17 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHUneven Idle, High Fuel Consumption - No Codes
Reported or suspected cause
small o-ring
Reported cause
No successful repair reported for this finding.
1
Identified cause
0
Suspected cause
0
Reported fixes
Read evidence and context
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Reported cause
In my case, it was the small o-ring on the brake booster connection on the driver's side intake plenum.
Leading evidence: 'small o-ring' (1 discussions report identifying this cause; no attributable repair resolution). Across 1 matching discussions, the most frequently mentioned possible cause is 'small o-ring' (1 discussions; 1 report identifying it and 0 suggest it). The most frequently discussed action is 'Clean maf and throttle body' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Performed · outcome unconfirmed · Original poster
Since the idle isn't crazy rough, I started with new coils + spark plugs, and just yesterday installed new engine mounts.
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Suggestion · Original poster
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.
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Suggestion · Original poster
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.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
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Suggestion · Original poster
Do the oscillations look irregular enough for me to want to replace the sensors?
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Suggestion · Original poster
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.
Counts describe captured discussions, not unique people or a diagnosis. Author identities are not reliably stored. Exact duplicate evidence is counted once across matching discussions; one discussion counts at most once per action/state. Multiple repairs reported together remain a combination. Suggestions and failed attempts do not establish a fix. Matches cover whole discussions and can include related faults, model variants and incomplete threads.
17 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
0 evidence records excluded for query context · 0 excluded by outcome qualification · 0 adjusted by outcome qualification.
1 acknowledgements have no verified action link; they do not establish a repair result.
Outcome without a verified action link
No improvement, so I'm starting to think it's a mixture issue.
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.
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
.
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.
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.
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.
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.
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.
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...
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.
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.
@smith365film
I would need to see the full list of actual values, like RPM and was ALL loads turned OFF during the capture, but it does look like the MAF value is a little high, but that could be from you having the A/c ON during the capture..
That said, here is some info I posted on another thread about fuel mileage that you will probably like
