jseklund

I am still trying to located the source of the rough idle in my mom's '01 A6 2.7T. I believe I've checked everything, but want to run some simple tests that I've never run before and need a little guidance -

1. Vacuum test. Fairly simple I think - just get a vacuum guage, some hose, and a t-fitting and insert in between a connection? The question is - where would you attach it? I was thinking that I'd run the hose/t fitting between the hose to the diverter valve and the fuel rail?? Where's the best place to connect this?

2. Fuel pressure - Seems like a similar test - get a fuel pressure guage, put a t-fitting in between the fuel line and the fuel rail. On the Audis, there's a screw connector - does anyone know what size these connectors are?

I've pressure tested the charge pressure side of things and it seems to hold pressure. Actually, it holds so well it kept blowing the boot off the Y-pipe at anything past 10 PSI (which I tried to avoid, but my regulator sucks for small amounts of pressure).

I have replaced most of the vacuum lines with silicone tubing, but would like to vacuum test everything to be sure it holds vacuum.

The injectors all fire, but I'd like to make sure that the pressure to the rail and from the regulator are good, just to be sure.

Thanks for the help.

bigdadi

"Have you replaced the CTS and fuel filter?

Lean run is generally a MAF or vac leak issue. Double check both.

Has you used a VAG-COM on it? If not get one and start looking at what the sensors are telling the engine."

Does your car has PCV?
"... Rough idle could be PCV problem. Mine has failed, codes but no CEL. Can you remove the oil filler cap when it's idling - mine was stuck on tight but was able too be easily removed when the car was new and didn't have the PCV problem..."

jseklund

CTS is newer and green. I am going to change the fuel filter. Vag is telling me that the car is running slightly rich, but I think it's the computer compensating. Haven't tried the oil cap, but I removed the spider hose and checked for flow and air moves through it and there are no cracks or leaks.

I have also swapped the MAF from my car with the one in her car with no change.

Thanks for the suggestions...

bigdadi

Could it be failed O2 sensor (pre-cat) saw from other post
The pre-cat O2 sensor has to be OEM due it does many things, the post-cat O2 sensor can be aftermarket due it just read data come out from the cat and report back to compare data before entering the cat for the cat's efficiency...

http://www.autodiagnosticsandpublish...or-testing.htm

https://www.audiworld.com/forums/sho...6#post24177816

Again ... failed O2 usually trigger CEL.

jseklund

bigdadi - this is a good thought - but I swapped the pre-cat O2 sensors in banks 1 & 2 to test this. I have been getting a slight misfire on cylinder 4 and the misfire stays on the same cylinder.

Thanks for the thought...

SloopJohnB@mac.com

Swap the coil on cylinder 4 with cylinder 5 and see if the misfire moves to 5.
If it does, the problem is with the coil.

jseklund

Haha, Thanks Sloop - I've already done all these things. I have another thread which was quite long trying to diagnose all this. I swapped the coil packs from 4 to 5, from 3 to 4, from 1 to 4, you name it. They've all been moved. I put a new plug in, swapped the plugs, swapped fuel injectors 2X, put all new lower rings on the injectors, pressure tested the charge air system, replaced a bunch of hoses, swapped the POS modules, checked compression (175 on all cylinders if I remember correctly), etc.

At this point, I think an official test of the fuel pressure is needed - but I don't know how to do this. I'd also like to vacuum test the lines to make sure they're holding vacuum (maybe there's a leak that I'm missing despite using propane and searching endlessly). I just don't know how to connect what to where...

Thanks for the thoughts though. No matter what I do, it idles like **** and has a subtle misfire on ONLY cylinder 4. The only 4 part has me confuzzled a bit - since I've changed out everything that I can that is specific to that cylinder other than the wiring harness (which I should also test) and the valves (but this is a reman'd head that the valves should have been checked on and I have compression). My other thought is that I have low fuel pressure and I'm not sure, but I wonder if Cylinder 4 is the last cylinder in the fuel rail, so it's just dropping low enough that by the time it reaches the end it isn't getting enough fuel, and then the computer overcompensates on that bank by adding a bunch of fuel?

I really don't know...

5v/cyl

What about a dirty injector on #4. It may be working, but the spray pattern is poor and would result in a misfire and unburned fuel which would give a rich run on that cylinder.

jseklund

I agree, but I swapped the injectors between 4 and 5 and then between 3 and 4 - so I've tried 3 injectors in the 4th cylinder with no change.

SloopJohnB@mac.com

If the fuel pressure was low enough for misfire on #4 that would also lead to a pretty white plug color.
Have you tried a leakdown test on that cylinder? Compression is one thing, leakdown another, although 175psi compression is usually a good sign if it's on all cylinders more or less.

Rather than a vacuum test, I'd try a smoker test...the tool is expensive so you might try to find a shop with one.
I had a vacuum leak on the back of one of my 4.2 engines that drove an Audi tech nuts until he smoked it out...he even put a new OEM O-ring in there and it still smoked. Had to go to an oversized non-Audi O-ring to cure the leak.

AT this point I have to think the problem is in the cylinder or port/head or perhaps camshaft lobes since everything feeding it has been swapped or changed.

Picked this up from the web:

If your fuel pump stops working (no noise, no line pressure), the first thing that should be checked is the pump's voltage supply and electrical connections. An open relay, blown fuse or loose wire may be all that's preventing the pump from working. Low battery voltage can also reduce the pump's ability to generate pressure by reducing the speed of the pump motor.

Measuring static output pressure and fuel delivery are the two standard diagnostic tests that can be used to determine a pump's ability to deliver fuel.

Static pressure is measured with a gauge attached to the fuel rail or teed into the fuel supply line with the engine off and pump energized. Fuel flow is measured by disconnecting the fuel supply line, energizing the pump for a specified number of seconds (engine off) and measuring the volume of fuel delivered into a container.

If static pressure or the volume of fuel delivered is less than your car manufacturer's specifications, your fuel pump needs to be replaced. Replacement would also be required if the pump's check valve has failed (inability to hold residual pressure in the system after the ignition is turned off).


However, I have a hard time believing it's fuel pressure since the fuel pump has generally more than enough oomph to fuel the engine at idle...more problems occur at higher fuel delivery rates when the pump can't keep up.