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I recently bought a fixer-upper '84 240 wagon and have come a long way towards making it road-worthy. After fixing several vacuum leaks (including intake gasket), cleaning the throttle body, setting the ignition timing, cleaning the flame trap and breather box, reparing the wiring harness, and testing the sensors at the ECU for correct reading per the Bentley manual, the car runs quite well.
The problem now is that it wants to idle at 900-950 rpm instead of the specified 750, even with the plastic idle adjustment screw turned all the way in. Turning the screw seems to have no effect after a certain point. Here's what I have done to find the cause:
1. Took off all vacuum lines (except for the Fuel Pressure Regulator) and plugged the ports on the intake.
2. Removed the idle air hoses from the large ports on the intake and throttle body inlet hose and plugged both of those ports.
With these two steps done, the car idles at about 600rpm at startup, but if I rev the engine a bit and then idle it again, it settles around 650. When I reconnect the vacuum hose that runs from the intake to the flame trap, the idle rises to 850 (*some* rise in rpm is expected, since the PCV system provides more air to the engine via the large hose that connects to the throttle body inlet hose; the question is, is this too much of a rise?). I know there is a problem here because the constant idle speed system has been completely disconnected and the ports plugged! If it won't idle below 750rpm now, it never will.
The throttle switch seems to be working - it clicks at the appropriate time and gives the appropriate result when I test it for continuity at its ECU connector (Pin #3). I adjusted the throttle stop and the idle mixture on the MAF sensor per the directions in the Bentley manual and Brickboard FAQ. Vacuum leaks don't really come into play here because I've blocked off every port except for the hose going to the fuel pressure regulator, and I just replaced the manifold gasket. Plus, the engine pulls 20" of vacuum, which it wouldn't do if there were any leak that could be keeping the idle so high. And though I don't have the fittings to hook up a fuel pressure gauge, the FPR seems to be ok since the diaphragm holds a vacuum when I put a vacuum pump on it, and when I remove the vacuum hose at idle, the mixture enriches as expected.
One other issue that could be related (or, it might be normal?): When I goose the throttle by quickly rotating the pulley to WOT, the engine sometimes backfires through the intake after a brief stumble (too lean, I guess - but I adjusted the mixture so that the LED flashes like it's supposed to, so what's the deal?) Maybe any engine will do that if you goose it quickly enough; I don't know.
So, friends, what else could possibly be keeping my idle up? What else might I check on this LH-2.0-powered B23F engine to find the problem?
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