TheCJ

Hi Everyone,

This is in reference to a 2004 A4 3.0 Quattro sedan
I do not have a VAG COM yet, planning to buy one

So this is apparently a typical A4 gremlin, rough idle... I'm a perfectionist and hate when anything isn't working correctly and am not afraid to take big steps in repair.

Symptoms:
When hot started and don't tap the gas, the engine runs rough, shudders, idle drops till the charging system light turns on and sometimes revs back up (or stalls) and continues to do this until I tap the acc pedal some or start moving. Actually this wasn't happening recently so this came suddenly. The problem does not exist at cold start.

My thoughts:
Could have been a vacuum leak or throttle body filthy, maybe idle control, PCV or EGR.

What I've done so far:
Replaced air filter with new MANN filter (helped a bit)
Throttle body cleaning (it was nasty) - this helped lessen the stalling and smoothed idle some.
Found that one of my combi-servos is leaking - temporarily disconnected and plugged the vac hose to it.
Checked the vacuum hoses on the upper parts of the engine - seem very nice, pliable, nothing hard or strange.

Tentative plans:
Check the PCV diapragm
remove and check the EGR
check idle control valve (wherever it is, if I could get a pic of one from from someone that would be awesome)
Replace fuel filter with best one I can find

This is still not resolving the idle issue and the car stalls out occasionally. I have heard that the VAG COM can be used to re-calibrate the throttle body but I'm not convinced that this is the issue.

Can you guys please list some possible ideas that come to mind on things I can check? Any questions for further info you may have?
I'm a relatively resourceful mechanic and don't mind difficult tasks so shoot away, I'm not going to the Audi mechanic. thanks in advance!

Steamboat

Could be a bad MAF sensor. With the engine running, unplug the MAF sensor. If the idle does not get worse with the MAF sensor unplugged you likely have a bad MAF sensor.

Could be a leak in the air duct between the MAF and the throttle body.

If you're going to keep this car a while and do the repairs yourself,definitely get VAGCOM.

TheCJ

Thanks, I'll check the MAF today. I don't think it's a leak in the ducting, I went through it this past weekend, took it all apart, cleaned, inspected and applied a light coat of assembly grease to the seals so it's all in good shape.

Definitely looking for a good VAG COMM unit though this particular symptom shouldn't need that to my knowledge but I'm slightly starting to think it could be an air temp sensor but no absolute proof or verification I've done indicates this. I might just change the temp sensors just for fun if I can get part numbers.

TheCJ

Follow up on this:
I tried unplugging the AFM (old name for MAF) and found very slight change. Strangely though the intake path diverter valve changed position to disengage. When I plugged the AFM back in it re-engaged. Also the automatic traction control light turned on and won't go out. The idle smoothed out for the time being so I'm suspecting a possible loose AFM connector even though it was click-locked and the engine was surging while I was working to remove it. Will follow up if there is any further development on this topic.
Hopefully the traction control light goes off.

turbo510

Also check or replace the Coolant Temp Sensor, as these CTS units are known to fail. The fact that the car runs well at idle suggests that the fuel mixture could be staying rich when it shouldn't be.

On the other hand, the CTS might be OK, but an air leak becomes too much to compensate for when the ECU attempts to run closed-loop. Take another look at the vacuum lines, since even small air leaks after the MAF will really lean it out. When I had the intake manifold off my 3.0, I found one or two of the 1/8" plastic vac lines broken.

The crankcase pressure control valve, which is the black disk-shaped thing by the throttle body, can fail. My car had an annoying surge until I changed that.

No idle control valve on these engines, unlike my old 12-valve 2.8. Same with EGR. And the fuel filter is unlikely to be the cause, unless it's internal pressure control valve isn't working.

TheCJ

Sounds like you've been down this path a few times, I really appreciate your time and input.

Is the CTS is this one? I'm not sure if this one is one that sends data to the dash meter or if this is used for the EFI system. If it's for the fuel, I'll change it anyways because the original is probably still there.


As for air hoses - Ok I'll change them all just for security and peace of mind. They all checked out ok but it seems like this is a common enough problem. I'm going to replace the hard plastic sections with brass pipe then. I already found one of the intake air diverter servos was leaky so I plugged the line temporarily.

Yea, I was thinking this problem could be the PCV valve too, the hoses to the valve covers looked kinda dirty. I'll pull it and give it a check and change it if it seems suspicious.

As for idle control valve nonexistent I suppose that makes sense since the intake butterfly could act in its place. I'm pretty sure there is an EGR though. Isn't this one?



I'll attack this again this weekend, hopefully have time to finish it all and report back on any findings.