Pinging under load, *sometimes*...

[Scott Fisher <sefisher@cisco.com>]

Here's a technical question that a quick perusal of the Bentley manual
doesn't solve, at least not at first glance.  Car in question is an '83
GT Coupe, 2144cc engine, CIS, naturally aspirated, solid-lifter head,
8.2:1 CR, 92-octane Chevron Supreme (California oxygenated fuel, *not*
with MBTE).

The only thing marring an otherwise glorious weekend with the Audi was a
tendency for the car to ping (knock, rattle, preignite) under *certain*
load conditions.

They're basically the standard ones -- high ambient temp, accelerating
uphill, from 3500 RPM or so on up till I'd let off the gas for fear of
knocking loose a piece of the edge of a piston.  (I've *got* such a
trophy already, when I learned that yep, you really CAN'T run 11.5:1 CR
on 92-octane gas, not in a BMC B-Series engine, anyway...)

Couple of observations:

When I let up on the gas, the pinging would stop, even if the RPMs
stayed pretty constant.

Sometimes the pinging would stop when the temp outside dropped (such as
entering a cooler portion of some valley), sometimes the pinging would
hang around a bit.

Usually it wouldn't ping when pointed downhill or on the flats.

When it pinged, there was noticeably less power.  When pinging
conditions ceased, full power returned.

So here are some of my thoughts on why this might be happening; I'd
appreciate other thoughts as well as confirmation/denial of the
probability of any of these:

a) excessive ignition advance.  Could be, and only when temperature
conditions cross the borderline do I get pinging.  I'll check this this
week; anyone have the specs for the '83 (my Bentley seems to stop at
'82, 6 +/- 2 degrees, or should I use that?)

b) an overly lean mixture.  This seems consistent with the intermittent
nature and the power loss, as though I'm not getting the WOT enrichment
all the time, and when it's already hot this leans it out enough to
cause the pinging.  Other reasons to suspect the WOT switch: when the
car is idling and I flick the microswitch for the WOT sensor, sometimes
the idle goes up and sometimes nothing happens.  (So... pointer to the
online FAQ for troubleshooting the WOT switch, anyone?)

c) too much heat in the cylinder head.  Not likely, as the cooling
system is now running remarkably well, and this happens when the temp
gauge is reading well within the low side of normal.

d) excess compression for the available octane.  Surely you jest; this
engine is only an 8.2:1 CR and I'm running the highest octane I can
obtain "over the counter."

e) injector blockage.  The car sat for long periods before I started
driving it, so the injectors could be partially blocked.  But...
intermittent and thermally sensitive?

This week, I'll put the timing light on it (any tricks there?) and
verify that I'm not running way advanced.  But because the WOT switch
seems to be intermittent, I'd appreciate anyone's pointer to the
official story on how to check and fix it.

Thanks,

--Scott Fisher