[T3] '73 Idle Test

William Jahn willjahn975 at gmail.com
Tue Aug 27 11:01:58 PDT 2019

I removed the temp or thermal switch from the cold start long ago. At one
time when I still had the cold start valve plug connected I ran a wire to a
switch on the dash so I could ground the cold start so I could use it at
will yet it caused a rich cold start. I keep the cold start valve unplugged
just the fuel line on it and checked it doesn't leak had it off with
pressure just to make sure.

 I'll check to see if the other wire believe it's #18 on the ECU is hot
when not cranking it also ends up in the test connector there is a + there
and it is hot off the battery and #50 . I do know at the test plug + is hot
all the time. If 50  is hot all the time then that's the problem I had the
starter off in 2003 when I pulled the engine and auto trans to repair the
trans lost all forward gears. I could have connected it wrong.

 If that's not it the I will check the connectors that power the ECU on the
left side forward of the engine. I imagine even corroded 1/4" male and
female terminals in those plastic covers could cause enough voltage drop to
lower the voltage to the ECU . I always wondered why # 18 was connected to
the ECU from the cold start and what it's function was.

 I'll let you know what I find. At least I have some possible fixes and are
not the engine  temp sensors at all, I'm not getting anywhere fiddling with
those. . I imagine even the contacts inside the power relay could cause low
voltage to the ECU mine is the original perhaps I should check for a
voltage drop there as well just to rule it out . A little drop here and a
little more in the connectors you pointed out add up. I never looked at
those all I ever did was clean the power relay connectors and it's ground
and fixed the engine grounds to the two 3 terminal trees found one in a
plastic cover that had two wires strands connected and couldn't pull the
male out of it it was green inside. I just cut it out and spliced and
soldered and heat shrink. It helped some.

 On the temp sensors.
 Yesterday working it the garage in over 100 degrees I removed TS2 and
found it was a VW # 022 906 041 Bosch # 0 280 180 012 that was used for
buses and T-4 . from what I could find a T-3 should have a bosch 0 280 130
003 I have an old one of those yet they do show a cross reference. I tried
the new on none Bosch yet it screwed in but I could rock in so it does not
fit as well as the Bosch ones and didn't want to bugger the threads in the
head. I know these TS2 might start out reading high yet drop off fast and I
was beginning to suspect TS1 . On the TS2 I have in it now the 012 I
noticed when I pulled back that clear plastic tube you know the one , that
the insulation on the actual wire was broken about 1/4" above where it
enters the sensor nipple and the strands were not broken yet tarnished. I
put it back in and found if I start it cold with TS1 connected the idle in
park is 850 same as started cold TS1 unplugged . The difference is  it runs
fine for say 5 minutes then begins the erratic miss. What I noticed is the
idle with TS1 connected did not climb to 1200 RPM it stayed at 850 only
when I unplug TS1 does the RPM also begin at 850 PM yet then rises to 1200
Only when the engine is hot does the idle 850 IN drive and 1000 in park . I
was going to raise the RPM up to 1000 hot engine in park and leave TS1
connected to see what happened yet I think I did this before and it still
had the erratic miss fire.

 Hopefully what you just suggested will fix this once and for all. I guess
wire #18 the the ECU tells it something yet do not know what other than
telling the ECU the cold start is in operation and alters injection pulses.

On Tue, Aug 27, 2019 at 7:15 AM Jim Adney <jadney at vwtype3.org> wrote:

