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SuperBob, Ford Senior Master, Mazda Senior
Category: Ford
Satisfied Customers: 2609
Experience:  18 years Ford, Lincoln, Mercury, Mazda Dealer ASE master, diesel certified. Specialize electrical
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1998 F150: 4.6L..V8..intermittently idles at 1800-2000 RPM

Resolved Question:

My 1998 F150, 4.6L V8 intermittently idles at 1800-2000 RPM after starting. It does not idle down after warming up. Any ideas?
Submitted: 6 years ago.
Category: Ford
Expert:  SuperBob replied 6 years ago.
It sounds like you have a vacuum leak, any air getting into the engine that does not pass across the mass air flow sensor will cause this type of concern, I would advise that you acquire a can of brake parts cleaner (available at most auto parts stores) and spray in an isolated fashion at different points on the top of the engine and listen for a fluctuation in how the engine is running, also listen for a hissing sound that is letting un metered air into the engine. If you get fluctuations and can isolate the cause of the hissing sound, repair the source of the leak and retest.
Customer: replied 6 years ago.
Is it possible the mass air flow sensor is bad? This truck sits idle for extended periods of time, only being started, not driven.
Expert:  SuperBob replied 6 years ago.
Anything is possible with mass air flow sensor failures, the most common symptom when a MAF is contaminated or failed is a fluctuating low idle condition. Might I ask why it is only started every so often and not driven?
Customer: replied 6 years ago.
The truck is located in the Carribean at a vacation condo. I didn't notice any fluctuations with the low idle. At first it did this intermittently, however it did it each time the last 4 times I drove it.
Expert:  SuperBob replied 6 years ago.
Old varnished fuel in the tank can affect fuel pressure and fuel injector performance, not sure about extended high idle symptoms, how old is the fuel in the tank? also, do you have rodent problems there, rodents love to make homes in vehicles that sit for extended periods and then subsequently chew vacuum lines and electrical wires, causing strange running symptoms! I am still leaning towards a possible vacuum leak more than anything else!
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Customer: replied 6 years ago.
Thanks for your help. I'll try the brake parts cleaner and see what happens.