[T3] On a 73 T-3 SB fuel injected aut trans pinging .

Jim Adney jadney at vwtype3.org
Fri Dec 8 20:35:11 PST 2017

On 8 Dec 2017 at 15:13, William J wrote:

>  The idea of the AAR is to allow air to bypass the throttle plate being 
> closed on cold start. This extra air causes the MPS to riched the mixture = 
> more fuel along with the Head temp sensor . Once the engine is warmed up the 
> AAR should close and the head temp sensor resistance has dropped it 
> basically is out of the picture. These are mainly warm up devices.

Not quite right. With more air, the MPS gives you more gas, but that's not 
necessarily richer, it's what the brain is programmed to give for that amount 
of air at that temp.

>  At this point the MPS is the main control of the fuel to air ratio and the 
> 5 pin TPS is what controls the full load and the 20 clicks is what add's 
> fuel as you advance speed which is  form of enrichment .

The MPS is ALWAYS the main control of the A/F ratio. Then that ratio is 
modified depending on the inputs from the temp sensors and the rpm, from 
the trigger points.

>   Any vacuum leak other than the AAR for cold start enrichment is going to 
> cause a rich mixture . Since the MPS is reading vacuum which is based on 
> load is it less vacuum in the IAD that is less vacuum to the MPS= more fuel 
> taking the TPS out of the picture?

It doesn't cause a rich mixture; it causes more gas to get mixed with the 
increased air, resulting in the same F/A ratio. That's the MPS's job.

The MPS will always know about any air coming into the IAD by any means. 
Yes, less vacuum (more pressure/more air) gets more gas. That's what it has 
to do to maintain the desired F/A ratio.  

>  I ask simply because my unplugging the air temp sensor richens the mix by 
> 10% across the board and the engine erratic miss stops tells me it's running 
> lean . Since from what I understand vacuum leaks cause rich mix is this 
> across the board ?

I suspect the 10% figure is an average, and a wild ass guess, too. 
Unplugging takes the resistance to infinity which corresponds to a very cold 
engine, which probably results in the max fuel for the MPS and rpm inputs. 
But the change you get is bound to differ if you unplug the sensor when the 
engine is a 160 F rather than 20 F.

> What am I missing here? Also what is confusing is why the idle does not 
> drop to set idle speed right away when I let off the gas? I know it used to. 
> Is this also caused by a vaccum leak?

It sure doesn't sound like a vacuum leak. A vacuum leak wouldn't always 
heal itself a few seconds after you let up on the throttle. Have you checked 
your carpet to make SURE it's not fouling the gas pedal? And are all the 
pivots at the gas pedal free? Problems here are common.  

I feel like you're putting too much emphasis on how the engine idles, as 
opposed to how it drives. These cars don't have the same level of idle 
stabilization that modern cars have. How does it do at speed?

Have you taken the distributor apart and made sure that everything's free to 
move and return? Given your apparent delight at delving into things, why not 
do that? Or I can rebuild it for you.

>  I've read no vacuum leaks are allowed . Does this include the PCV system 
> this 73 has , meaning as far as I can tell the PCV closes at idle if I watch 
> it with the small cap off the crank case vent can then as I open the 
> throttle I see it open a bit.

The L-jet system is particularly intolerant of vacuum leaks, because most 
leaks bypass the air measuring flap. On our D-jet systems, any leak into the 
IAD is treated as just more air and accomodated by the MPS. Air leaks that 
happen farther downstream cause problems because those don't affect the 
pressure that the MPS sees quite as much as they would if that same air got 
in the IAD; plus downstream leaks will be just for individual cylinders, 
tending to lean them out.

I would have expected the PCV valve to open at idle, as that's when the 
manifold vacuum is at its max. It will actually be higher if you're driving and 
take your foot off the throttle, but that's hard to see.

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

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