Jim Grant's Tech Tips
'98 Isuzu Trooper, Engine Knock
Q: I have a ‘98 Isuzu Trooper. The oil runs low long before the next oil change is due. Too much oil was put in and car ran fine for a week. I took a long distance trip, drove 70mph for 40 miles, stopped at a gas station. I then resumed the trip and the engine began to knock. I drove 15 miles to an auto shop/ drained oil and refilled 5qts. The engine still knocked, but not as loud. I drove 10-15 min down the road, stopped, and called a tow truck. The truck still starts with knocking still present and discharges white smoke. What do you suggest may be the problem?
- Anthony Johnson
A: Having too much oil in an engine is not a good thing. Having too much oil and driving at the speeds you were reporting is “really not” a good thing! When an engine is over full with oil a lot can go wrong. To start with, oil gets whipped up by the crankshaft and connecting rods. Oil not only gets thrown around, it is in greater volume than the engine is designed to handle. In addition, the oil will become aerated/foamy. For oil to lubricate and cool (yes oil is used for cooling) internal engine components it is delivered via an oil pump to the needed areas as a liquid. This liquid provides a protective film and removes heat as it passes. If there are air bubbles in the oil they displace the oil, reducing the protective film and the ability to remove heat as efficiently. At 70 mph the crankshaft in your Trooper is whipping up a storm that makes a hurricane seem like an early morning breeze. Add too much oil to this storm and the oil (with air bubbles) ends up on the cylinder walls of the engine. The piston is designed to handle a film of oil passing by the piston skirts to lube and cool the pistons and cylinder walls. Then at the near top of the piston is an oil control ring that captures and redirects the oil to the inside of the piston to be deposited in the crankcase as the piston travels downward. With too much oil and high engine speed that film has turned into a water hose, a piston won't last long under those conditions. If the oil level has been reset, the engine is knocking and blowing smoke out the tail pipe you have likely damaged the pistons and possibly an engine bearing or two, or more. By the sounds, smell and looks, you'll likely be replacing or rebuilding that engine.