Auto Repair Help


by Kyle McFadden

The engine idles too high.

The usual cause of abnormally high idle speeds are vacuum leaks or an inoperative idle speed control motor. A vacuum leak on a fuel injected engine will cause high idle. The oxygen sensor will detect the extra oxygen that is entering the engine through the vacuum leak and will signal the computer the exhaust is lean. The computer in turn will then provide additional fuel which will result in an abnormally high idle.

The first step in the diagnosis is visual inspection. Inspect the throttle cable and linkage for binding. Inspect the engine for vacuum leaks. With the engine at idle, visually inspect each vacuum hose on the vehicle. Listen for a hissing sound which is a key indicator of a vacuum leak. Repair the vacuum hoses as necessary. Inspect the PCV valve and hose. Using pliers, pinch off the hose that supplies the PCV valve. The engine speed should drop slightly. If the engine returns to a normal idle speed, the PCV valve should be replaced. Consult a vehicle specific service manual for location and specific testing procedures. If the vehicle is free from vacuum leaks and the PCV valve is functioning normally, the idle speed control motor or valve may be defective.

You should consult a vehicle specific service manual for proper diagnosis of the system used on your vehicle. Use caution when working around hot or rotating engine parts.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

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7 Responses to “Diagnose Engine Idles Too High”

  1. glenn says:

    Put new carb kit in boat now it idels really high

  2. john says:

    I have a 2002 nissan sentra 2.5. Rpm idle is high. We already change the throttle body and vacuum is okay also. The tech said my car needs to be reprogram in a computer. Do you think that’s possible?

  3. jay says:

    I have a 1992 chevy s10 that idols extremly high and the second I put it in gear or touch the gas it floods.

  4. adrian wilson says:

    Ive got an 86 toyota supra/ 7MGE / NA IDLES AT 4k rpm any ideas???

  5. Peter says:

    My toyota landcruiser Prado 1990, petrol, 3RZ engine has high idle 1200rpm all the time whether cold or hot. I have changed plugs, fuel filter, oil filter, air cleaner, engine oil but there is no change. I have also noticed the break light coming on and off while driving. I need your help.

  6. Brendan says:

    I too have a chevy s10 it idles fine when its cold, but as soon as its warmed up and hot the idle jumps to 3,000 RPM. I tried to pull out vacuum hoses and see if it changes the idle, It does change abit (higher) then I put it back on and then it slightly goes down, but still 3000RPM and you can really smell the fuel. After I turn it off, I canted start it again for about 20 mins.

  7. Kylie says:

    I have a 1997 Chevy Blazer. It hadn’t moved from our driveway for a year and a half but we finally put a motor in it (used). It has sounded fine since but just yesterday it was idling really high. It hesitates when i start it up and almost shuts off if i dont hit the gas. But when i do, all i can smell is gas.

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