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

Jim Adney jadney at vwtype3.org
Tue Apr 2 21:46:07 PDT 2019

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 

> 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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