> William,
> I note that 2 of your observations seem to point in opposite directions.
> Unplugging the air temp sensor makes the idle settle down, making us think
> that the mixture was lean.
> Using a quarter tank of gas in 20 miles sounds like an extremely rich
> mixture.
> I suggest that you take the car for a long drive, like 100 miles, at
> reasonable
> speeds to see what's really going on. Pick a time of day and a destination
> that avoids heavy traffic so you can stay in 4th gear most of the time.
> Start
> with a full tank fill-up and record your odometer reading. End with the
> same
> and record the end fill-up gallons. Calculate your actual gas mileage.
> (mi/gal = (odo2-odo1)/(gal used)
> Before you do this, here's one more check you can do: There's a wire that
> runs to the temp switch under the IAD. Pull that wire off and measure the
> voltage between that wire and ground. It should be zero with the engine
> off
> or running. It should be 12 V while starting. That's assuming your Cold
> Start
> Valve is plugged in.
> If you get 12 V with the engine off or running, you've got a misconnected
> wire on the starter. Did you have any work done on the starter around the
> time when this problem started? The solenoid terminal on the starter also
> gets a wire to the brain and the Cold Start Valve. If that wire is
> connected to
> the hot terminal on the starter, it will run the system rich all the time,
> even
> with the Cold Start Valve disconnected.
> Do you notice a change in the idle when you connect and disconnect the
> temp switch wire? There should be no change.
> I wonder if you might actually be running rich most of the time. Inspect
> the 2
> wires that carry 12 V power into the FI wiring harness in the LF corner of
> the
> engine compartment. Look for a wire that is frayed, with only a few
> strands
> still connected. Extreme richness could be caused by low system voltage,
> but you've got a new VR, so the VR is not the problem. But a high
> resistance
> in the wires that feed the FI could cause a reduction in the supply
> voltage to
> the FI. I've seen this problem on another car.
> Jim
> On 26 Aug 2019 at 10:54, William Jahn wrote:
> > It has the 048 . The only thing I can add is the gas millage is not very
> > good. Of course most of the driving here is stop and go yet it's been
> like
> > this for a while and it used to get better millage. I can go through a
> 1/4
> > tank in 20 miles .  From the way it acts it seems to start out rich
> because
> > of a high TS2 reading then leans out since the TS2 in my mind drops to
> > fast. This seems to be why connecting TS1 then leans it out more since by
> > now it's temp has risen dropping it's resistance.The reason I feel it's
> > leaned out  on rennlist an old tech trick was to unplug it to richen the
> > mix . My thinking is if I change TS2 with a new none Bosch part since
> Bosch
> > no longer makes these and this part is reading much more in line with
> what
> > it should perhaps it won't start out at such a high resistance then nose
> > dive to 77.5 ohm in 2 minutes then it just might allow TS1 to be more in
> > line and works as it should since TS1 seems to work proper. I just don't
> > feel running to lean is a good idea. I drives ok with TS1 connected and
> > unconnected because it has a load I can't feel it. Yet if I try to  read
> > charging system out put it's next to impossible to get a steady RPM
> reading
> > with TS1 connected it causes the RPM's to rise and fall a few hundred rpm
> > all while holding the throttle steady  from  idle to 2300 RPM then above
> > that it does read steady. This is also what I hear and feel even at idle
> > and raising it standing at the rear of the car I feel this what I call
> > erratic miss hand by exhaust tip and hear this slight skipping as one
> would
> > hear a miss fire.
> >
> >   This is what I found looking further .
> >
> > "
> > *Q: What is the effect of disconnecting the TS1 sensor? A:* The TS1
> sensor
> > produces a fairly weak effect on the mixture, to compensate for the
> varying
> > density of air with temperature. Removing the sensor will slightly richen
> > the mixture. This is a common mechanics "trick" for an old engine that is
> > running lean due to wear or vacuum leaks. See the PL analysis above for a
> > detailed explanation.
> >
> > I measured the effect of a disconnected TS1 sensor (see the question
> above
> > for the setup conditions). Disconnecting the sensor increased the
> injection
> > pulse width to 7.1 ms - an 11% increase, almost exactly what the analysis
> > of this effect predicted (see the PL analysis section). Note that this
> > enrichment effect is variable as the temperature changes - stronger for
> hot
> > temperatures, weaker for cold - due to the elimination of the
> compensation
> > for air density as a function of temperature."        I found this here .
> > https://members.rennlist.com/pbanders/ecu.htm#ICM
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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