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

William J catnine09 at dslextreme.com
Sat Dec 9 00:02:50 PST 2017

----- Original Message ----- 
From: "Jim Adney" <jadney at vwtype3.org>
To: <type3 at vwtype3.org>
Sent: Friday, December 08, 2017 8:35 PM
Subject: Re: [T3] On a 73 T-3 SB fuel injected aut trans pinging .

> 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 lifted the carpet not long ago and vacuumed out the entire area , the 
bottm end of the gas pedal rubber sort of a flap was broken off . Everything 
else was free . In my case letting off the throttle whether at the gas pedal 
or at the throttle itself makes no differnce in how the engine comes back to 
set idle speed. When I was checking the timing I was of cause using the 
throttle on the IAD. I did notice the gas pedla felt easir to move once I 
cleaned the area.
> 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?

  It was fine at speed before ,it's was not something that stood out.  The 
ping was what brought me to this point. Before that I was not all that 
concerned . It did concern me that the idle was not stable simply because 
the I could hear as well as feel the exhaust was not close to a steady beat 
. It's not like a steady one cylinder miss fire or the engine rocking and 
shaking. This is why I refer to it as an erratic missfire and so far the 
ONLY thing that corrects this is unplugging the air temp sensor on the right 
side of the IAD . Also I know how it ran before 2009 when this began. Before 
that if I was near the rear of the car I could hear the exhaust was even and 
steady like a heart beat now it's like heart palpitations on a regular 

 I've been trying to describe this for a while. It's next to impossible to 
use words . Now since the air temp senser may or may not add 10% more fuel 
or change the fuel to air ratio the point I've been trying to express the 
second I unplug it every single miss fire is gone in an instant . This is 
not a good example , but pull one plug wire on any air cooled VW what do you 
notice then put it back on . This sensor being plugged in then unplugged has 
the same instant affect. I am trying to find out why. I know what it does, I 
know the high and low ohm readings are within spec for what that's worth and 
I know it is not shorted internal or via the 2 wires to the ECU. I also know 
when the engine is cold which here can range from 65 F to 95 F sitting all 
night when I first fire it up it runs fine no miss fire , within at least 2 
minutes the miss fire begins and stays there . If I unplug it when cold it 
runs better plugged in yet only for the 2 minutes . Adding to this when it's 
miss firing hand near the tail pipe I feel weak pulses and strong ones , I 
unplug the sensor they are all even . Also plugged in I slowly yet steady 
give it gas in park no engine load idle at 900 the RPM's do not rise smooth 
they hang and even might reach 1500 then drop down like it's fighting itself 
until it reaches 2300 to 2500 RPM at that point the response is smooth . If 
I just gun it fast no problem . Unplug the sensor and it responds every time 
smoothly no matter how slow or fast I push down the gas. I don't feel this 
driving at any speed . My main concern is if it's lean for what ever reason 
I very well may burn a valve and when the ping stated burn a hole in the 
piston . I've even tried with the AAR plugged off and it still reacts the 
exact same way . The same missfire is there plugged in and gone unplugged. I 
have also had the ECU out and unplugged and checked every single wire to 
every componant and to ground and find not a single issue . I tried another 
ECU I have off the 72 it didn't have the EGR yet with it in it acts the same 
way . Before years ago when the engine ran perfect I used the 72 ECU and had 
it connected to the dyno at work had the exact same readings on both I was 
trying to pass smog . It was just a bit to rich at idle and if I changed any 
setting point gap or timing ballast the HTS it was worse. I can also add 
resistance to the head temp sensor around 600 ohms and leave the air temp in 
circuit and get a similar affect meaning the erratic miss clears up yet not 
quite as good .

 If I had a bad plug or wire or valve  it would not change like it does .

> 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.

 Yes I said I rebuilt the AH Dist and it did help meaning the timing would 
return to 5 BTDC within a second instead of 2 minutes. Perhaps you forgot . 
I realize you get lots of emails and keeping track is not so easy. I don't 
find delight in this , I'd much prefer it ran like it should . I don't 
expect it to run like a modern FI memory holding computer car where 
everything has built in compensation for every condition.

 This is the first and only FI car I ever had . Sure it's over 40 years old 
It still should run even . You have a few T-3's if you had the engine 
running and heard this erratic missfire and uneven exhaust pulse wouldn't 
you try to find out what it was.

 This is my only car and that's my problem . I only have a certain amount of 
spare parts  and if I knew they would not be available I would have stocked 
up. I didn't think ahead or plan for this to become my only transport.

 I joined Samba in 2009 when this issue of the way this car runs came about 
. I've gotten assorted answers and ideas most of all no vacuum leaks allowed 
to rule evevything out ign first then vacuum leaks , valve adjustment , temp 
sensor hot and cold readings , fuel pressure , injector leaks then adjust 
the MPS.

 In 2006 and all the years before this car ran great . In 2009 this started 
and yes the fuel . Perhaps in that time frame it's the fuel that is crap and 
the root of all these issues . I can't test that , I have no clue what's 
even in it. Many have no issues with the fuel , many have tossed the FI and 
gone to carbs..

 The last thing I want to do is toss parts at it hoping to fix it. If I 
could make a video or just record the sound of the miss and post it somehow 
perhaps someone would understand what I can't express in words.
>>  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.

 The PCV does open at idle just not a lot.. I haven't played around with it 
much not enough to be concerned as long as it vents the crank case and isn't 
sucking in oil which it doesn't.
> -- 
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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