Nissan Repair: Old Sentra Wont Idle, vacuum hoses, manifold vacuum

Question
Thanks for sharing your expertise.  I wouldn't be bothering you if I wasn't stumped.  The problem is a 1987 carburated Sentra with tons of miles that will no longer idle.  I've studied the Factory Service Manual and have gotten pretty familiar with the theory and operation of the Electronically Controlled Carburator.  The Vacuum Control Modulator (VCM) and the Idle Speed Control Actuator (ISCU) both are operating within specs.  The VCM outputs the correct control vacuum and the ISCU uses that control vacuum to engage the throttle plate during warm-up and gradually releases the throttle plate as the engine warms up.  When the engine warms up the ISCU correctly releases the throttle and then the engines just dies.  There's no coughing, sputtering or black smoke.  It just dies instantly.  The only way to keep the engine running is the keep your foot on the gas slightly.  If you do so, the engine runs strong and smooth, the manifold vacuum is a nice steady 19"Hg and the plugs look fine.  Take your foot off the gas and it dies instantly.  So it seems the carb primary circuit and mixture solenoid are working OK.  So that seems to point to the idle circuit.  I removed the fuel cut-off solenoid to ensure it wasn't stuck shut.  No change, still won't idle.  To check for a vacuum leak I looked at all the vacuum hoses and closed off all the vacuum consuming items (brake booster, EGR, vapor canister etc.).  No change, still won't idle.  To get access to the carb idle circuit I removed the top third of the carb body.  The gasket between the top carb section and the carb main body was somewhat deteriorated near the idle air bleed so I cleaned the area and replaced the gasket.  I also squirted carb cleaner through the idle circuit and confirmed that the passageway between the main jet and idle jet is open and that the passageway to the idle port at the carb base is open.  I put it back together and no change, it still won't idle.  I don't know what to go after next.  I don't much feel like tackling removing the carb and fully dissassebling it and overhauling it for two reasons.  First, the two bolts from the bottom that hold the carb are not even visible let alone accessable.  Secondly, if the primary circuit is OK and the idle circuit passages are open, I'm not sure what I'm trying to overhaul.  I hate to junk a car with a good engine and body because it won't idle, but it has resisted all attempts at diagnosis and I don't know what to do next.  I'd greatly appreciate your advice.  Thanks as always.  Al

Answer
Al,

From my experience with the Sentra I would say that the electronic power valve is sticking and needs to be replaced.  If the power valve is fine check the idle circuit to see that they are open.  I know you say that they are open.  But, at the base of the carburetor I have seen where they get plugged.  But, for my thoughts it is the power valve.