P0102 and P1857 but only at idle

[fyremanpat]

Im getting these 2 codes and the ESP light only at idle. Under normal driving conditions or even WOT there are no issues. Ive had 3 different MAFs in it so its definitely not a bad MAF. Its been boost leak tested many many times with absolutely zero leaks. At idle im sitting at 21-22mmHg and boosting around 22psi. Ive searched and searched with no luck. Is there anything Im over looking? Thanks!

[old guy]

What is your MAF g/sec reading at idle? If you are getting a low reading with three different MAF sensors it's either a problem with the MAF scaling in your tuning or you are getting un-metered air into the intake manifold at idle. If it has been there all along i would suspect the scaling. If it is a recent occurrence it's probably un-metered air.

You can have a vacuum leak in the block breathers that doesn't show up with a boost leak test. If the leak is on the "checked" side of the check valve you won't hear a boost leak but you will be sucking in un-metered air.

Check your MAF g/s at idle. If you are getting a low MAF g/s at idle just clamp off the vacuum lines connected to the intake manifold one at a time until you see the g/s increase. Then you know where to start.

Good luck!

Edit: I wasn't familiar with the P1857 so I researched it in the Bentley. Quite frankly I'm not sure why you are getting this code. P1857 is an error code for a load signal error that the ECM passes through the CAN bus lead to the TCM. The ECM diagnoses the malfunction for the component in question (in your case I would assume the MAF sensor) and sends the information to the TCM.

Since you don't have a CVT or 01V automatic transmission there would be no TCM for which to send the signal. I'm guessing the DTC is being generated because of the low g/s MAF reading which would translate into a low load signal and the ECM is giving the code even without anywhere to send it.

Again, Good luck!

[fyremanpat]

3.71-3.86 g/s at 800 RPM idle.

[old guy]

3.71-3.86 g/s at 800 RPM idle.

Sounds pretty normal. Maybe a touch on the high side for a manual transmission. What is your engine load at idle? Typically i see loads at 16-18 but my idle g/s is usually just under 3 g/s. If I turn on a bunch of stuff that drags on the engine my idle g/s goes up to around 3.4 and the engine load goes to a max of 20.25

How does that compare to your engine load reading?

The reason I'm asking is because from what I can gather out of the Bentley is that the P1857 has to do with expected versus actual engine load in conjunction with the automatic transmission. Again, I don't know why you would be getting a P1857 but if your engine load at idle is significantly different than mine at idle it may help in figuring out what is causing the problem.

Edit: And did you try clamping off the vacuum lines and looking for a difference in g/s?

[fyremanpat]

That was at cold idle, if it makes a difference. It was about 40 degrees out. I'll let it warm up to normal operating temp today and check it again. I will also try clamping off vacuum lines and report back. Thanks old guy!

[fyremanpat]

Just drove it around to warm it up and its idling at 800 RPM, g/s at 3.04- 4.12 and 19.5% load

Right as I posted this I watched it drop to 0, ESP light came on, idle changed, then it went back to 3.14, idle smoothed out and light went off. I didnt touch a thing when it did this. Now its looping every 30 seconds or so. WTF? This thing is possessed! I think I need a priest

[fyremanpat]

I started to wonder before if it was the wiring before, so I stripped it at the plug and checked all the way to the firewall and found nothing. Just no when it started acting up I hit the MAF plug a few times and it seemed to straighten out....guess I'll be doing some soldering soon!

[old guy]

Sure sounds like a loose wire somewhere. I think you are on the right track.

[customa4]

Like OG said, probably a loose wire but here's how to diagnose if the wire isn't the problem. Click

Some more info from OG from this thread. Click

First check and make sure you have power to the MAF sensor. Terminal 2 should be battery voltage. With the engine started you should see battery voltage between terminal 2 and 3. Terminal 3 is a ground. A check between terminals 3 and 4 should produce 5 volts.

[fyremanpat]

Like OG said, probably a loose wire but here's how to diagnose if the wire isn't the problem. Click

Some more info from OG from this thread. Click

Thanks custom, I went through all that a while ago and never found an issue. I think its a wire thats about to break the rest of the way. I had a similar issue with the n75 plug when I first got the car.

[customa4]

Thanks custom, I went through all that a while ago and never found an issue. I think its a wire thats about to break the rest of the way. I had a similar issue with the n75 plug when I first got the car.

Sorry, should have read the OP before posting!

[fyremanpat]

Well...It wasnt a broken wire. I took the connector apart and inspected the wires. So now Im back to square one. Dangit

[diagnosticator]

Are the terminals in the connector in good "like new" condition? If any of the terminals are bent out of shape, it's likely they won't maintain constant contact with the mating terminal pins. If any of the terminals are bent out of shape, replace the terminals using repair wires from the dealer with the correct terminals already crimped onto the wires and splice the repair wires with the new terminals onto the wiring harness.

Do not use solder joints to do wiring repair work. Audi prohibits solder repairs. This is because solder joints are not vibration resistant, and will fail. Only use properly crimped connections for vehicle wiring work.

There is an intermittent contact somewhere in the circuit. That must be found and fixed.