[T3] Still trying to figure out the 73 T-3 FI issue

Jim Adney jadney at vwtype3.org
Mon Jan 1 15:24:47 PST 2018

On 1 Jan 2018 at 13:48, William J wrote:

>  I know more air = more fuel not richer like a vacuum leak would cause.

Vacuum leak = more air = more fuel, so NOT richer. That's the whole point.

>  Is it the time the head temp sensor heats up added to the time the
> AAR is open so you have more fuel which is richer and more air because
> of the AAR being open. 

No, NOT richer. The MAP sensor and the ECU try to keep it the same. The 
compensation may not be perfect, but they try. (It's a primitive system, after 

> This is the part that does not make sense. Cold the AAR is open =
> more air yet why would the idle increase 200 RPM not right away yet
> after a few minutes? 

If your AAR is sticking, which is common, it may take a couple minutes of 
vibration and some warming to break it free. Just guessing here.

> I know there is some spec at what point the HTC at certain temps
> reads an ohm drop and some spec on how long an electric AAR stays open
> and gradually closes. 

OTOH, the temp sensors DO change the richness.

> The AAR seems to be closing yet not in the 12 minute so called time
> frame . It seems to vary since it may stick. I'm not even certain the
> RPM sould rise at all with the AAR open . Some say it will and others
> say it is only supposed to allow more air to compensate for the HTC
> richer mix. 

The AAR is trying to compensate for the increased friction in a cold engine. 
In a perfect world, it would not change the idle speed, but in reality it over 
compensates on some engines and undercompensates in others. It's just a 
matter of manufacturing tolerances and tolerance buildup.

None of this is anything to worry about.

> Also while the runners are off replace the oil cooler seals which is no
> something I look foreward to. I haven't had the tin off in a long time
> and can't recall if the left upper tin can come off just by remoing the
> screws on the fire wall tin just enough to sneak the left upper tin
> out. 

That's a big job and probably unnecessary since you rebuilt this engine once  
with new seals. If you decide to go ahead, in addition to the air runners and 
injectors, you'll also need to remove the distributor and the generator, but 
getting the tin off is the hard part. While you're in there, you should make 
sure that your oil cooler has the foam pad that keeps cooling air from 
bypassing over the top of the cooler. And check the fins on the cooler to 
make sure they aren't clogged with dirt.

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

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