Tom M'Guinn

Sorry, I was speaking in general about intake leaks, but you are right for pre-turbo post AFM leaks of course.

67King

Okay, so I've deduced that "AOS" means something in addition to open diff S2 trans. I'm guessing that's not what y'all are talking about. So what is an AOS in this situation?

One thing to point out. Okay, two things. First, if air is moving through a pipe, and the pipe has a hole in it, it will create a venturi affect, which is a vacuum at that hole. So it will suck in air. Second, the AFM is a resistor, meaning it will create a pressure drop across it to make air flow. That means the pressure behind it is less than the pressure in front of it. That's another source for vacuum at that leak. So it will probably suck in a little unmetered air.

Actually, I have a third point. In closed loop, the O2 sensor will correct those leaks anyway.

AScholtes

And you are also correct... with an exception... a major boost leak at the fuel pressure regulator. I learned the hard way on an old turbo car I had. Vac line split at FPR and car went extremely lean while under full boost in a long 4th gear pull (non Porsche motor)... well ended up with holes in the top of #2 and 3 pistons...

AScholtes

AOS means Air Oil Seperator

Im not sure how the O2 sensor setups in our cars work, but for most vehicles this is only true within a very small window based on the design of the O2 sensor (narrowband). And under full boost or WOT (when the mixture richens) the computer ignores the O2 sensor input showing a rich condition....so in essence, no it will not correct for the leaks...

Tom M'Guinn

Well, that's very true also -- clearly I should have left it to you to answer these questions Same thing happened to me, though I got away with a mere blown HG.

ehall

Okay, wait a minute!

My AOS is totally separated at the top of the cylinder. THAT is a lot of air going to an open hole. Next, that hole is before the turbo inlet, (via a smaller hose) so the AFM signal is significantly different from the amount of air actually entering the turbo.
NOW, how does that ACTUALLY affect the DME's fuel flow decision?

I'm running way rich. My boost hits at 4k on a 26/6. I have chased EVERY vac leak and exhaust leak.

We're down to a very few veriables!


I'm thinking that my trashed AOS os causing a HUGE leak of air.

I would love some real ideas on this...not that the discussion isn't great, but it keeps going in opposite directions...too me anyway.

ehall

okay, NO! The leak is AFTER the AFM and the meter. The hose to the AOS is near the rear of the J boot ASFAIK., but before the turbo.
So, MORE air is being read at the meter, than is ACTUALLY entering the turbo.

That should mean more fuel for less air, IF the fuel is being metered based on the AFM's air load...YES?

Bottom line is that there is more "metered air" than is actually entering the system. The meter is not reading ACTUAL air entering the turbo. It's reading the air it THINKS is entering the turbo.

Right?

Paulyy

Mine did that also. same turbo. 15 psi at 4k rpm. and had no leaks of any kind. so i pulled everything off the engine, turbo exhaust intake ect. couldn't find the problem. timing was in order, good compression. everything was okay. i gave up and spent some $$$$ on replacing the turbo, exhaust, injectors and other small things insted of putting things back on. best excuse to upgrade.

If you find out why the turbo lags so much, id really like to know.

ehall

Here's the thing. I just put on a head that has Larger intake valves, and is otherwise totally new, with no other mods, so I should actually be seeing EARLIER spool than I was. I used to spool at about 3100, HARD. Now it's 3900 to 4100. That doesn't add up. Somewhere I've got a hole in my engine.

Paulyy

Did it happen like oneday out of the blue the turbo changed it's mind when it wanted to make boost or as soon as you changed the head?

ehall

nope. It was after replacing the Head and the HG. I've checked every vac hose ane reconnected. I ran seafoam through to check exhaust leaks. I used ether to check vac leaks...AGAIN!...I'm down to a few Ideas.
1. I KNOW that the entire top of the AOS can be lifted rigt off the cylinder without any effort...it's just laying there!
2. Timing was set a tooth or two retarded,
3.FQS switch is set too rich for the 3", no cat exhaust and better flowing head, than before I changed them.

All changes were made at same time.

ehall

I'm running same Vitesse AFM chips and stock turbo, with 3 bar AFR.

AScholtes

Now, Im not totally familiar with the entire strategy of the DME in these cars. But it seems that in most turbo cars of this vintage, that the computer goes off of a base fuel map when WOT and boost kicks in and uses very little information from flow sensors and such...meaning, it has a base fuel flow that it uses when the TPS shows WOT...

This does not explain why you are running rich... that, I have no idea without knowing exactly what has gone on with your car. We can sit here and guess all day, but without seeing what is going on is like looking for a needle in a stack of needles... its all guessing..

AScholtes

Oh c'mon now... no reason to take it that way. I thought this was a tech board and tech was the topic of discussion? I'm just trying to help the guy out with accurate data....

Either way, I wish it would have only been a blown headgasket in mine...

ehall

ok no, I'm not certain either. What I'm reading is that you are saying that more air is entering the turbo than is being read at the AFm. The hole to the AOS is after the AFM but before the turbo, so why would you think that more is getting to the turbo? The air is bleeding out of the tube towards the aos and not getting to the turbo. Are you saying that the turbo is pulling air from the AOS? If so, from where? Why would that affect the fuel load? HOW would that affect the fuel load? What part of the computer is going to read that and adjust for it?
A wide open AOS is a gaping no-pressure hole, venting air away from the turbo inlet....is it not? It's a path of least resistance.
Are you saying that the turbo is creating a venturi effect across that orifice?

There is ZERO question that I'm running rich, BTW. WAY rich!