why would someone tweak the vane spring pressure? is there a perceived advantage?

Tweaking the spring pressure is a "cheap" way to"tune".

By reducing the spring pressure you can trick the DME into adding more fuel. Adding spring pressure will reduce fuel. This "works" because the Fuel Injection system is "Load Based," which is just another way of saying the DME uses intake airflow to determine how long to keep the injectors open.

interesting.

FRWilk's site on the Air Flow Meter (AFM) is very infomative. Before I fiddled around with the tension of the helical spring, I would read this and understand it. I followed it to the letter and it improved things measurably

www.frwilk.com/944dme/afm.htm

Thanks again, fr!!

Hoss,
what was the CO2 number? This number is indicative of the combustion efficiency. Normally above 13 is acceptable but 14~15 is good. The more efficient the less the cat has to work to clean up the dirty exhaust.

I feel for you. I have a 88 bmw with similar Bosch vane flow air meter system that the PO tampered with. I had to reset the base CO (before CAT) by disconncecting the O2 to a specified %. Fortunately I had the use of a 5 gas exhaust analyzer. For my tampered bmw I had to re-adjust the tension and the co adjustment screw on the vane air flow meter. The base co % spec for 944(from factory service manual) is .6%+-.2%. Clockwise for richer mixture and counter clockwise for leaner mixture. The RPM should be 840+-20. Be sure to use a wire and short B and C on the diagnostic plug.

NOX is created by combustion temperature higher then 2300degrees and high pressure. So make sure your cooling system is in good condition (coolant and pressure test - coolant resevoir cap does go bad) and if have to turn heat and fan on to hot and max during smog test.
NOX goes up sharply as a/f ratio go past stoichiometric(14.7(air) to 1part fuel). 16, 17, 18 = high high nox.

Looking at your second set of number after the install of new cat, I would say that is lean misfire. Which means the a/f is so lean that the a/f mixture was not ignited and passed out of the combustion chamber and into cat. That is why HC is so high and NOX is high. The new cat is working though so the CO is cleaned up in the CAT. CO is partially burned fuel. I bet the CO2 number for your second smog test is around 10~12. Please post your CO2 numbers for both. I like to see. Thanks and good luck. Please keep us posted. I am very interested in these smog reading.

CO2 with old cat was 14.9 @ 15mph and 14.9 @25mph
CO2 with new cat was 14.7 @ 15mph and 14.9 @ 25mph

How many miles on vehicle?
Have you measure fuel pressure?
Air leak around fuel injector? (as someone else has mentioned)
Fuel injector issues? partially clogged? damage?
Base CO?(someone else has mentioned)
Air leak - Evap system - air leak from lines connecting charcoal canister? line from temperature valve(thermo vacuum valve)?
varify no excess carbon on back of intake valve?
Time to double check and verify.
James

The car has 117K miles.

I have not(don't know how either) measured the fuel pressure.

The car performs quite well and I've run BG and techron fuel cleaners through it. Smooth acceleration, no hesitation, etc.

I procured some of the tools I needed today(vacuum gauge, etc) and will be checking the timing with a timing gun and looking over everything tomorrow. I also plan on checking the 02 sensor as porsche944 recommended.

I must admit I'm not familiar with the charcoal canister or the temp. valve...could you elaborate.

I have a 944 Haynes manual so some research is being done as we speak.

Thanks guys.

can you really? Cuz if mine fails one more time, I could see me taking you up on that, mr. Don't say it if you don't mean it.http://www.pelicanparts.com/support/...s/a_frusty.gif

It doesn't work well, tho. Can't just go around mucking around with air/fuel mixtures w/o losing eff. of motor. To much fuel is as bad as not enough. (not an expert, but read a couple Bosch books)

Auntie Wilk's tip-o-the day: "On an almost empty tank, drop a bottle of rubbing alcohol in the tank. Take it to get smog cert. Alcohol lowers the combustion chamber temp, just a hair, but maybe enough to get tags."

If you need it, PM me. As you can imagine, my connection is pretty skiddish, but it shouldn't be a big deal.

You are correct. The engine will only idle within a very narrow band of AFM swing, so tweaking the spring pressure needs to be balanced between the by-pass and mechanaical idle (TB by-pass) settings.

However, I have seen more than one AFM that had been "tweaked", causing the engine to run like crap. The last one I encountered had been "tweaked" to compensate for leaking fuel injector O-Rings. 3 out of 4 cylinders were running lean, so some "mechanic" slacked off on the spring tension to "fool" the DME into seeing more air flow, there by richening the mixture.

Fortunately my trusty vacuum gage lead me right to the bad O-Rings. Once I had that fixed, it was a simple matter of putting the AFM spring tension back where it should be.

While we are on the subject it might not be a bad idea to check the AFM flapper to verify that it moves freely with no binding and that the by-pass is not open too far.

The Air Flow Sensor is supposed to be 'calibrated' for each car -- since no two engines are identical. That is why it is adjustable. For example, I gave my old '83 AFS to Don944LA when he couldn't pass smog. Somehow mine was set properly and his old one wasn't -- he passed with flying colors!

Winner Winner Chicken Dinner!
 

WOOoooo HOOooo!!!

Passed baby!


After a long winter/spring where other priorities/cash availablity were issues....


I took the car in to a local independent Porsche shop.

Needless to say I should have gone this route from the beginning, but I learned alot, and was able to take care of some other issues with the car in the intervening down time.

The Porsche tech fixed the smog issue for me....cost $65, lol. $15 of which was for a fuel additive called Blue Sky.

He told me that the fuel mixture was about 3X where it should be and with a simple turn of an allen wrench I was dialed in.

Here are the final numbers for future reference, measure only.

15mph @ 1801rpm

HC: 61
CO%: 0.10
NO: 785(max 807...still close)

25mph @ 2051rpm

HC: 0.65
CO%: 0.02
NO: 169

Thanks to everyone
 

Thanks to everyone who offered advice/help throughout this thread. I see that several of you really had good ideas, but at the time I had limited knowledge/ability to check and test some of what you were saying.

It sure would be nice to have a dyno and exhaust gas analyzer in the garage, wouldn't it.;)

Mine just passed PA emissions with FRWilks chip in the "stock" position..

Re: Winner Winner Chicken Dinner!
 

Interesting thread - so did the Blue Sky additive help? Or did it pass because of the work done by the Tech? I also am failing emmisions HC=600PPM at idle. Only CO and HC are tested (No Dyno) in my area. My CO number was to low .5% I have taken this up to .8% and this should reduce my HC number, I'm just running to lean.

So is Blue Sky worth using?

I think the majority of the difference was in the tunning, but the additive does work according to the Porsche mechanic(and this guy knows his stuff).

Also, regular tune up stuff like plugs, an oil change, etc, were taken care of.

I think the majority of the difference was in the tunning, but the additive does work according to the Porsche mechanic(and this guy knows his stuff).

Also, regular tune up stuff like plugs, an oil change, etc, were taken care of.