rich mixture/ oil fuel dilution problem

[audi_guy]

Hi all,

I've got a oil fuel dilution problem with my 2000 A4 1.8t (ATW) Quattro Tiptronic 91K miles. The car is unchipped and unmodified. I did a Blackstone Labs and Dyson Analysis on my last OCI of 4800 miles on M1 0w40. The UOA report was all good except there was .5% fuel dilution. I thought I had determined the problem in that the front O2 sensor, during that OCI, was giving VAG codes of resistance too high intermittent. The 02 sensor was replaced at 2000 miles into the OCI. Also the spark plugs were on the last 5000 miles of their suggested 30K service life - also replace at 91K. The air filter, MANN, was replaced at the beginning of that OCI.

There are no other codes and both 02 sensors are giving normal reading according to the VAG-COM program.

To verify that everything was now ok, I ran a VAG log on Group 002 and 032 as suggested in the "Fuel Trim Info" paper posted else where on AW forums and on www-ross-tech.com. I found that the MAF appears to be fine with 132.58 g/s at 6320 rpm. However the Group 032 Partial Throttle percentage, second field (Multiplicative), is showing a constant -0.8 to -1.6 from idle to 6000pm. If I'm understanding the "Fuel Trim Info" article correctly, it appears that the engine is continuously running rich which may account for the oil fuel dilution. Does this sound correct?

If so, does anyone have any suggestions as to what next to check in order to troubleshoot and correct the rich mixture and oil fuel dilution problem?

 

[FowVay]

If those are your long term fuel trim numbers then they are perfectly fine. It isn't until your long term fuel trim settles into the 10% range that you have problems.

The .5% doesn't seem excessive either. This engine is known to run a bit rich. How do your other wear numbers look in the analysis?

If the vag-com numbers represent your long term fuel trim then you don't have a running problem. You may have a leaky injector during shut down but I don't see a problem during hot temperature operation.

edit for a thought: monitor the fuel pressure on the rail after shut down and see how long it takes to bleed down. This may lead to a clue. The manual states that residual pressure after 10 minutes should be 2.5 bar.

Post exactly what DTC's you have gotten from your VAG-COM.

 

[audi_guy]

FowVay,

Thanks for the info. Here's the UOA.

http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=003063

Terry suggested I needed to get the oil fuel dilution under control. As for DTC, the only code I've received was for the 02 sensor internal resistance to high - intermittent. I don't have access to the exact number at the moment. That was solved with the O2 sensor replacement.

After doing a little more research, I found that a faulty coolant temp sensor may at times cause a rich mixture. The coolant temp after the engine was completely warmed up and during the test runs was 75C. The Bentley manual as I recall listed somewhere around 87C as normal.

I guess either the coolant temp sensor is giving false readings or the thermostat is malfunctioning. Does that sound correct?

 

[1 FMF]

1/2 percent fuel dilution is a problem? that's news to me. I could understand > 2% and maybe even > 1% for late model fuel injection. i'm no expert but your uoa looks to be inline with the rest of the 0w-40 uoa's: viscosity in high 30w range (typical), all wear metals normal, insoluables are low, flashpoint is lower than what is expected but by 10 deg, and you have 5k miles on a turbo motor. Just time to change the oil.

if you floor the gas pedal when the car is cold and accelerate hard, then I wouldn't be surprised to see some fuel in the oil.

75C = 167F and 87C = 190F (i need temps in F)

my understanding is the computer doesn't go into closed loop mode using the O2 sensors to adjust A/F ratio to lean until the coolant temperature reaches a certain point. If a faulty thermostat or malfunctioning sensor is keeping the computer from going into closed loop under normal driving then the A/F will be richer, and not necessarily cause fuel in the oil but will increase emissions. But depending on your driving conditions and drive time, the engine may never get into closed loop and also not get hot enough to burn off fuel and combustion blowby which could cause the problems you're seeing. And then again, doesn't the 1.8 have a small sump, something like < 4 qts? If so then could just be something you have to live with. And also being a turbo motor, coolant temps might be kept colder than the usual 190-200F used on non-turbo cars, so maybe the 75C number is not incorrect- definitely worth verifying.

 

[FowVay]

1FMF, on VW's if they don't go into closed loop within a prescribed timeframe they will set a trouble code. Personally I believe that he may have a very slight injector leak but nothing that I would lose any sleep over.

The analysis looks nice to me. His oil thinned to approximately 11.5 cSt's (64.7 SUS) which isn't bad at all for Mobil's 0W-40. I'd personally use their 5W-40 because I don't like the PAO base used in 0W oils but that's purely a personal thing.

My only comment is that he doesn't have a fuel trim problem. A leaky injector is a slight possibliity but his fuel dilution is inline with what would be experienced in a short-trip driver. If it were my car I'd use either GC 0W-30 or Mobil 5W-40 and see how it faired. I'm not overly impressed with Mobil-1's 0W-40.

Audiguy, if your vag-com is on a laptop then take a sunday drive and watch the coolant temperature as the engine heats up. You may find that there's a fault in the coolant temp sensor. They're super cheap and easy to replace so it might be a $10 saturday project just for fun.

