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Rough Idle (87 Toyota Celica)



 
 
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Old March 8th 05, 11:46 PM
Thomas Hildebrandt
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Default Rough Idle (87 Toyota Celica)

I'm having a difficult time diagnosing a rough idle problem on my 1987
Toyota Celica GT-S.

When the engine is at normal operating temperature and the idle air
bypass valve closes and the RPM drops, the idle quality becomes poor.
The idle bounces up and down, and it feels like it about to stall
(though it has not stalled as of yet). This is worst when the engine is
in gear, but random missing is sometimes heard when in park or neutral.
The idle speed is correctly set at 750 RPM. When accelerating from a
stop, there is noticeable hesitation, but there are no problems when
cruising.

Ruled out:
o Spark plugs - they look fine
o Spark plug wires - swapped with another set
o Idle air bypass valve - when warm, RPM does not drop when the bypass
hose is pinched
o Oxygen sensor - replaced
o Low compression - readings are fine

Unlikely:
o Exhaust restriction - runs fine at higher speeds
o Fuel pressure - runs fine at higher speeds

Possibilities:
o Vacuum leaks - Everything I've read suggests that this is the most
likely possibility, but I've been unable to locate one.
o EGR valve leak - I removed and inspected this. The pintle inside
moves back with some force, and the diaphragm holds vacuum, so
I'd assume this is fine. However, when cleaning it, I noticed that
the valve was not water tight when closed.
o Distributor signal issue - I've been getting a code 12 "RPM signal"
from the ECU, which is "No NE or G signal within several seconds
after cranking." I swapped the distributor, reset the ECU, and am
still getting this code. The distributor pickups look fine. The
dash tachometer is working.

Any ideas for areas to concentrate on? I do not know if the "RPM
signal" trouble code is severe or whether it could be causing my
idle problem.

Keep in mind that the problem is not apparent when the car is started
cold; I figure either because the ECU operates in a different mode,
or the higher idle masks the problem.

Thanks,
Thomas
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