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

William Jahn willjahn975 at gmail.com
Sat Apr 6 10:07:42 PDT 2019

I guess I'll have to replace the runner hoses and a few other vacuum lines
to see if this helps. All I can conclude is it must be running on the lean
side , at least at idle , Have the timing set @ 5* BTDC . Before I do lines
I will check if the new ignition switch may have changed anything , easy to
do just plug in that temp sensor , if it  acts the same way then I'll
change out any vacuum line I have not in a while runner hoses, IAD to auto
trans and the heads to oil bath for the PCV . Those seem or at least look
fine and are still connected at the heads and oil bath.


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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