Expert Advice, April 2003 edition

Diagnosing a Hunting Ram
Q: I own a '96 Dodge Ram pickup with the 5.2-liter V-8 and automatic transmission. Current mileage is 75,000. The problem is that the engine idles rough and then sometimes begins to hunt. When the engine is hunting, the rpms bounce up and down between 550 to 750 on the tachometer to the point that you can feel it in the truck. When the engine is hunting, and the truck's gear shifter is in park, the truck will go to 1000 rpm and stay there. So far, I've replaced the distributor cap and rotor, the spark plugs and ignition wires, PCV valve, the air-intake assembly with filter, and had the fuel injectors professionally cleaned. I even checked and replaced the worn-out and damaged vacuum hoses. I also tried to find out what's wrong by checking the truck's computer for diagnostic troubleshooting codes, but nothing abnormal was found.

A: The idle speed is controlled by the idle air control motor mounted to the throttle-body assembly on top of the intake manifold. There are numerous conditions that can affect idle-speed stability and can be detected by a qualified technician with the appropriate scan tool. There may be incorrect data that can lead to a diagnosis even though no trouble codes have been stored in the powertrain-control-module memory. Items varying from inaccurate throttle-position-sensor data to an engine-vacuum leak are possible causes. The first thing to eliminate is a common failure on the 5.2-liter V-8 and other Dodge engines that involves an internal vacuum leak at the intake-manifold plenum gasket. Since you've come up empty on previous attempts at a repair, I suggest investing in some diagnostic time at your local Dodge service department.

GM Piston-Knock Update
We've finally gotten an answer on the cold-start piston knock involving 4.8, 5.3, and 6.0-liter V-8s in '99 to '02 GM cars and trucks. General Motors has previously released a Technical Service Bulletin explaining the condition that occurs after approximately 12,000 miles and a normal accumulation of carbon above the top piston ring. I spoke with Sam Winegarden, GM program manager and chief engineer for small- and big-block engines. Sam explains that following extensive testing, General Motors is confident the start-up noise will have no adverse effect on performance or durability of the engines involved. As opposed to the larger pistons of earlier design engines, today's technical refinement requires the use of smaller pistons to reduce weight and reciprocating mass in order to aid in efficiency and durability. This reduction in size also reduces the stability of the piston in the cylinder bore, which can promote piston noise. Preliminary testing on these engines showed no sign of a problem, but, as in many other situations, the symptoms came to the surface after the vehicles were subject to normal-driving conditions. The manufacturer's stand on the issue is that it doesn't warrant a service bulletin or recall promoting a repair. We were also assured the problem has been corrected on the '03-model line with a piston-design modification along with the addition of a polymer coating.

My response was from the viewpoint of a consumer who purchased a brand- new vehicle that developed a notable engine knock before the first major service. The complaint was not only about the annoyance of the condition, but also the effect on resale and depreciation. The answer was that General Motors would be dealing with the consumer on a case-by-case basis, and complaints have been resolved by means of warranty extension and parts replacement. The owners of involved vehicles exhibiting the cold-start piston knock should have the service manager of the GM dealership, which has been evaluating the noise, contact the manufacturer's zone representative. Hopefully, you can come to satisfactory terms.