brian_d

Ok, got this limping car for a few hundred bucks, figured I could fix it. Mass air flow sensor was dirty causing bad idle, MAF Low Input Voltage code. Cleaned MAF and that all went away. But, although running better, this car could not get out of it's own way.

Found a seriously bad catalytic converter, and a cracked turbo housing at the waste gate.

Replaced turbo, dv (not holding vacuum,) and cat - and WOW this car has power!

However, under boost, a sudden drop in power (bucking) and a P103 Mass Air Flow Circuit High Input. At this point, the car runs very rich and less power, bucking. A quick shut down/restart clears the problem (sometimes for a day, sometimes for 5 minutes) but it comes back.

Observations: it seems to happen at higher loads like 4th or 5th gear pulls. Usually happens above 3k RPM. If I drive easily on gas pedal, I could drive all day with no issue - it's just under boost/load.

Torque app confirms the High Input - Graphing the MAF shows immediate high input (pegged) and stays there no matter what I do, until I shut down/restart car.

Any ideas? Thanks in advance.

brian_d

By the way it's a '99 with 1.8

Prospeeder

Sounds like overboost, The old ones have no boost pressure sensor. So they make a calculated guess from the mass air volume to know roughly how much boost its making. If its overboosting, the sensor will go out of range or "high" signal and then kick itself into the sort of limp mode your experiencing. At that point it would be the N75 boost control valve, its plumbing attached to it, or the wastegate sticking. Or theres the possibility it could be that the MAF really is no good and is falsely reporting a too high signal back.

brian_d

Thank you for the reply. I think you may be right about the over boost. Boost on torque app showed upwards of 16 lbs boost before the maf showed the error. Should be more like 10 or 12?

Drove the car on a 200 mile trip today keeping rpm low and watching not to over boost. Then, out of nowhere, suddenly the same error from the maf just cruising along with no boost. Had to shut down/restart to clear then it was ok again.

Then started getting zero voltage from maf/running rough again periodically.. especially at idle.

Going to check wiring/plug to maf (again) and possibly re wire it tomorrow. If that doesn't work, I probably have a bad maf along with the over boost problem.

Seems like when I shift gears the boost drops out abruptly with a shuttering sound. Also abruptly drops out when I am under boost and let off the gas.. not smooth at all.

brian_d

Here's what I found with the MAF. Checked wiring harness and no problem. Probed 12v and signal wires while engine running and wiggling wires and could not get the MAF to fail. Did the same with both ground wires so all seemed good electrically. Driving with engine up to full temp, however, brought symptoms of rough idle, no signal, high signal, etc so I figured it had to be a bad MAF... which it was.

Got a reman from Advance Auto for about $150 and drove it 200 miles back home with ZERO issues except the over boost in 5th gear pulling. Idles perfect no matter what the temp, and my long term fuel trim dropped from about 15% or 16% to zero. And short term fuel trim hovering at about +- 2% to 3%. Big difference!

I will look into over boost and compressor surge tomorrow. N75 actuates with 12v applied so I will probably be back for more info. Thanks!

brian_d

I'm posting this reply in hopes that it helps someone in the future. Having high spikes of boost (upwards of 22psi under high load, causing way too much air through the MAF and triggering P103) it turns out it was a simple fix.

I pulled the N75 valve to test it and it worked perfectly. Just below the N75 I noticed that the hose to the waste gate actuator seemed a little loose (like the clamp was not completely tight.)

I grabbed the hose and it pulled right off the nipple. Must have missed that when replacing he turbo. Could have been a costly mistake. I got in there and tightened the clamp nice and secure and the problem is solved.

I was losing boost pressure from the N75 valve which prevented the waste gate actuator from working at all.

Under high load now only producing about 8 psi max boost wide open throttle. I'd like a bit more, but given the age and mileage of this car (over 264k miles,) I'm very happy... and there's still plenty of power.

And I must say, for a car of this age/mileage, it runs extremely smooth and quiet.. very smooth idle too.

A4 turbo

I am experiencing a similar problems have the maf unplugged n drive it around like that but the old maf gave me error code I tried to clean it n cel still popped on I bought a maf off ebay installed it n now I have error code high voltage from the maf unplugged runs fine but not an experienced audi or turbo driver but with the new one I have in n plugged it it idles rough n cel turns on instantly. I have a k and N cold air intake coming in today that I'm putting in but will it cause me problems driving around with maf unplugged or should I get new maf n diverter vavle?

brian_d

A4 Turbo,"

I had the exact same issue.. I bought an inexpensive MAF from car parts.com for around $50 or so and it never worked from the beginning. And believe me, this made troubleshooting much more difficult since I thought I had eliminated the MAF as a suspect.. boy was I wrong.

A reman MAF from advance auto runs around $150 with your core (trade in) and just may solve your issue once and for all..

I had the same symptoms - MAF high voltage, low (or no) voltage, rough idle, black smoke from exhaust on rough idle.

This extra fuel delivery (rough idle, black smoke) can ruin your catalytic converter in time. I had to replace mine ($120 shipped eBay.) And about 3 hours of headache in the driveway lol.

After tracing wires back to control unit (eliminating loose or broken wires as possible suspects, I finally decided that the MAF just had to be bad - and it was. Plugged in new Maf and the rough idle, black smoke was gone.

Your diverter valve is probably ok, unless you're experiencing other issues as well.

On a side note, you will be able to drive with the MAF unplugged, but you will never run efficiently since the computer tries to adjust fuel delivery based on your O2 sensor only (after the fact.)

With a good MAF in working order the computer makes calculations based on air flow into the engine before ignition and fine tunes based on O2 sensor (rich/lean) voltages and adjusts your short term fuel trim accordingly.

So in short, I would try a quality MAF along with your new cold air intake.

Hope this helps some.

A4 turbo

Ok yeah I just put in the intake in n it works great I hear the diverter vavle working great n the turbo not really sure it doesn't make a high pitch sound more of a roar kinda but I fell the pull should it have a high pitch sound I'm not sure doesn't sound bad tho but I'm a turbo rookie thanks for the help tho I'll get new maf n try that wires seem fine just learning as I go is there anything else I can do for maintenance on turbo besides the normal I appreciate any help thanks

brian_d

Glad that works.. with my old turbo I could always hear it spool up. But with the new one it is much more quiet. I put in a K03 just like the stock one.. serious pulling power with a manual boost controller getting about 16 lbs of boost WOT. I also have cold air intake with k&n filter and can hear the air rushing in under load.

Best maintenance for your turbo is probably to run full synthetic oil like Mobil1. And change regularly. I live in a mountainous area and the last 4 miles of my commute is up a steep mountain pass. One time I got to the top of the mountain and pulled over to check something under the hood and noticed the turbo glowing orange! Since then I run full synthetic and let the engine idle for a bit to cool down before shutting it off.