Causes of highly elevated idle speed

I was fixing up the Reader's Rides pages, but Photoshop is unaccountably
slow today so I gave up. This happens from time to time and I have no idea
why. It cures itself in time.

Anyway, we've all read of reports of people with idle speeds that are way
too high, or fluctuating idle. With that in mnind, I've been studying my
intake system and diagrams, and considering the idea of the effect of low
coolant levels on hot idle speed.

It turns out (on my car at least), that there are no less than *four* extra
possible sources for additional intake air besides the throttle plate:

1) EACV
2) Fast Idle Valve
3) Fast Idle Control Solenoid Valve (A/T only)
4) Air Boost Valve

If any of these stick open, excess air will be incorrectly admitted to the
intake manifold, and idle when warm will be elevated, or will cycle.

Of these, the EACV and the Fast Idle Valve are warmed by coolant. Their
inlet and outlets are respectively connected to the discharge and suction
sides of the cooling system, so flow through these items appears positive,
and self-purging of air.

Coolant feed through the EACV appears to be there solely to prevent icing
of the EACV's air passages. Coolant temperature does not control the EACV
in any way.

The Fast Idle Valve *is* controlled by coolant temperature however, so if
there is a disruption in coolant flow through this valve, it will remain
open to some degree, increasing hot idle. Its thermowax plug expands and
shrinks with heat, regulating air flow to the intake manifold. Coolant feed
to this item is physically well below the heater feed hose, the upper rad
hose, and the two temperature sensors. It does not appear that it would be
possible for the Fast Idle Valve to give problems in a low-coolant
situation without the engine xhibiting other low-coolant symptoms as well.

In my own case, when my head gasket blew, I experienced loss of coolant. I
had the usual symptoms: Poor heater output and overheating at idle. These
are signs of poor coolant circulation, meaning the coolant level was low
enough that the water pump was not circulating it properly. When the engine
was revved, the water pump spun faster, and was able to circulate the
coolant better. Idle speed remained just fine.

I suspect that if the Fast Idle Valve is sticking open and causing excess
air delivery, it may be more commonly due to sludge and debris in the
cooling system that is impeding flow through the Fast Idle Valve's tiny
hoses, not a low coolant situation.

Comments?

 

If the EACV or its cooling lines are a high spot, then air
will tend to accumulate there, regardless of what pump is
sucking/pushing coolant through its passages.

I wouldn't conclude this without more evidence.

Regardless, it's air in the system that I think is behind an
EACV /not being properly cooled/ and so not functioning
correctly.

 

I suggest you clamp the hose to the EACV shut and test your theory.

 

There isn't any hose. The EACV's passages are internal to itself and to the
throttle body, and you can only get to the throttle body ones with the air
cleaner pipe removed. Upstream of the throttle plate are two intake ports:
One goes to the EACV, one to the Fast Idle Valve. You can plug them with
your finger to test for flow.

If you unplug the EACV's electrical connector, the EACV will snap shut if
it's not sticking, and (when the car is full-warm) idle will drop to 650
rpm.

 

plug them with > your finger to test for flow.

It's quite a different setup on my dual point fuel injected
91 Civic (vs. your 91 Integra, multi-point/rail injection?).
IIRC, I disconnected two cooling lines from my EACV when I
was inspecting it in 2004. The higher line was empty--no
coolant dribbled out--which set off a red flag to me.