 

[audi_guy]

1 FMF & FowVay,

Thanks for the replies and info.

Yes the driving is 80% in town with less that 10 mile trips with my light footed wife doing the driving. I doubt the oil ever gets to full operating temperature for any sustained period of time. On the highway it another matter since I'm doing the driving and I do like to stretch the A4's legs.

The 1.8t has a 4.3 qt oil capacity with the new Audi/VW recommend large (monster size

) oil filter. With to old oil filter it was 3.7 gts.

Yep. I looked at a bunch of the 1.8t uoa reports and did see most had fuel dilution as high or higher than my uoa. There were some uoa reports with negative fuel dilution. I don't know if the tester's on these considered < .5 % negative or if in fact where was no fuel dilution.

Yes my uoa didn't look bad at all when compared to the other 1.8t uoa posts.

I will check for a leaking ejector with the leak down test. I won't loose sleep over it but I'm rather picky over my Audi. My wife would say I anal.

Yep I've already switched to GC 0w30 and will do a another uoa at the end of this 5k oci. I've got a 5 year stash of GC 0w30 green at the moment. Of couse that stash will drop if I pick up a 00-02 2.7t S4 I've been lusting over.

Yes my vag-com is on a laptop. I'll log a test on the coolant temp. I've already looked into the coolant temp sensor and thermostat and can handle those.

Thanks again guys.

 

[audi_guy]

I've made good headway into solving the rich mixture issue/problem. I'll have to wait until my next OCI and UOA to see if the oil fuel dilution is lower.

As reported earlier the coolant temp sensor was reporting 75C at engine full warm up. I replaced the sensor and the coolant temp was 87C at full warm up. I ran another Fuel Trim test with my VAG-COM and found that the coolant temp ran from 75C to 84C at highway speeds with the short term fuel trim at 0.2% and long term up to 3.1%. So there was still a problem with the coolant temp.

I changed the thermostat. I tested the old OEM thermostat and it began opening at 81C when the spec should be 87C. The new German Wahler thermostat begins opening at 87C. The next Fuel trim test shows the short term fuel trim at 0.2% and long term at 0.8% and the coolant temp ranging from 87C to 92C.

With the coolant temp, fuel trim/mixture, and replacement of a secondary air injection system pressure hose (discussed in another topic here on BITOG), the engine seemed more responsive with better acceleration but I just thought that might be because I wanted it to run better. My observation was confirmed however when my wife said she thought the car was running better because she didn't have to press accelerator as much to get the car moving or during normal driving. This statement is from someone who doesn't notice anything about a car and how it performs unless it doesn't start when she turns the ignition key!

I'll post an update on the fuel dilution at my next UOA.

 

[audi_guy]

Correction: the fuel trim values are negative. -0.2%, -3.1%, -0.8% are the correct values.

 

[moribundman]

Do you do lot of short distance commuting? I monitor my O2 sensors while driving and I know that my 2.8l V6 Audi engine runs somewhat rich during warm-up. Not a problem, because 99% of all times I take the car out, I get the engine fully up to operating temperature.

 

[audi_guy]

quote:

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Do you do lot of short distance commuting?

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moribundman,

Yes. 80-90% in town driving with trips usually less than 10 miles each. The engine is stock unchipped so I was concerned by the .5% oil fuel dilution on my UOA. Terry suggested I needed to take a look at solving issue. From what I've read, I expect some fuel dilution from a Stage 1 chipped 1.8t but not from a stock unit. I know that the engine will run rich during warm up. From the fuel trim numbers I'm now seeing with the new coolant temp sensor and thermostat, Audi seems to be using a slightly rich mixture to keep the combustion chamber temperatures under control. It is a technique used in aircraft piston engines as well (military aircraft during the piston engined era). The other technique is to use a lean mixture for combustion chamber cooling (used by the airlines during, you got it, the piston engine era). I assume with the rich mixture to a point you'll have more power and with the lean mixture less. I guess Audi is going for the power side.

At any rate, a malfunctioning coolant temp sensor and thermostat can't be a good thing for engine performance and fuel economy. So I'm happy that this is taken care of. A more responsive car is icing on the cake. And if the fuel dilution is under control on my next UOA, so will that.

 

[moribundman]

So your coolant temp sensor was defective? On the A4 it does affect the air/fuel mix. Also, if your O2 sensors or MAF sensor are not operating properly, the ECU will use fuel maps and those tend to be on the rich side for safety reasons.

You should definitely drive the engine until fully warmed up and then some (40 minutes at highway speeds) at least once a week. That should help a bit with fuel dilution issues and is generally recommended anyway.

 

[audi_guy]

quote:

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So your coolant temp sensor was defective? ... O2 sensors or MAF sensor...

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Yep. The coolant temp sensor was giving erratic readings as well as reading 75C as full operating temp. The MAF was replaces under warranty and is giving correct readings. Both O2 Sensors are giving correct readings. I previously replaced the front (before the catalytic converter) 02 sensor since it was giving a DTC code of "Internal Resistance Too High - Intermittent". There are no DTC codes for any system including the emissions systems. I'm pretty sure everything is now in good working order.

Yeah, I'll have to restart the old tradition of going for a Sunday afternoon drive.