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.
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
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.
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.
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.
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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