[T3] I'm a bit confused as to why unplugging this IAD temp sensor clears things up.

William Jahn willjahn975 at gmail.com
Wed Apr 3 11:50:28 PDT 2019

Jim I am using all the 73 parts except the IAD , Iam using the one off the
72, the only difference I could find at all was the 72 did not have the EGR
tube /port on the top yet the 72 does have that flat area where on the 73
it has the one hole and the 2 bolt flange which when I did have it on the
engine I blocked that tube since the EGR was just rusted junk. Other than
that the 72 IAD has a vacuum port for the dual cans retard line which I
have blocked off. The rest of both IAD's are the same. I've had the 72 IAD
on this engine since I rebuilt it in 97 and back then I didn't have this

 I might have a vacuum leak some where since the runner hoses are all oil
soaked and when I placed clamps on them I did notice a change.

 My main point about asking about the vacuum advance since I am now using
the 73 AH distributor and that vacuum can is shot , it does nothing other
than hold the advance plate from moving on it's own because of it's
internal return spring. Since the vacuum can is shot and when this issue I
try to describe is when I have the trans  in park the engine is warmed up
and AAR closed the idle is 950 , as I slowly open the throttle from idle
watching the tach the rpm transition is erratic , it's not a smooth rpm
rise. What it does is become erratic. Say it's 1,000 rpm as I open the
throttle a bit more just to gain 200 rpm more the engine will raise say 100
rpm then hang there then try to speed up a bit and will drop down than up .
It does this until the rpm reaches 2,100 then it becomes a smooth
transition . I noticed this first when checking the charging system output
by slowly raising the RPM to 2,000 just to see what it put out. With  this
said the reason I asked if the vacuum advance might be the cause is I'm not
driving the car yet the throttle is opening enough at that point to allow
the vacuum advance to have vacuum applied . Whether driving or not you can
see when the vacuum advance comes in using a timing light and you are @
1,300 engine RPM for example the mechanical advance will advance it a
certain amount and with no load you then pull the vacuum line off the can
and see the advance is now less . The mechanical advance is based  engine
RPM only not load influenced . My question is since the vacuum can is doing
nothing even not driving the car with a working can  it will still advance
the timing a certain amount , I'm thinking this is why the rpm rise is so
unstable until I unplug the IAD temp sensor, does this sound reasonable?
Since the IAD sensor does richen the mix a certain % and @ 1,200 rpm no
vacuum advance then it's mech advance only and may need more fuel to air ,
if the can worked it would be a bit more advanced and might need less fuel
to air mix. I realize driving load is different yet the vacuum advance with
still come in depending on how far throttle is open. Or do you feel it's
more likely a vacuum leak some where I have not found?

 I have even opened the throttle on the engine and hear this unstable rpm
rise , as soon as I unplug the IAD sensor that instant it becomes very
smooth meaning closed throttle idle sensor plugged in bring up rpm slow and
it is an unstable rise in rpm unplug it and it's very stable rpm rise or
call it idle 950 to 1,200 unsteady transition.


On Tue, Apr 2, 2019 at 9:45 PM Jim Adney <jadney at vwtype3.org> wrote:

> On 2 Apr 2019 at 19:32, William Jahn wrote:
> > From what I´ve read vacuum leaks tend to make the system run richer and
> > I imagine this may cause different issues depending on just where they
> > might be. Also it´s seems that this temp sensor unplugged enriches the
> > mixture this is also what rennlist states as an old mechanics trick.
> You need to be really careful here, because vacuum leaks have
> drastically different effects on d-jet, L-jet, and K-jet FI systems,
> as well as being different depending on where the leak occurs. Our
> systems are d-jet, which is probably the least commonly discussed
> system online.
> On our d-jet systems, leaks in the hose to the pressure sensor on in
> the pressure sensor itself will make the system run richer. All other
> leaks will have either no effect or will lean the mixture. It's not
> simple. You have to think about how the leak effects the pressure in
> the pressure sensor compared to the pressure in the IAD, the air
> runners, and the cylinder head inlets.
> Keep in mind that leaks directly into the IAD are no different from
> air let in by the throttle butterfly, so they have no effect on
> richness at all.
> > I´ve tested the TPS and it reads what it should when it should yet is
> > it possible since I have not had it off for a good long time or have
> > adjusted it, it could be part of the issue?
> In a word, no. A worn out TPS can cause a certain symptom, but not
> your symptoms. You can do a quick check of your TPS by simply
> unplugging it and seeing if anything changes. If you car then runs
> more smoothly at cruising, then you may need a new TPS. If unplugging
> your TPS fixes your idle problem, then we can consider looking into
> the '73 ECUs and changing a resistor there.
> > my vacuum ?dvance can is shot and it advances the timing only on a
> > light load which is the condition I get this unsteady RP? from 1,000
> > RPM to 2,100 RPM then it has a smooth engine speed transition.
> You will NEVER see any vacuum advance with your car standing still.
> There's no way to get it to work during a tuneup other than attaching
> a hose to it and sucking on that hose. It's either good or it leaks
> and needs to be replaced. Test for leaks by sucking on that hose.
> > if I plug that sensor in from 1,000 to 2,100 RPM no load the rpm rise
> > is not steady or smooth.
> Nothing about this rings a bell in my memory. But I'm assuming that
> you've checked that the vacuum advance is working smoothly and not
> sticking.
> > I realize the vacuum advance is an econo?y device meaning with
> > less engine load it will advance the timing/spark sooner , wouldnTMt
> > this apply to just sitting at ldle when there is no load then trying
> > to bring the RPMTMs up that the timing is just to far retarded?
> No, the vacuum signal for our d-jet vac adv comes via a small
> drilling that has been carefully positioned to be just where air will
> be passing the edge of the throttle butterfly when the car is
> cruising. At idle, that port is on the atmospheric side of the
> butterfly. When you open the throttle slightly, with the car standing
> still, that drilling is still on the atmospheric side. Cruising is
> medium load, requiring a throttle opening that would over-rev the
> engine if there was no load. Only at that medium load throttle
> opening is the butterfly in position over the drilling and able to
> supply vacuum to the vacuum advance.
> > AS far as I know on some type 3 years like 72 they retarded the timing
> > with a dual advance unit to cut down o? emissions( lean it out)  yet
> > once you gave it some gas the advance would take over . it was only at
> > idle.
> Timing has no effect on the rich/lean. It does have an effect on how
> combustion progresses and the emissions that result.
> Yes, that was '72-only. That's a '72-only distributor with a '72-only
> vacuum can. If you're still trying to use that distributor with your
> '73 system, even if you're not using the '72 retard side of the vac
> can, you might be better off with your '73 distributor and vac can.
> (A check of the advance curves in the Bentley indicates that they are
> pretty much the same, so it might be okay.)
> If by some chance, you're using the '72 distributor with the '72 IAD,
> so you have both vacuum advance and retard, then you need to be VERY
> careful to set the timing by the special '72 instructions in the
> Bentley. No other reference gets it right. And you have to make SURE
> that the vac retard diaphram is good.
> If possible, you should be using distributor, IAD, and ECU all from
> '73. They are all different from those supplied for '72s.
> Make sure you're setting the timing according to the Bentley '73
> instructions. '73s get timed to 5 deg BTDC at idle, not TDC. If you
> don't have a vacuum retard and have been setting your timing to TDC
> at idle, that's 5 deg less than you need. That would make the engine
> run poorly thruout the RPM range.
> --
> *******************************
> Jim Adney, jadney at vwtype3.org
> Madison, Wisconsin, USA
> *******************************
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