[T3] Bad day at the (FI) office

Jim Adney jadney at VWType3.org
Tue Mar 29 13:58:13 PDT 2016

On 29 Mar 2016 at 7:15, Daniel Nohejl wrote:

> Well, on Sunday my sensor read 87 ohms without the resistor so then
> wouldn´t I need a resistor in order for the ECU to not read the sensor
> as a short? Or am I misunderstanding what the effect of the ECU
> reading the sensor as a short would be? In other words, it sounds bad
> to me but perhaps you´re saying that below 100 ohms just registers as
> warmed up and it doesnTMt matter if itTMs 99 ohms or 1 ohm. 

I suspect there's a difference between 1 Ohm and 99 Ohms, but I'm not 
sure how much it is. I'm pretty sure zero Ohms is bad, because I had 
the temp sensor short out on my '68 many years ago, and that killed 
the engine, and it wouldn't restart.

It sounds like this time I wasn't clear. My theory was that as the 
sensor heated up some connection inside the CHTS became intermittent, 
so the resistance could suddenly pop to infinity. In other words, it 
would go open, not shorted. A sudden jump from ~75 Ohms to infinity 
would make it suddenly go super rich, which would give you the 
bucking and missing you described earlier. The rich mixture, and the 
non-combustion, would cool the temp sensor down to where it worked 
again. So you would be in a cycle of decent driving and bad driving, 
with poor overall gas mileage.

It's just a theory, but you can test it by swapping your other temp 
sensor back in. The cracks in the insulation do no harm unless they 
allow the center wire to touch something that's grounded. Or you can 
send it to me and I'll put heat shrink over it. I also have some 
other temp sensors here that you could try, but if you want to try a 
series of these tests, I recommend getting a 13 mm tube wrench. (A 
tube wrench is like the one that came with your Square: the one for 
the spark plugs and lug bolts.) I can give you a reference if you 
can't find one in Brooklyn. Shouldn't be expensive.

> Ray Greenwood has been telling me something different for the last few
> months now....at least since November when I first put a resistor inline
> with the temp sensor. He says that the CHTS shouldn´t read below 70
> ohms because then there´s no room for it to offer any additional
> enrichment on, say, a hot day in August at 70 mph on the Thruway. In
> other words, he suggests that the CHTS continues to influence the
> mix?ure well after warmup. ITMm not really sure which is true: either
> it does or it doesnTMt (or maybe it partly does?or partly doesnTMt). As
> mentioned, I can?use a variety of low ohm resistors and see what
> effect that seems to have on the AFR at idle and while driving after
> warm up. 

I think we're all in agreement that when the engine is cold, the temp 
sensor richens the mixture. Then, as the head warms up, the sensor 
leans it out. Ray seems to be saying that at some point, the response 
to less resistance is a richer mixture. In other words, the 
resistance is a monotonic response to temperature, but the mixture 
dips to a minimum at some point and turns around. I'm skeptical about 
that, because I don't know how you'd implement that in an analog 
circuit. I'll look at the circuit on the rennlist and see if I can 
understand that part, but that's not for our B brain. If anything, it 
would be more advanced than the B brain.

> Anyway, in general, things seem to be better. We went away over the
> weekend and on the way out of town, we leaned out the MPS a wee bit
> closer towards its original position. Our highway cruise AFR is kind
> of lean....anywhere from 14.5 to 16.5 but our MPG over the 260 miles was
> a modest 25.8. I imagine if we were too lean, our MPG would have been
> much higher. 

Yes, that sounds quite a bit better. What resistor was this with? 
What setting for the MPS: \  |  /  ?

What kind of driving and at what speed?

> I´ve yet to have a set of points go bad on me in only 1600 miles.
> Admittedly, I only have about 70,000 miles of VW driving under my belt
> but still....Part of me wonders if repeatedly having the key in the
> ignition in the on position while toggling the fuel pump prime switch
> hastened the death of these points. Maybe I can power the sw?tch from
> an always hot fuse and see what happens? 

Your use of the priming switch should only be for maybe 10 seconds. I 
don't think that's enough to hurt the points, but I agree that 1600 
miles on a set of points is pathetic. Is your coil one of the black 
Bosch OE coils?

I actually never set the points with a feeler gauge; I just use the 
dwell meter. That is a pain, because you have to put everything back 
together to check your work, because the dwell reading you get when 
running is different from the one you get while cranking with the 
starter. Nevertheless, the dwell meter gets the final say.

The tit on the points actually causes no problem at all, other than 
interfering with a feeler gauge. It's not a problem, because opposite 
the tit is an exactly matching divot in the facing contact.

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

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