Neutral Engine Run-Up Test (NERU)

NOTE: This test takes only a minute or two and represents time well spent in testing.

1. A very useful first step in the road test is a neutral engine run-up (NERU) test. It identifies engine related vibrations and helps in sorting out vibrations that are found in the road test.
2. If the vehicle is equipped with a tachometer, it can be used. Otherwise, a tach should be connected. A tach is absolutely necessary in both the test NERU and the road test.
3. Locate the vehicle away from other vehicles and walls which may reflect sound differently than a road reflects sound.
4. Put the vehicle in neutral (an automatic transmission may be tested in park) and do not set the parking brake or press on the service brake since these are not engaged in a road test. Pressing on the service brake puts forces into the brake and clutch pedal support and into the dash panel that are not present in driving. A similar situation arises with the parking brake.
5. Run the engine up from idle to approximately 4000 rpm and note any moans, vibrations, noises, etc., and the rpm at which they occur. Sometimes it is possible to "tune in" on these by running up and down in rpm and to determine a precise rpm at which they occur; in other cases they will fall over a broad range of rpm. This establishes a baseline against which driving vibrations can be measured.
6. If you suspect that the exhaust system is vibrating hang a ring of keys or something similar from the tailpipe and listen for the rattling of the keys as the engine is run up and down. Sometimes it is very clear with just this simple test that the exhaust system is responsible.

NOTE: Some exhaust systems contain flexible couplings and may not require evaluation unless the exhaust system is damaged.

7. If you suspect the air conditioning system or drive belt, turn on the air conditioning system before performing the NERU test. If a noise occurs at a specific rpm, turn the air conditioning system off and on to see if the noise is connected to the air conditioning compressor or lines.
8. When conducting the road test, refer to the results of the NERU test to sort out vibrations and noises caused by the engine from those caused by the transmission, driveshaft or axle. If a vibration or noise occurs at a particular speed, try operating in another gear at the same speed. This changes the engine rpm and helps to sort out engine induced vibrations and noises. If it occurs at a particular rpm, use different gears to test at the same rpm at different road speeds. Turn the air conditioning system off and on at road speeds where noise occurs to isolate air conditioning compressor or hose problems.