[T3] Temp sensor and MPS problems on new motor

[Jim Adney]

Jessica and Dan,

'69 FI w/ B brain

Before you got your new engine, I thought we had pinned your problem 
down to something intermittent in the brain. Have you fixed that 
problem? I don't recommend messing with the pressure sensor; you have 
to get your brain problems sorted out first. Last I heard, you had 
one brain that was intermittent and 2 brains that seemed good except 
for the fact that they didn't control the fuel pump properly.

What's your current brain status? I have good used B brains here if 
you need one.

The ideal (stotiometric) A/F is 14.7, so most of your numbers are 
slightly rich. If you see changes at times after the engine operation 
conditions change, this could just be something temporary due to 
changes in exhaust temp. Also, does your A/F sensor have 2 terminals, 
or does it use the exhaust system body ground as the second terminal? 
If the latter, changes in temperature could be causing intermittent 
grounds and/or galvanic voltages between some of the parts.

I don't know how your A/F sensor works, but some of them can be 
fooled by excess unburned hydrocarbons. I've got zero experience in 
this area, so I really can't advise you on this, but it's important 
to understand the limitations of your instrumentation. It may be 
lying to you.  

I admit that most of these are long shots, but AFAIK you are the only 
ones to be doing this sort of looking, so it's hard to know what's 
good data or even what correct data should look like. Keep in mind 
that the '68-9 D-Jet system ran a bit leaner and got slightly better 
mileage than the later versions. Keith's experience with A/F is with 
his '71, which runs richer. If you're driving at moderate speeds and 
the weather isn't too cold, then 28 mi/gal sounds perfectly normal to 
me, for a '69. In fact, when those cars were new, VW guaranteed 28 
mi/gal, but then they backtracked a bit and said that this was for 55 
mi/hr (or was it 65...) no wind, no stops, etc. My '68, with the same 
engine and FI system would consistently get this kind of mileage as 
long as I kept my speed down.

The fact that you get the same outcome with different pressure 
sensors indicates that the problem is NOT in the pressure sensors.

You have a new electronic voltage regulator, so I don't think that's 
a problem.

I haven't ever seen a symptom that I would call "lean hunt." It 
sounds like what I call "loping" but that only occurs at idle and is 
simply caused by the idle speed being set while the AAR is still cold 
and open. You have to set the idle with the engine warm and the AAR 
closed. You know the AAR is closed when the idle doesn't drop when 
you stop its hose with your thumb. If the idle drops when you plug 
the hose, the AAR is still open.

And, of course, you have to be sure that your throttle is closing 
fully when you take your foot off the gas. There are lots of ways 
this can go wrong if you don't get all the details right.

If you adjust the idle with the AAR open, when it closes, the warm 
idle will be too slow and the engine will die. Is it possible that 
this is what you're seeing. A sticking throttle cable can also cloud 
this picture.

One other possibility: When my '68 was new, with an A brain, it would 
sometimes start to lose power while crusing on the highway. I took 
this to be a sign of overheating. When this happened, I would take my 
foot off the gas and let the car slow down a bit, letting the car 
drive the engine for a few seconds, so the fan would have a chance to 
cool things while there was no combustion happening. Then I would put 
the tranny in neutral and rev the engine up and down 2-3 times, just 
to get more fan action, but still at minimal load on the engine.  

In worst cases, which seemed to happen more in warm/hot weather, I 
might have to repeat this every 15-30 minutes. Once I replaced the A 
brain with a B, I don't think this ever happened again. The A brain 
also tended to flood when starting, even without the CSV, which my 
'68 didn't have. That problem also went away with the B brain.

I understand that you're "once burned, twice shy" so you're very 
tuned into any potential problem, but I'm not sure you have any real 
problems here. If the "lean hunt" is just an idle problem, then I'm 
tempted to ignore the A/F readings and concentrate on figuring out 
how to get the idle adjusted correctly.

Keep in mind that the D-jet system is an open loop system: There's no 
feedback, 
so the FI has no way to know how well it's doing. Consequently, there 
are bound to be times when the A/F goes sideways. The D-jet FI is 
simply programmed to do certain things under certain conditions of 
temp, rpm, and manifold vacuum. It's primitive and stupid, and yet 
significantly more advanced than a carb. It's the granddaddy of all 
of today's cars, so even with a new engine, it's not going to work as 
well as a modern car.  

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