[T3] Temp sensor and MPS problems on new motor
Jim Adney
jadney at VWType3.org
Fri Feb 19 07:34:30 PST 2016
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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