[T3] Bad day at the (FI) office

[Jim Adney]

VW/Bentley call the cyl head temp sensor #1, but Bosch calls it #2, 
so I try to avoid using those numbers to identify them. Call them by 
their locations. The resistances of the both sensors goes down as the 
temp goes up. Variations of +/- 20% are acceptable.

The cyl head temp sensor should read somewhere around 1000 Ohms at a 
temp of 122 F, which is not very warmed up, so expect it to be much 
lower.

The case/intake temp sensor should read around 130 Ohms at 122 F, 
which is still not completely warmed up.

When fully warmed up, I'd expect both sensors to be well below the 
resistances above.

One of your readings is low by a factor of 10. Looks like you got 
your scales mixed up, below, but I can't tell which reading is 
mistaken because I don't know which is which. If what you typed is 
correct, that may be part of your problem.

Read on below....

On 24 Mar 2016 at 19:27, Daniel Nohejl wrote:

> I tried out two more brains (one A and one B) and they both behave
> exactly like the one I reported on the other day (CUX10): with 100
> ohms of resistance at TS2, the motor seems stuck in warm up mode even
> though the 150-165 ohm range I saw over the last few days is already
> pretty low. With the resistor removed, the idle is very lean and it
> hunts. Acceleration is poor and very lean as well. Resistance at the
> sensor with the resistor removed was 64 ohms when warmed up, which is
> the lowest ITMve seen it. Interestingly, the resistance reading of the
> intake air temp sensor was also 64 ohms. The K manual says the lowest
> should be 80 ohms so mine is off by 20%. Does this matter? ItTMs also
> only about 45* out and while ITMve driven around a fair amount this
> evening, I didnTMt do any heavy pushing, speeding, or long highway runs
> so I wonder why ITMm seeing the lowest readings ITMve seen of each
> sensor. Does anyone out there have any resistance readings of their
> sensors when the engine is warm? 
> 
> If it matters, I left the B unit installed as it was marginally less
> lean than the other two. 

Keep in mind that we don't have any actual F/A data on the '68-9 
D-jet engines, so, even assuming that your meter is accurate, you may 
be going astray on an assumption of what to expect for F/A. And, BTW, 
this is D-jet, not K-jet, so stick with the D-jet manual.

Yes, it's reasonable to worry about some part of the engine getting 
too hot, the heat load on an engine is a function of many things. 
But, there's almost no way to overheat an engine at idle, since 
there's no load and no output then. You can get away with some really 
"bad" things at idle that you wouldn't dare do under heavy sustained 
load. (It's the "same" as how much advance you can tolerate while 
crusing as opposed to under full load.) Plus, we're talking about a 
low compression engine here. If you put together high compression 
with sustained max load at high rpm, that would be different.

> I also verified that my AAR is more or less all the way closed.
> Pinching off the hose between the AAR and IAD didn´t discernibly
> change the idle speed or the idle AFR at all. I figure if it was
> hanging open, then pinching it would have a significant effect on both
> idle speed and AFR as I´ve observed in the past. 

I recommend doing this test by pulling off the INPUT hose to the AAR 
and just putting a thumb over the opening. Pull off the end that goes 
to the air cleaner for the easiest, and least damaging process. 
Pinching a new hose won't cause any problems, but if you get in this 
habit, you'll be replacing that hose one of these days. It's not a 
big deal, but when it happens, you won't have a spare and you'll wish 
it hadn't.  

Right now I'd say that you're caught between 2 sets of conflicting 
data. Your F/A meter is telling you that you're too lean; your gas 
consumption is telling you that you're too rich. Your temp sensors 
seem to be saying that they think things are still not getting fully 
warmed up, so it's running rich.  What should you believe? Unless 
you're leaking fuel out on the ground somewhere, the fuel consumption 
calculation is so straightforward that there's little chance of 
significant error.

So, my suggestion is to remove the 100 Ohm resistor and see what 
happens to your mi/gal. Sure, adjust your idle if you need to. Then, 
unless your consumption has gone over 25 mi/gal, try returning the 
pressure sensor screw to the / position where it started. Then 
measure your mi/gal again.

I'm skeptical of the FAR readings you're getting, and also aware that 
we don't really know WHAT readings would be "correct." Yes, we know 
what the "ideal" is, but should we expect the first generation EFI to 
achieve this in an open loop system under all conditions? That would 
be extremely unlikely.



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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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