[Vwdiesel] OT: remote diagnosis

Val Christian val at swamps.roc.ny.us
Sat Jan 22 08:07:47 EST 2005


I've been using a passive pyrometer for several years, and own a 
couple of them.  Technically, they are long wave IR radiometers,
and even the $40 ones perform well.  (Sample testing of a $40 model
showed an absolute error of about 4F, however, the relative readings
were within 2F.  I consider that excellent over a -40F to 500F range.

These inexpensive radiometers have some shortcomings.  The primary
limitation is that they assume an emmisivity of 0.95.  For most
painted surfaces, that's about right.  Skin, rusted metal, plants,
and other objects will have different emmisivities.  However,
you'll find that the measurements are still close.  Relative measurements
are very accurate.  For example, measuring temperature across the
tread of a tire, to find alignment/inflation temperature differentials
yields very accurate relative readings.  The material is the same.

In your mechanic's application, assuming the manifold has somewhat 
uniform rust/paint/whatever, I'd bank on the measurements.  What I
don't understand is the rapid fluctuations, which you thought might
coincide with cylinder firing.  Like Mark The Miser, I would suspect
that the thermal mass of the manifold would be sufficient to smooth
out the temperature.  So I'm wondering if an object (or something)
is interfering with the measurement.

Mark the Miser's idea of swapping injectors is good.  It helps rule 
out injector problems.  You could also have sticking valve, injector
pump issue, or some other problem.  Typically fuel problems will 
uniformly affect all cylinders, as you already know.

BTW, we use the passive IR pyrometer to debug airplane ignition problems,
where there might be a rough running mag.  Two spark plugs for each
cylinder.  One on top, and the other on the bottom.  The bottom ones
like to foul with lead, and sometimes with oil.  Cycling the mags,
with the pyrometer pointed at the exhaust manifold (merely SS sheet 
metal on these puppies) yields the answer.  The old mechanic trick,
was run on the rough mag, and go around with spit on your finger,
and find the manifold (pipe) which doesn't sizzle as much.

I always wanted to find the one which melted Limburger cheese the 


>     Sorry for the semi-OT, but the pool of expertise here is too
> astute not to ask.
>     My assigned plow truck is a 1990 IH/ Navstar with their DT 466
> inline-6, converted from a garbage packer chassis to a dump truck.
> Engine has been exhibiting a slight miss which becomes conspicuous
> under load, i.e.- when fully loaded with material and applying power.
>     We changed fuel filters, with only a slight improvement in overall
> running.  One of our mechanics mentioned that he had a IR thermometer
> with a laser sensor, so we took a reading of the exhaust manifolds at
> the head, on the five cylinders we could access with the hood lifted.
> Engine was cold and idling.
>     The IR thermometer was impressive.  On four of the cylinders, it
> transmitted readings quickly enough to show a 40F +- fluctuation which
> we assumed to be a temp fluctuation between strokes of the engine
> cycle.  On four cylinders, the reading was between 190 and 230F at as
> close to the head as I could shine the beam.  On one cylinder, the
> reading was a fairly constant 120F.
>     I can't vouch for the accuracy of the thermometer, but what was
> striking was the differential between the 4 fairly constant cylinders
> and the one that was conspicuously and constantly low.  It seemed
> fairly apparent to me that a good starting point would be to examine
> seriously the injector in the low-temp cylinder.  Unfortunately, shop
> management claims that the problem is faulty fuel- a known issue.
> Their non-solution is to drive it until it quits running, and then to
> replace the entire aged unit. I disagree, but before I go over their
> heads, I'd appreciate some opinions from the group.
>     TIA,
>     Scott Kair
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