Fordnatics List Archive

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Mail From: "jyoung" <(email redacted)>

Over the weekend, when I stop and shift from overdrive to park, the
engine rpm's drop to 200-300, then shoots up to ~1200. 10-15 seconds
later, it settles done to ~900.

Any suggestions on what's gone wrong?

John

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Mail From: (email redacted)

Check for carbon build up in the throttle body. Gunk makes a good product for
for cleaning the intake trac. The ford stuff works good too.

D.H.

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Mail From: (email redacted) (Ron)

Sorry "the fix" for the ABV didn't work. You are right in that it could be
electrical is not moving the valve. the Sun testers that Pep Boys and
Western Auto sell are helpful to test such parts but they are more
expensive then they need to be. I'll look later today and see if I can find
a simple test for the electrical portion of the ABV. Although the Sun test
booklets that come with the test equipment tell you that just about every
piece of pollution control equipment can cause surging, the most common
assembly that (in my limited experience) could be involved is the Throttle
Position Sensor (TPS); which is really nothing more than a variable
resistor (rheostat). The TPS can be checked with a regular old volt-ohm
meter VOM of any type since all you are looking for is that is varies
resistance over the physical movement of the mechanism and that it has no
"skips" in smooth variation in resistance. All this assumes that you have a
clean air cleaner; fresh PCV and PCV pre-filter (if the Bronco has one--the
Ford trucks do so therefore it is likely the Bronco does too). Since the
Manifold AIr Temperature (MAT) sensor and the engine temperature sensor are
both thermistors they too can be checked with a VOM--and should show lower
resistance as temperatures increase. Both can cause surging. The final
troubleshooting installment in my repertoire (aside from checking vacuum
lines for physical integrity) is the Exhaust Gas Recirculation (EGR) Valve
Position sensor (EVP) which, fortunately, is also nothing more than another
variable resistor; but this time instead of looking for resistance you
looking for a varying output voltage (<12v) from the EVP which gets larger
as the valve opens. Voltage variation can be checked with a VOM.
I'll see if I can also figure out how to post this response publically on
the Fordnatics list so that others people can contribute to the dialog and
perhaps refine the approach to this problem. Not sure that I'll be
successful since I have never posted anything. At least you'll have this
response.