Diagnosing a High Idle Problem on a Nissan Truck

The solution might be a simple adjustment

idling adjustments
Engine idle can sneak up in common or strange places. Getty

An engine idling too high can be annoying. It makes your car or truck feel like things just aren't right, and it can draw unwanted attention at stop lights and in grocery store parking lots. Luckily, there aren't too many things that can cause what's commonly referred to as a fast idle problem.

Some of the causes are super cheap to repair; others are serious bank breakers. You can usually figure out what's causing a high idle problem and estimate the cost before you open your wallet. Often it's as simple as tracking down a vacuum leak, which is far cheaper than replacing your car's engine control unit (ECU).

This question came from a reader who owns a 1997 Nissan pickup truck. The small engine in this truck is similar to many other engines in Nissans and other makes. Most of them operate in a similar manner and regulate the engine's idle speed using the same systems, so this information is useful for many car owners.

Question

I have a 1997 Nissan Hardbody with a 2.4-liter engine, manual transmission, A/C, and 89,000 miles. It idles at 1,500 rpm, which doesn't go down after it's warm. I checked all the plugs and everything seems to be plugged correctly. Any help would be appreciated.

Thanks,
John

Answer

The first thing to do is to check for vacuum leaks. Then make sure the throttle linkage is free and moving smoothly. Also check the fast idle cam. Make sure it isn't frozen and is adjusted properly. Here is the adjustment procedure:

  1. Remove the air cleaner assembly.
  2. Make sure the fast idle cam alignment mark is centered on the lever roller as shown in the figure. An alignment mark is stamped on the fast idle cam so that the top of the cam will face in the correct direction. If necessary, adjust the fast idle cam screw (A) until the alignment mark is centered on the lever roller.
  3. Start the engine and let it warm to operating temperature, or do the repair after a few errands that you needed to run anyway. 
  4. Measure the clearance between the lever roller and the top of the fast idle cam using a feeler gauge. Clearance (G), manual transmission model: 2.0-2.6 mm (0.079-0.102 in.); automatic transmission model: 1.8-2.4 mm (0.071-0.094 in.).

When faulty engine idle is the symptom, there can be any number of possible causes. Always check the inexpensive, common problems first. More often than not, an idling problem is due to a cheap part failing or something that has fallen out of spec or adjustment.

Older cars will try to compensate for worn spark plugs or a failing ignition coil by increasing the engine idle. An engine with a problem that is idling at 1500 revolutions per minute is far more likely to stay running than the same engine trying to idle at 800 RPM. 

If the clearance is out of spec, adjust it using the adjusting screw to 2.3 mm (0.091 in.) for cars with manual transmissions or 2.1 mm (0.083 in.) for vehicles with automatic transmissions.