Need help with 1.8t high idle

[Ur20v]

Customer brought me his '02 A4q with a few problems... Failed OBD2 emissions inspection, CEL, coolant leak and high idle. Addressed the O2 sensor issue, fixed the leak, but the high idle has me stumped.

Engine starts easily but constantly idles at 1250-1325 rpm. Adjusting the idle via VCDS has zero effect. I ran throttle adaptation several times to no avail. I removed and thoroughly cleaned the throttle body and used a fresh gasket as the old one was torn and ran throttle adaptation again with no change. I plugged each port on the intake manifold and still no results. Opening the oil cap while the engine is running makes no difference, and covering the MAF with my hand while the engine is running causes the hose from the intercooler to the tb to collapse and will cause the engine to stall.

Driving the car, the boost climbs to 5 psi max, though I have a feeling this is due to how the vac lines are (mis)routed. This will be addressed as soon as I find all the routing diagrams, but again, I think its a separate issue from the high idle altogether.

The only two DTCs were for B1 S2 O2 sensor (fixed) and radiator outlet temp sensor signal too small, so no insight as to what could be causing it there.

[EErie B6]

All 3 of your problems are most definitely related. The car is only seeing 5psi because the ECU is in "limp mode" trying to protectotself from something.

High idle can be associated with an air leak post throttlebody. You already say the vacuum lines are incorrectly routed, that is more than likely your problem.

Sort your vacuum line problems out and your other problems will sort themselves.

Here is a good thread on the vacuum system and what/how the various parts of it operate. In it they also show you how to bypass or simplify certain parts of it. Remember that doing so can cause you to fail emissions inspection so i don't recommend just ripping it all out. What this thread will do is give you a good understanding of the systems, what they do, so you can better understand how to fix them.

http://www.audizine.com/forum/showth...Simplification

Enjoy.

[Ur20v]

Thanks, but as I stated above, I isolated the motor from all the vacuum lines. I capped each port on the intake manifold and plugged each port on the TIP. The vac lines being messed up definitely account for the 5 psi max boost as there are no codes to cause a hard or soft limp.

[diagnosticator]

The vacuum hose system is not involved with the boost control. The only hoses for the boost control are from the compressor discharge to the N75 solinoid center port, and from the N75 short side port to the waste gate actuator. The long leg port on the N75 inserts into the compressor inlet hose/pipe. The idle speed you report is normal for post cold startup. After 30 seconds or so, drops to a normal idle speed of about 820 +/- RPM.

[EErie B6]

^ no, but a hole in the vacuum line running to the WG would cause an over-boost condition (but this would show up as an overboost code) and then 5psi limp mode. If screw clamps have been used on these hoses, there is a decent chance this could be the case as the clamp tends to cut into the smaller hoses like those used for the WG.

high idle could also be caused by a leaking gasket... possible culprits could include the injector seats, the gasket between the intake & head or TB & intake, or even then IAT sensor. A quick way to find it or rule it out would be to give the various suspected spots a quick spray with carb cleaner or similar with the engine running. If the engine hesitates or revs then you found your leak.

[Ur20v]

OK, after swapping in a third MAF element the car idles! The connectors for the SAI and bypass valve solenoids were swapped and now both function as they should. The bypass valve itself operates normally and the upper chamber holds vacuum. The N75 valve measures at 15 ohms and it works running the output tests on VCDS. But the car still will not boost beyond 5 psi. The only DTC is 17698, but I can't imagine how that would put the car into limp mode. I attempted to pressurize the intake tract to 20 psi to check for boost leaks, but my cap was made for my B5 S4 and wouldn't hold pressure.

[old guy]

Possibly a weak N75.

058-906-283-C 20-30 ohm
058-906-283-F 25-35 ohm
034-906-283-H 25-45 ohm
034-906-283-J 20-40 ohm
034-906-283-K 20-40 ohm

Try clamping off the boost signal line from the N75 to the turbo and see if you get full boost. If you don't you probably need to replace the waste gate. If you do then consider replacing the N75.

[Ur20v]

Oh I forgot to mention I pulled the signal line from the N75 and went for a spin - the boost shot up to 20 pounds.

