hose clamps you need & sizes and head to Home Depot to pick them up.

T fitting: I don’t think i have the same vacuum setup as you or maybe I do & don’t remember off the top of my head; but I know the T fitting has a check valve in it so I wouldn’t skimp out and get a cheap one.

- - - Updated - - -

What color is that in the background of the picture?

[old guy]

That picture is pretty interesting. That setup is not what came on a 2005 1.8T. It came on the later '02 and early '03 model years. By 2005 several of the components were changed. The PCV and associated check valve were incorporated into one unit and the metal drop tube on the back of the engine was replaced with a plastic tube. Personally I would maintain what you have. the plastic drop tube is what I would consider a downgrade.

[A4 Quattro]

Old guy suggested using Teflon tape for injector cups, if you search around this forum you’ll see his post,
Basically go around couple times counter clockwise.

They should have a Pancake valve instock locally, it would be a good idea to replace the hose that connects to the pancake valve, mine was really dried out and had developed a decent size crack/hole

Crankcase breather: given that it broke you should piece it back together to see if there’s any large plastic pieces missing that potentially fell in while removing it.

Onetime hose clamps are anoyying to remove haha, but now would be a good time to get a count on how many hose clamps you need & sizes and head to Home Depot to pick them up.

T fitting: I don’t think i have the same vacuum setup as you or maybe I do & don’t remember off the top of my head; but I know the T fitting has a check valve in it so I wouldn’t skimp out and get a cheap one.

- - - Updated - - -

What color is that in the background of the picture?

I'm going to pick up everything locally if I can, or buy everything online at the same time. No sense in replacing the pancake valve if I can't replace everything else, ya know?

I'm pretty sure I have all the big pieces of the crankcase breather. If anything fell in, it would have to be a pretty small piece.

Also, the color in the background is a metallic purple wrap. That's my other broken B6 😂😂. I'm always down to whore my car, peep the clicky click HERE.

That picture is pretty interesting. That setup is not what came on a 2005 1.8T. It came on the later '02 and early '03 model years. By 2005 several of the components were changed. The PCV and associated check valve were incorporated into one unit and the metal drop tube on the back of the engine was replaced with a plastic tube. Personally I would maintain what you have. the plastic drop tube is what I would consider a downgrade.

I was also surprised, since this isn't what I thought I would be looking for. I was also a little curious why people were complaining about the metal tube breaking when they take it out haha. Maybe this has been addressed before, and Audi did the work using older style parts and awful one time use clamps? Hmmmm...

[A4 Quattro]

I wanted to update this thread again one last time. The car is officially starting, idling, and driving the best it has in the year I've owned it. I ended up replacing the PCV T fitting, elbow, and the pancake valve I broke taking everything out. Also while the manifold was out, I replaced the injector seats/cups and injector o-rings. I had a few unexpected hangups along the way, but all said and done, it was a pretty easy fix. I got mad at the car about a lot of things, cut my hands up a couple times, and learned a lot about the car. Thank you all for your inputs. I hope someone down the road can learn something from this thread.

Happy wrenching

[Audishiit]

When you did the injector inserts what did you use on the threads?

[bushyhippo]

if this is leaking, will it cause misfires?

[Ol Dirty Noodle]

if this is leaking, will it cause misfires?

Yes