[T3] lean exhaust

Jim Adney jadney at vwtype3.org
Sat May 6 10:05:59 PDT 2017


On 5 May 2017 at 19:44, KENNETH ROGERS wrote:

> Question. Does FI vw's type 3's run leaner than carbs in vw's Type
> 3's? On carbs, it is 14 to 1 lean air to fuel ratio. My 71 FI is
> getting 15.1 to 1 lean air to fuel ratio at idle. While accelerating
> it goes all the way off the scale super lean, then deceleration it
> goes back to 15.1 to 1. 

I don't think there's an easy answer to this. The '67 OE carbed engines got 
better gas mileage than the later FI cars, but they may also have had a bit 
more power. (Note the maybe!) It's likely that they ran leaner under some 
conditions and richer under others. The only thing we know for sure is that 
they polluted more.

Please put your AFR meter away for now. This is a rather pointless exercise 
since NO ONE KNOWS what the correct target ratio is, and the AFR 
someone else gets may not be what is correct for your car/engine/FI. And it's 
completely pointless as long as your system is made of mismatched parts.

Daniel is correct that you need to start with a correctly matched set of FI 
components. You're not there yet. I'll send you info on getting a correct 
pressure sensor separately.

Daniel is incorrect when he says that having matching part # suffixes will get 
you a matching/compatable set of components. It's not that simple. Your 
brain is a D, but a D brain needs a C pressure sensor.

Your current pressure sensor is a B, which is correct for the '68-9 system 
where full load enrichment was done by a pressure switch, which you don't 
have. The completely different '70-1 system moved the full load enrichment 
function from the pressure switch to the pressure sensor. (FYI: For '72-3, full 
load enrichment was moved again, from the pressure sensor to the throttle 
position sensor.)

Daniel failed to mention that a significant number of his problems were due 
to bad connections at the FI component connectors. Much of this was due to 
his many swappings of parts, which eventually led to loss of spring tension 
of the female connectors. NOTHING works right if the connectors are not 
tight. That part is obvious, but what's less obvious is that swapping pressure 
sensors 10-15 times can leave you with bad connections. So don't do this 
unless you're prepared to re-tighten the female pins, which is a real pain to 
do.

Kenny, the very first thing you MUST do is figure out why your fuel pump is 
staying on. This may seem unimportant, but if it's due to a misconnection at 
the starter solenoid, it is confusing the brain in more ways than just keeping 
the pump on; it's also richening the mixture. (It's signalling the brain that 
you're running the starter, so it richens the mixture for starting.)  

I've already told you one way you could check to see if this is your problem. 
Here's another easy test: Turn the key ON, but don't start the engine. Note 
that you can hear the pump running. Crawl under the car and unplug the 
wire that runs from the starter solenoid into the engine compartment. If the 
pump then stops, you know that the wire was connected to an incorrect 
terminal. It needs to plug onto a terminal that is ONLY HOT while starting. 
The FI cars came with a solenoid that had an extra terminal for that purpose. 
If your starter has been replaced, you probably don't have that terminal any 
more. You'll need to find another way to connect that wire. It should connect 
to the same terminal as the wire that comes from your hot start relay.

Once you've got the pump to shut off after ~1 second, AND have the timing 
right, AND have the valves adjusted (cold) to .004/.006 (or .006/.006) 
Intake/Exhaust, AND have readjusted your fuel pressure up to compensate 
for your increased displacement, AND have the right pressure sensor, AND 
have the idle adjusted right, THEN it might be interesting to see what your 
AFR meter reads.

Note that I said interesting. It's not something that you should use to go back 
and make "golden screwdriver" adjustments for. We don't know what the 
target reading should be and you don't know how well your meter is 
calibrated. (And nobody else knows how well their meter is calibrated either.)

And, once you've got things pretty close, you should probably change your 
hot spark plugs back to the correct heat range.

I really want to make the point that the AFR meter is not particularly useful 
here. Yes, it's interesting, and yes, we all wish we knew how we were doing, 
but nobody used these meters when these cars were new and no one, 
outside of Bosch and the EPA, perhaps, knew what a proper reading should 
be (or even if there WAS a "proper" reading.) And, I also want to emphasize 
that I THINK these meters are sensitive to changes in exhaust temperature 
changes. That is, they don't respond correctly when the exhaust 
temperature, and the sensor temperature, is changing. It's possible that 
really expensive systems can do accurate dynamic readings,but most of 
them are accurate only when they are at thermal equilibrium.

Daniel has a nice system with the sensor placed early in the exhaust path. 
His numbers are interesting, but they get really confused when he's 
accelerating or decelerating. I suspect those numbers are just wrong, which 
led Daniel to do a lot of work and make a lot of changes that turned out to be 
counterproductive. Please don't make the same mistake.   

-- 
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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