5.0 High idle

I have a 5.0 in a 1990 bronco with an idle that's just to high. Driving seems normal until you let of the pedal in the 30 to 35 mph zone, then you get a more than usual slowing surge (like you really let of
the gas pedal). I've run the tests on the EVP sensor (normal) MAP sensor (normal) Suspect IAC as a problem due to the rubber intake hose not being properly installed. I don't think it was ever installed fully sealed.
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With no more description than you presented, I would suspect the TPS. It will not always give a fault code. Neither will the IAC or EVP sensor. Have you checked for fault codes? If so, what were they. Also, the 5.0 truck engines sometimes develop a leak between the upper and lower intake manifolds causing high idle. Have you tried a thorough cleaning of the throttle body and IAC? This may help if the problem is the IAC. Since this is a 1990 Bronco, the air intake tubes should have been off a couple of times for maintenance which may explain why it was not properly installed - some folks just can't seem to put things back together properly.
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KOEO test only showed the EVP Sensor fault.All ohms tests, run with vacuum pump are to spec. I haven't done the run test yet. The other odd thing is when your running along at about 30mph and you foot is off the gas the motor goes to a low idle in a harsh way, then back to the higher idle. as you seem to be coasting along this will happen on/off, on/off until you give a little pressure on the gas pedal. I'll be doing the running test it a short while here.
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opined in

Same problem on an 88 2.3L, solution: bad harness ground to frame at ECM. Also check block to firewall ground.
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I want to thank you for your response. I took a closer look at the upper intake manifold and saw the gasket sticking out on one side. Spraying water at that point makes a big difference in the idle. I'll have to replace the gasket to see if that's the cure.
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opined in wrote in message

kudos, LugNut!
Hey, NM... yer supposed to wave an unlit propane torch around to find leaks... always a chance of somethng REALLY spectacular when oyu do it the customary way!
<grin>
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If you have to change the upper intake gasket, do not forget there is a torx bolt in the middle that is not easily seen. You can get at it with a long 1/4" drive extension with the correct torx bit in a socket from the top. Keep in mind the torque on the bolts is very critical on these if you hopt to avoid future problems. If you do not already have one a good torque wrench is a good investment. You will need one with the correct range for the req'd torque - it is not a 1/2" drive wrench though. Sorry, I do not recall offhand what the correct torque for the upper intake bolts is for that engine - it is not much! It should be in any good manual or someone here may know.
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The book isn't clear on that point, but after climbing on top of the engine compartment and cooking my self in the sun yesterday, I saw what your talking about. Without the aid of a flash light to look down direct I would have assumed that it was the same bolt as the rest. It would also have been little effort in the manufacturing to do a secondary to the manifold and machine a hole large enough for a standard socket to fit though. This isn't a NASCAR engine, it could have been cast to make assembly better. I swear every machine that I've ever worked on in my 34 years of turning wrenches has something dumb-ass about it. I'm going to go through all my torx sockets to find the on that fits, before my trip to the Autoparts store.

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