erratic problem fyi

Had a somewhat strange problem recently that I thought the group might
like for their collective knowledge base.

My Miata would start fine when cold, or had sat for a while, and would
run fine for short trips. When it was fully warmed up, it would
occasionally develop a erratic idle. It could be too high, or drop down
and stall out. When this condition occurred, it would also run a little
rough. The most noticeable problem would be that it would bog down on
quick acceleration. If it was turned off when these symptoms were
present, it would be very hard to start. I would act as if flooded, and
the throttle would have to be held to the floor to get it to start.
sometimes it would then take a while for the idle to smooth out again
to be driveable.

Well it finally got to the point where it the condition was consistent
after it was warmed up. The most noticeable oddity was that the idle
would go to about 1500rpm with the shift in neutral, but would drop
back down near normal if put in a gear. Until the engine was to full
operating temp, it would be pretty normal acting, with maybe a couple
of odd spots during acceleration.

So, definitely temperature related. But, there were no error codes
reported by the ecu when trying to do diagnostics. Tried an alternate
air flow meter with no real change. Then changed the engine temperature
sensor. The one on the back of the head by the coil packs. Not the fan
temp sensor, or the instrument temperature gauge sensor. That did the
trick. Now runs like it should.

So it appears the engine temp sensor can have a failure mode that does
not register with the diagnostics. It appears it would short out when
the temperature was very near the normal operating temperature and
would then act like it was very cold. (Could be an open condition
also.)
Result was it would run a rich mixture causing the other problems.
Just a troubleshooting note to add to the collective knowledge. Will
post to the maita.net forums when I get a chance.

--
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white '93 189k mi.
since a baby

Good to know! Thanks for the info, I've not had any run ins with that one
........yet!

Thanks again,
Chris
99BBB