Thanks for those numbers I'll check the p/n tomorrow. I ohmed out the valve on my S4 and it was closer to 28 ohms, but Google said 15 ohms was a common reading. I'll check the new one on my TT as well

[old guy]

Oh I forgot to mention I pulled the signal line from the N75 and went for a spin - the boost shot up to 20 pounds.

Thanks for those numbers I'll check the p/n tomorrow. I ohmed out the valve on my S4 and it was closer to 28 ohms, but Google said 15 ohms was a common reading. I'll check the new one on my TT as well

OK. That indicates that the turbo/waste gate is working as it should. Now it's beginning to sound like an issue with the N75 not dumping the boost signal. I have to ask this dumb question. You are clearing all codes after each diagnostic session, correct?

[Ur20v]

Yes, i clear the codes as the pop up.

I swapped in 2 known good N75 valves today and neither made any difference, the boost gauge holds steady around 5 psi. Additionally, i cannot force readiness on the cat, and even though both O2 sensors show OK, readiness still shows them as failed or incomplete. This car is really starting to test my patience!

[old guy]

So you know the turbo is capable of boost. You got at least 20psi with the waste gate disconnected. You know you have a functioning N75. That pretty much confirms that the ECM wants the motor in limp mode for some reason. I have never had any luck trying to force readiness on the cat. I just gave up and let it self correct after several hot/cold drive cycles. But I don't think that has anything to do with the limp mode.

Random thoughts: Maybe the MAP sensor on the intercooler is defective and reading a higher than actual pressure causing the ECM to read it as an overboost condition? Maybe log the requested versus actual boost and see if that tells you anything. You can also confirm limp mode by logging the N75 duty cycle.

[diagnosticator]

Are you sure the N75 is not connected wrong? You can hear the N75 buzzing during output test, right?

[Ur20v]

I'll do some datalogging tomorrow, thanks for the advice.

Yes, i have the N75 oriented the correct way- short nipple to wastegate actuator, long to TIP and middle to charge pressure. It clicks about twice a second when triggered in the output test.

[old guy]

I'll do some datalogging tomorrow.

While you are logging be sure to check the fuel trims. If for some reason the A/F mix is leaning out it will go into limp mode to protect the motor.

[Casius]

I've got some issues with mine as well, I get normal idle while the engine is cool, but once it reaches temp the thing idles at ~1000rpm and also throws a high idle CEL.
I don't want to hijack the thread though, so if you want I can make a new one.

[Ur20v]

Well, it seems that all the progress i made with this car has evaporated. The brand new B1S2 o2 sensor threw 2 codes (the same codes that caused it to be replaced), the idle has made its way back up to 1250, there is a new DTC for a small evap leak and we are back at square one. I pressurized the intake tract to 25psi and found no leaks, did a bunch of datalogging and found no anomalies or abnormalities. This car is starting to drive me nuts!

[old guy]

Is the idle staying high or is it cycling up and down again?

[Ur20v]

It holds steady around 1250-1300. I'm beginning to wonder if the ECU itself is bad. When I opened the box it was dry but there was plenty of dirt in it and the relays were rusty (replaced both). I just wish it would throw a damn code related to why it is in limp mode!

[old guy]

It holds steady around 1250-1300.

It sure sounds like you are sucking in excess air. Even though you ran a boost pressure test it could still be a vacuum leak somewhere. The leak could be on the upstream side of a check valve. Consequently when you run a boost pressure test the check valve does its job and stops the boost pressure but once under vacuum the false gets sucked back through the check valve. The easiest way to find that type of leak is to clamp off the vacuum lines leaving the manifold one at a time. If clamping one of them drops your idle significantly you just have to clamp further down that particular line until you find the problem.

Again, Good Luck!

[Ur20v]

No excess air... I turned my propane torch on but did not light it and ran it along the intake tract, each Y and check valve, and the idle did not change a bit. Very frustrating.

[old guy]

No excess air... I turned my propane torch on but did not light it and ran it along the intake tract, each Y and check valve, and the idle did not change a bit. Very frustrating.

I would still try clamping off the vacuum lines at the manifold. The propane test isn't going to pick up a problem such as too much air through the block breathers (PCV/PRV) or the evap system.