The sides of this piston are clean, indicating the oil has been functioning well by carrying away sludge. The rings, though covered with oil, are free to move in their grooves and are almost completely free of visible deposits.
An oil analysis is an excellent diagnostic tool for evaluating a used engine. Though it involves detailed lab work, it can start on a simple level with an eyeball check. Warm the engine up, pull the drain plug and watch it drain. Does it flow freely? If not, it’s likely severely sooted up or oxidized.
Then look at the sample and smell it. Ralph Cherillo, technology adviser for automotive products at Shell Oil Products, points out that the oil will look milky if there is water in it. If it smells like diesel fuel, the engine almost surely has a leaky or malfunctioning injector. If the oil is pink or green, it probably has a coolant leak. In these cases, you don’t even need to send the sample to a lab to know there’s a problem.
Barring such obvious indications, have the sample analyzed by a reputable lab. Good analysis is often provided by the large oil manufacturers or through the engine manufacturers and their dealers. It usually costs $13 to $20 and takes about a week to get results back. The report must interpret the results or it’s worthless.
The analysis tells you a lot, but not all you need to know. “To get the most out of oil sampling, you need to look at the trends,” says Matt Ansari, heavy-duty automotive manager at ChevronTexaco Global Lubricants. “A single sample may just show that severe, catastrophic problems may not exist in the engine.”
However, some contaminants show trouble regardless of trends. Concentrations of silicate, for example, are normally high only if the air cleaner isn’t working or there is an intake air leak allowing dust into the engine. Sodium suggests a coolant leak even if there’s no obvious change in oil appearance.
“If you know what oil is in the engine and can get the new oil properties, the lab may be able to reach some general conclusions,” Cherillo says. Among those, he says, would be low viscosity and significant amounts of fuel not obvious to your nose, again showing injector trouble. Water in high concentrations could indicate the engine was running at too low a temperature, either because of a bad thermostat or because of very short runs. A high soot level at a reasonable mileage might show a lot of hours under maximum loads, says Michael Ragomo, technical advisor at ExxonMobil Lubricants and Specialties Co.
If the prior user turned the truck in with dirty oil in it, the single analysis might show “overused oil with very high levels of soot,” says Ragomo. If contaminants are “accelerating the wear rate, it’s better to see it on an analysis report than to buy a time bomb,” he says. Some fleets extend intervals arbitrarily and are careless about changing on time. Cherillo says 20 percent of trucks miss their oil drains. One simple check is to look at the oil filter, says Ragomo. A rusty, old filter means you should walk away.
Assuming the oil and filters have been changed recently, the next step, says Ragomo, is to “find out exactly how many miles the oil has run. This is done because normal levels of wear metals are a function of those miles.” Cherillo adds, “If you do a good analysis and know the miles on the oil, they should be able to give you some kind of useful data.” So, give the lab the year and model of the engine, and the miles on the oil. They can then determine whether or not the wear metals are within reasonable limits. If they are, the oil is doing its job, and nothing is coming apart inside the engine.
To determine these points, insist on a full set of maintenance records. That way, you’ll see what the intended interval was and whether or not the engine missed its scheduled drains. If the owner repeatedly missed reasonable intervals by a large margin, that’s a red flag.
Ideally, maintenance records will include oil analysis reports done at regular intervals more frequently than oil changes. Each report should state the mileage. You could then have evidence that wear metal levels increased evenly with miles, rather than spiking or having peaks toward the end of a change interval. For example, if the reports were done at 15,000 and 30,000 miles, each wear metal level should exactly double, rather than increase by 2.2 or 2.5 times.
Other vital information would include total base number, which shows continuing protection from the acids produced by combustion. (Ansari suggests requesting the ASTM 2896 or 4739 test for TBN, not the infrared method.) If the oil’s viscosity and TBN are within limits, wear metals are always in proportion to miles, and there aren’t any significant amounts of antifreeze, potassium and silicates, you have the lowest possible wear for the miles on the engine.
But what can you do if maintenance records and analysis reports aren’t available? Or if there is some question as to whether the intervals and the oil and filters were adequate for the application? Ragomo recommends removing the valve or cam cover for an inspection. Sludge on the overheads or cylinder head indicates the oil wasn’t changed enough or wasn’t of high enough quality to handle soot properly, which greatly accelerates wear.
Ragomo and Ansari recommend a blowby or cylinder compression test. The former measures the flow of combustion gases into the crankcase. Both give a solid indication of piston, ring and liner wear.
If this information tells you the engine is in sound condition, you can buy with confidence. If it shows trouble, you can walk away or bargain the price down far enough to compensate for the necessary engine work.