smith365film

I recently purchased a 2002 C4S, and am having issues diagnosing a somewhat uneven idle. No codes have been thrown, and the engine seems to run well otherwise, but my fuel economy has been pretty terrible (a little under 12mpg around town). Since the idle isn't crazy rough, I started with new coils + spark plugs, and just yesterday installed new engine mounts. While I had the airbox off to do the mounts, I took the time to replace the air filter and clean the MAF and throttle body. No improvement, so I'm starting to think it's a mixture issue.

Just hooked up the Durametric, which had my FRA values for bank 1 and bank 2 at 1.11 and 1.07. The respective RKAT values were 0.47% and 1.08%. There was a stored code from when I unplugged the MAF while the engine was running yesterday, so I cleared it. After clearing, the RKAT values crept all the way up to 3.70% and 3.38%. I can feel the engine stumbling at idle, but it doesn't deviate much from 1000rpm. It also smells very rich when I first start it up, but this might be normal when cold starting these cars.

The next step will be performing a leak test with a can of carb cleaner, and maybe a smoke machine if I can source one. Anyone have any experience with a no-code rough idle? I should mention, my car has an FSI 4.0L engine in it (I'm the second owner since the engine rebuild), but I don't think that's related to my issues.

smith365film

I also noticed that the O2 sensors ahead of cats are constantly pinging between 10v and 75v. Is this normal? Here's a video of the reading on Bank 1 and Bank 2.

gbarron

I am not familiar with everything included in an FSI 4.0 build, but I assume there would be camshaft work which may be causing the uneven idle. My 3.4L X51 package with different cams from the factory has a bit of a lumpy idle as well, it's a trade off for the higher-end performance. Regarding MPG, I doubt a 4.0 package was designed to deliver good MPG, your observed numbers may very well be on spec for the build. Hopefully someone else with a FSI 4.0 build can chime in.

binaryaudax

I don't have an FSI build, and I can't comment on the Durametric readings, but I experienced similar issues once on my 3.4L. The idle was a bit lumpy, mileage was poor, and throttle response was delayed immediately after a cold start -- but no codes were thrown. In the end, it was an air leak that I could only find with a smoke machine. In my case, it was the small o-ring on the brake booster connection on the driver's side intake plenum.

islaTurbine

1. Don’t use carb cleaner to find leaks. I had a Porsche Master Tech do that and he then burned $5k of my money with nothing to show for it because he couldn’t find the root issues. I took the car home, smoked it myself and resolved numerous air leaks. It was an expensive lesson.

2. If you have an FSI engine, I highly suggest you open a ticket with FSI to see if they will give you any feedback.

smith365film

Yeah, the carb cleaner method didn't find anything. I'm going to test my MAF voltage at different rpms just to rule that out, and will eventually get a smoke machine to confirm whether there's a leak or not. I'll open a ticket with FSI, but they aren't fully supportive of subsequent owners unfortunately.

hardtailer

Some target values for your reference, hth.
ETA: Sorry, cannot upload pictures from my device, it seems.

wdb

No camshaft work on an FSI 4.0 unless it is an "R" engine. 1K RPM idle is normal and intentional. My FSI 4.0 gets 3MPG better on the highway than the old engine got.

Maybe @Porschetech3 can chime in on the Durametric readings.

I think you're on the right track with troubleshooting air leaks.

Porschetech3

I agree with whats already been said, I'll just add that 9 times out of 10 if you have a real problem with MPG it is usually the 02 Sensors are aged...

That said it looks like your fuel trims are all rich ( meaning there is a vacuum leak) and you must do a smoke test to correct any and all vacuum leaks....Correcting vacuum leaks will bring the fuel trims back down...

smith365film

Thanks! I have a smoke machine on the way, but will try plotting the switch rate of my precat O2 sensors today.

smith365film

Okay, here are the voltage readings for my O2 sensors at idle (1000rpm), as well as under load (around 2900rpm is where I was holding it). Do the oscillations look irregular enough for me to want to replace the sensors? The Bank 2 oxygen sensor seems to be worst of the two.

smith365film

And here are the MAF numbers. It sits around 20-21 kg/h at idle, and I took it up the RPM range. The ending spike is at roughly 5000rpm.

De Jeeper

Im pretty sure the maf should be closer to 16 at idle. Trust but verify my memory .

smith365film

@Porschetech3 Anything seem obviously off to you? Appreciate anyone else's insight as well!

De Jeeper

I think the o2s look fine. Im pretty sure the high maf readings will add fuel, low readings create a lean condition. Again trust but verify. I just dont know if this is a symptom or a cause.