Date: Sun, 26 Aug 2012 18:31:16 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Poor Hot Start Discovery - midway through solution
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At 01:52 PM 8/26/2012, Skip wrote:
>I also thought that by stepping on the accelerator, that would
>inhibit the amount of fuel being pushed into the fuel injectors,
Doesn't work that way. The combined action of the fuel pump and fuel
pressure regulator result in a constant pressure across the injection
orifice, so that no matter what the engine conditions, equal open
time means equal fuel injected.
> thereby reducing the impact of "flooding". This action (by me)
> actually helped in cases where the van would not start when the
> engine was hot. Though the engine would turn over alot, if I
> depressed the accelerator that engine would finally begin running.
This does work in at least mild flooding. The injectors are no doubt
being commanded to inject more fuel, but the increased airflow
through the engine at cranking speed seems to more than compensate for it.
Conclusions: 1. By fiddling with the Temp II connector this
morning, it decided to work when the engine is hot, or 2. I didn't
get the engine hot enough, or ambient temperature not being in the
nineties allowed the radiator to keep the engine at a less than
really hot temperature. In other words, I'm not totally convinced
that my Temp II sensor is the root cause of this "hot/no start"
issue...but I'm thinking maybe it is.
>What say you?
Connection problem highly likely. Clean up the connection post,
tighten the female connector a tad, slap a bit of grease over it
afterwards to keep the weather out.
>Also, my whole theory here is that the Temp II sensor (when the
>engine is hot) sends a "low ohm signal" to the ECU, and the ECU
>instructs the injectors (via the relay in the black box) not to
>administer (too much) fuel to the engine, thereby preventing "the
>engine gets itself flooded when "hot starting".
>
> Is this correct?
It's about as correct as it could be while still being totally wrong.
;-) The relay in the box controls the fuel pump, not the injectors.
The ECU takes a number of factors into account when calculating how
much fuel to inject: air-flow meter vane position, intake air
temperature, engine temperature, engine rpm, throttle-position
switch. Once the engine is up to temp and the oxygen sensor is also
working, it changes to a servo- or closed-loop mode of operation
where it continually adjusts the mixture richer when the O2 sensor
indicates lean, and leaner when the sensor indicates rich. So on a
hot start it will know that the engine is up to working temperature
and will do little or none of the enrichment that would be required
to get a cold engine to run. Unless, of course, T-II is unplugged,
in which case it starts thinking in terms of really extreme cold and
huge enrichment.
The T-II sender has a longstanding reputation for being flaky and
causing mysterious ailments. Having taken a number of them apart, my
conclusions are:
The characteristics of the thermistor pellet may be out of spec from
the outset, more likely in cheap imitations than the fancier brands
but not impossible in either. May lead to less-than-optimum running
under certain conditions, but not likely serious and not an issue in
closed-loop operation. Determining this requires testing at various
temperatures as a sender may be in spec at one temperature and not at others.
The sender may be simply open-circuit as received, instant gross failure.
A sender that was in spec when new may change characteristics
slightly and drift out of spec, but is extremely unlikely to
mysteriously go open, or behave grossly differently hot than cold -
unless its seal (from brass housing to insulator) fails and moisture
gets inside, in which case all bets are off. Of the five or six I've
taken apart, one of the cheaply-made ones had allowed a trace of
moisture inside, although not enough to cause any problems as of when
I disassembled it.
The great bulk of T-II alleged failures are actually connection problems.
Yours,
d
