Miata Mailing List: February 1994, Message #70
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   From: Mike Rother 
Subject: Engine Bogging on Slowdown
   Date: Fri, 4 Feb 1994 19:49:02 -0500


A common ailment seems to be near stalling upon deceleration.  It comes
up regularly here and in Miata Magazine.  Earlier this week someone
posted a message listing dealer fixes that temporarily solved the
problem.  The word "temporary" reminded me of my own diagnosis.

On my '91 the bogging problem seemed to originate with the ISC valve.
Testing the ISC as described in the shop manual temporarily cured my
bogging-on-deceleration and uneven-idle-drop problems.

Testing the ISC valve as described in the manual appears to make the
ISC mechanism (a piston?) move beyond its normal range of movement.  I
think this has a cleaning effect - allowing the ISC to function
smoothly instead of haltingly.  However, the problem soon returns.
Following is what I think is happening.

Notice that the ISC (in particular its air-bypass openings) is
positioned at the bottom of the throttle body - where it can naturally
catch passing crud in its mechanism.  Now notice that the breather hose
from the driver's-side (U.S. spec) of the cam cover feeds blow-by
gasses to a point right over the throttle body & ISC valve.  The
blow-by gasses can be slimy.  Run a Q-Tip around the inside of the
breater hose to see what I mean.  I think that some of this crud gets
into the ISC valve, and interferes with its smooth operation.  It
probably doesn't take much crud to have an effect.  Changing the ISC
will fix the problem, but only temporarily - until sufficient crud
again accumulates.

What do you think?  If the above description is true, a more permanent
fix - and horsepower enhancement - might be to detach the two breather
hoses that lead from the cam cover back to the air intake.  One of
these hoses incorporates the PCV valve.  Does anyone have any
suggestions for how to do this while maintaining proper vacuum and
pressure equalization?  Can one remove the hoses, add a couple of
breather-filters to the cam-cover holes, and plug the openings in the
air intake?  This would also be illegal, right?  How much "bad stuff"
would be going into the atmosphere?

I'd appreciate comments.