mdecsi

Hello,

I'm new to this forum. I spent my last two weeks browsing through the archives here and I must admit, I learned more in these fourteen days than in the whole time I own my car (2 years). This place is really amazing and I can't thank you enough for it.

Anyway, I have a '96 Audi A6 2.8Q (AAH). It has 260.000 km (that is about 160k miles) and I think I am a very fortunate person, because the car is Euro spec, so it doesn't have EGR. As I read, it is a real PITA to do clean, repair or do anything with it.

So, according to "best practice" I read here, I made the following to my car:

- cleaned injectors and replaced all of the O rings (poor man's method: http://www.youtube.com/watch?v=VFJlTfHyrUk )

- changed fuel filter

- replaced all vacuum lines, as I don't have EGR, there are substantially less

- tracked down and fixed all vacuum leaks in connecting parts (cracked vacuum reservoir nipple, etc...)

- did the phone cord fix (amazing, easy and very effective thank you VAP)

- checked and cleaned the two pipes connecting the valve cover and plenum, they are OK

- made a Vag-Com interface to communicate with the ECU, no error codes

- checked spark plugs, they are OK

- checked all the fuel lines, no leak anywhere

- bought a vacuum meter

Ok, car creates about 18.5-19 inHg vacuum now. As the car has more than 160k miles I think it is an OK value. With the "propane torch method" I could detect a small leak on the intake manifold on the back of the drivers side, but I do believe it is just a minor one. I know that a leak is a leak, it must be fixed, I plan to do it, but that 19 inHg indicates, it must be a really small leak.

The problem is gas mileage. Car consumes about 12-14 l/100km in the city, which is about 19-17mpg. I see, some of you has 32 mpg, which seems unbelievable to me.

My question is: can I do anything to improve my mileage, or the only thing left is removing the IM, replace all the gaskets, and do the matching?

Thanks,

Mike

mdecsi

Almost forgot:

- cleaned MAF with CR MAF cleaner spray
- checked O2 sensors, they do bounce from 0.2V to 0.8V, and they are fast

turbo510

I don't know about 32mpg with a 2.8, maybe under some ideal condition, but if your engine is running about as good as it can, and that sounds like yours, about the only things to affect mileage are those things that relate to friction: tires, engine oil, draging brakes, wheel alignment, driving habits. If you have an automatic, the converter lockup has to be working.

mdecsi

OF course, my right leg sometimes feels heavy, but I try to drive economically, shifting at low RPMs.

Oil: I use Castrol Magnatec 10w-40. I know it is not the best oil around there, maybe at the next oil change I will try another, better oil (will browse the archives for what others recommend).

Tires: i use factory recommended pressure, sometimes slightly more.

The brakes are OK.
Car is manual.

One more interesting detail about MAF:

According to Ross-Tech the MAF should read 0.8*full HP of the engine when monitoring:

"Here's a good sanity check for the status of your MAF. Do a full-throttle run all the way to redline in a single gear (second works fine). Group 002 usually shows air mass in g/s (in many modern ECU's, consult your Factory Repair Manual for the specific group for your particular vehicle). Your peak airflow should be roughly 0.80 times your horsepower if you are close to sea level. So, if you have a stock 150 hp 1.8T, expect around 120 g/s. If you see significantly less than that, you MAF may be on the way out."

http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info

As my car has (had) 174HP, my max. MAF value should be around 140 g/s.
But the highest value I could measure was slightly more the 100 g/s in 2nd WOT at 6k rmp.

Where is the missing 40g/s? There has to be an enormous vacuum leak where undetered air can enter, but trust me, there is not, I checked everywhere.

Searched the archives for values but found only one message indicating actual values:

https://www.audiworld.com/forums/sho...68&postcount=4

So, according VAP's measurements "A stock 2.8 12V moves about [..] 96 g/s at 6k rpm's in first and 102 g/s at 6k in the top of second."

Question is: Which value is right?

According to Ross-Tech I should read about 140 g/s
According to VAP about 100 g/s

Could my lower (cca. 100 g/s) value means my MAF is bad and it causes all my problems?

Mike

PS: For the record: I cleaned my MAF 1 month ago with a whole can of CRC MAF cleaner.

Lago Blue

Certainly you can, I'm sure there are more besides these, perhaps some others will pipe in here with more, but just quickly:

After your cleaning -

- are the injectors closing completely and leak-free under pressure?

- are the injectors spraying the best possible pattern when open? &

- are they all flowing the same amount?

- if you have as you say a small vacuum leak detectable by you, perhaps the ECU also detects it, and defaults to a safe/rich mixture mode, rather than believe the MAF's signal;

- when light-footing it in an attempt to improve fuel economy, you are running at very low throttle settings which your stock MAF may have trouble reading correctly, not that there is anything wrong with it, but it is reading air which has just tumbled through the sharp-edged filter-box lid's air exit opening. One of VAP's velocity-stacks may help improve even the stock MAF's ability to send a more accurate and appropriate signal; particularly under light-load conditions.

- certainly any un-metered (by the MAF) air leaks effects are magnified at low-throttle settings, when total airflow is low, and the leaks share of that total flow is highest;

I've not used a Vag-com, but you may be able to cross-reference your MAF outputs against actual voltages.

Two other common faults found:

- is your coolant T-stat closing such that your engine does warm sufficiently?

- is your ECT sensor sending a signal that indicates the engine is fully warm?

turbo510

Lago had in his pointers one often overlooked, the thermostat. They go bad and can hang open, which causes extra heat loss to the coolant, reducing mileage. Also, the MAF is less important than the oxygen sensors during cruise, but you said the O2s look OK.

mdecsi

Regarding the thermostat: I changed it about six months ago, because I suspected the old one was defective.

Engine coolant temp reached 80-90C (176-194F), but sometimes it dropped back to 50-60C (122-140F) on the highway. So I bought a new Meile brand thermostat and had it replaced, but I've experienced nearly the same results since then. In the winter sometimes the coolant could not reach even 60C (140F), but as I have a new thermostat, I believe it is normal. Am I right?

I have a spare used thermostat from the junkyard, I will try to drop it into a pot of water, heat it until it is boiling and if it works, I will replace the thermostat again, just to be sure.

According to Vag-Com ECT sensor is ok, it shows the correct values.

Can the small IM leak (between the upper and lower parts) be fixed somehow without taking it of? Like smear some silicon sealant on it. Of course only for as a temporary solution.

I will try to make a velocity-stack as you recommended, there are plenty of info about it on older topics I suppose.

Loudgoldwing

"In the winter sometimes the coolant could not reach even 60C (140F), but as I have a new thermostat, I believe it is normal. Am I right?"

No. You have likely thrown an oxidation clot from your radiator, and your new thermostat is sticking open (again).

At speed, your engine should NEVER drop below 180 degrees F. If it does, the thermostat should partially close back up, because that's what it's designed to do. It SLOWS the flow of coolant so your engine stays warm.

This is absolutely, positively, without a doubt in my mind the #1 contributor to your issue.

mdecsi

This is my second 12v V6 Audi, but the previous one ('91 Audi 100 2.6, ABC) behaved just the same, the coolant temp reached normal values then dropped on the highway. I though it was normal. I know thermostats often fail, but *this* often? And why in the open position?

Never mind. I am going to replace my thermostat today and check mileage in the following days/weeks.

I wish it was that easy.

Loudgoldwing

Thermostat sticking open = too much coolant circulation. Engine not reaching full temperature. The injection system runs rich to achieve stoich 14.7. Poor fuel mileage.

Thermostat sticking closed = o coolant circulation. Engine overheating and suffering an untimely death.

You are fortunate.