2111 code....check engine light

[Doyt W. Echelberger]

My 87 5ktq is running GREAT again but shows a check engine light just for a 
few seconds when I accelerate.

The great running and the check engine light only appeared immediately 
after I replaced the ignition wire loom and the spark plugs (because the 
engine performance had degenerated into a rough running condition with loss 
of power and missing.)

OK, so when the check engine light started showing up I pulled the codes 
and got 2111.....engine speed sensor.

Correct resistance is 1000 ohms.  Mine is 100,000 ohms.

Bad engine speed sensor.

But why didn't the check engine light come on before the tune-up, when the 
engine was running rough and missing?

Now some more data:........after I tested the ohms and determined that the 
sensor was out of range (faulty) I got back in the car and drove it for 
about 20 minutes.  The check engine light came on less frequently during 
acceleration, and didn't stay on more than a few seconds. The longer I 
drove, the less often I saw the yellow light. An hour later, after more 
driving, it almost never comes on when I accelerate.

How do I explain this?   Is the speed sensor working better?  Is the ECU 
getting less sensitive to the readings of the faulty sensor?  Or is the 
check engine light come on for a completely different reason that has 
nothing to do with the speed sensor fault?

Saying it another way.......do I have a faulty speed sensor that actually 
had nothing to do with the check engine light coming on temporarily during 
acceleration? Could it be that the light was coming on to indicate the 
automatic retard of the timing due to knock, and the engine isn't knocking 
as easily or as often now that I have driven it for an hour?

The plugs that came out were black with carbon. The ignition wires were 
old, very old. The new wires and plugs MIGHT be burning out the carbon that 
had built up in the combustion chambers, and that burning out process was 
causing some knock that progressively became less frequent as the carbon 
accumulation goy blown out.

The carbon build-up is a well known characteristic of my engine, and 
hundreds of messages in the archives document the problems of other people 
with the same thing, including how they got rid of the buildup.  Winding 
out the engine with really hard driving for an hour or so seems to be the 
most successful solution.

OK.....that makes sense.  I'll go out and drive it hard for an hour and see 
if the check engine light still comes on. I really suspect that the faulty 
speed sensor is just a coincidental condition that has nothing to do with 
my check engine light coming on.       (Or not.)

Off to the highway to test this hypothesis. If the engine blows up, I was 
wrong.    More data to follow, if anybody cares.

Doyt Echelberger
still playing with an old 87 5ktq in Ohio.