[T3] Bad day at the (FI) office

[Jim Adney]

On 25 Mar 2016 at 12:30, Daniel Nohejl wrote:

> A few years ago, Joe (harness builder and engineer) came up with an
> equation based on these numbers 
> 
> =-50.92*ln(64)+469.8
> 
> where the number in parentheses is the resistance for the CHTS. I
> dropped the equation into Wolframalpha.com <http://wolframalpha.com/>
> and at 64 ohms, the temperature would be 258* which seems reasonable
> on the top of the head and close to an exhaust port on a warmed up
> motor at idle. I know it´s not exact, but that´s all I have. HereTMs
> the graph Joe made as well: 
> 
> http://i.imgur.com/25qfIVM.jpg <http://i.imgur.com/25qfIVM.jpg> 
> 
> I get low resistance numbers on both of the CHTS I´ve used since
> installing the new motor.  One of them was used on the previous motor
> and gave the same low resistance after warm up. 

Okay, Daniel, you've convinced me. I was wrong about how low that 
resistance could come. While it starts out 10x higher, the much 
higher temp of the head enables it to come down to the same range as 
the case/oil temp sensor. I agree that ~250 F is not too hot for a 
head temp. Thanks, I needed that.  ;-)

> Interestingly, in the fall, I discovered that the sheath on the wire of
> the CHTS I had been using for almost three years was torn and leaving
> the wire exposed in two places, so I swapped that sensor ?ut for a new
> one. Right away, I had lean idle hunting and and difficult acceleration
> (hesitation, some bucking). Since the only change ITMd made was to the
> CHTS, I had to conclude that it was the source of my issues. I measured
> resistance of 58 ohms fully warm whereas on the old sensor it was
> around 100 or so. I added resistance i?line, made a slight MPS tweak,
> and drove that motor until installing the new one this winter.

I've gotten a lot of temp sensors with an abused or cut off wire. As 
long as it's in good shape for the last 1/8", I just replace the wire 
and use good heat shrink over the joint. I've done a lot of these, 
just to have spares, but I've seldom needed to replace one.

Years ago, I cut apart one of those temp sensors to see what was 
inside. I've still got the parts somewhere. It's interesting that the 
sensor element is a small semiconductor slug, maybe 4 mm long by 4 mm 
dia. The connections to it are just spring loaded pressure contacts 
to the ends, so everything depends on good physical contact. Now I've 
never had much trouble with any of those sensors, but I knew someone 
once who claimed to have replaced dozens of them over the years.

Okay, just speculating now: What if your intermittent bucking problem 
wasn't caused by lean running, but by an intermittent internal (or 
external) connection problem. A bad connection would make the 
resistance go high, which would make for overrich running, which 
would make the engine buck and hesitate once it was warmed up. Adding 
the resistor had the effect of making the engine run richer and 
cooler, so it's possible that keeping the temp sensor just a little 
bit cooler kept it away from the temp where it started to lose a good 
connection. So the richness has managed to keep the head cool and the 
temp sensor conductive, but at the cost of poor gas mileage.

So here's my suggestion: Try some different temp sensors. There's no 
reason to believe that a new one is any better than a good old one. 
I've got a bunch here that I could loan you. Make sure they have the 
copper washer under them, and some conductive grease there wouldn't 
hurt. The only downside to trying this is that these are a bit of a 
pain to R/R. (I use a 13 mm tube wrench to do this; makes the job 
easy.)   

> All the CHTS I´ve ever bought were sold under Bosch part # 0280130012.
> I wonder if over the last few years there´s been a change in how
> they´re manufactured so that they can perhaps be used in a wider
> variety of applications with different cars? 

I checked and '68-9s came with 0 280 130 003 (VW 311 906 041) which 
had a long wire to reach up from underneath. Starting in '70, 0 280 
130 012 (VW 311 906 041A) which had a short wire, was supplied. My 
impression is that the length of the wire is/was the only difference. 
HOWEVER, looking thru my stash, I find 003s with the A marking and 
012s with long wires. My guess is that Bosch eventually just started 
selling 012s for all years, by making them with long wires so they 
would reach from either location, and VW at some point did much the 
same by buying 003s with long wires and marking them with the A part 
#.

The case/IAD sensors are 001/006, but I have an 014 that's brass for 
Mercedes, Volvo, etc, probably anything water cooled. All those 
numbers are still listed in my 1990 Bosch price list, but those are 
the only parts in the 000-014 range.

> Anyway, yes, all of the readings I´m getting are with the MPS turned
> CCW (rich) from the pre-set position. Thus, if I keep the resistor
> inline, I´d have to lean out a bit by turning CW and back toward the
> original position. If I remove the resistor, then I´d have to go
> further CCW to avoid running way too lean. 

My hope is that something intermittent and temp sensitive in the temp 
sensor is the problem, and that a different temp sensor would fix 
this. Then you could adjust the MPS back to let everything run at 
normal temps (hotter) and with normal gas mileage.  

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