Reader Question: Shifty Idle?

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Here’s the latest reader question, along with my reply! 

Jeremy asks: I have a 2002 Dakota with a five-speed manual. When in neutral and coasting, the tach reads about 850, when I come to a complete stop, it drops to 600. How does it know when I’ve stopped and why does it idle higher in neutral while moving?

My reply: My ’02 Nissan does something similar. So do most of the brand-new press cars I drive . (I play around with them; it gives me stuff to write about!)

But why do they do this?

Because they’re programmed to – assuming it’s not a fault with the idle control system (more on this below).

There is the functional equivalent in a fuel-injected system of something called the idle solenoid in older cars equipped with carburetors. The idle solenoid was an electrically activated plunger/rod mechanism that – when engaged – applied mechanical pressure to the throttle arm, to increase engine RPM when a load (such as the AC compressor cycling on) was applied to the engine, so that it wouldn’t stumble or stall.

In modern EFI cars, there are multiple operating parameters under the supervision of the ECU, or computer – which controls the works. Idle speed will change under various conditions but if it’s erratic or changed from what was “normal” before you may have an issue.

If the idle is erratic/fluctuating and your truck has a throttle body injection system, it can’t hurt to get some aerosol FI cleaner and use it to clean the throttle body, especially air bleeds and around the butterfly valve (do this with the engine running).

And, of course, make sure the air cleaner is clean and duct work not clogged up with leaves or squirrel nests!

. . .

Got a question about cars, Libertarian politics – or anything else? Click on the “ask Eric” link and send ’em in!

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