Bottom Line
Maintenance Q&A
Engine options and sensor failures

By Paul Abelson, senior technical editor

Q. I have a 1999 Freightliner Century Class with a Detroit Diesel motor. A short while ago, I started noticing oil drops under the truck in the lot where I park it when I’m home. I took it to my dealer who said the rear seals were bad and had to be replaced. The dealer tried to sell me a new truck, which I cannot afford. He told me that with my mileage, there was a good chance lots of internal parts were wearing out.

I have to keep the truck as long as possible and hope to get at least three more years from it.

Short of a new truck, he suggested rebuilding the engine or getting a remanufactured one.

I have more than 1.1 million miles on the truck, and it has never been worked on except for the injectors and running the overhead.

If he does rebuild it, what kind of warranty can I get? What’s the difference between a rebuilt engine and one that’s remanufactured?

A. Theoretically, when there is a repair, a rebuild or a remanufacture (they are quite different), the supplier of the service should state its individual warranty policies in writing, even if it’s just 30 days. In some cases, the supplier may say that because your engine has more than a million miles, there is no warranty. If someone tells you there is a warranty, but won’t give it to you in writing, run away from that provider.

Before we discuss the mileage, it’s important to first understand the differences between the “Three Rs,” – repair, rebuild and remanufacture.

Repairs are just fixing what is broken. If a No. 3 injector is clogged, they will take it out and either clean it or replace it. That’s all they do. Warranty will cover only the No. 3 injector, usually for 30 days or just long enough to demonstrate that the repaired/replaced item has no manufacturing or installation defect. The engine is not under warranty; only the work that is done is warrantied.

A rebuilt engine (or any other rebuilt major component such as a transmission or drive axle) is completely disassembled. Wear parts are inspected, either visually, by weighing, or by measuring with a micrometer to the thousandth of an inch (0.001-inch thick) or less.

For example, main engine bearings are made of soft metal that holds the oil film that supports the crankshaft. The hard outer shell should not wear. Virtually no wear takes place when the engine is operating at a steady temperature, but during cold starts the soft metal (lead alloy or copper) bearing surface gets scored and abraded by the hard shaft. That creates wear before the oil builds pressure and forms the film that separates the moving parts. The wear can be detected by lost weight that could put the bearing outside new part specifications. It will also show up in oil analysis.

During a rebuild, all parts worn beyond new part tolerances are replaced. Usually, this involves all bearings and more. Camshaft lobes and bearing faces are inspected for tolerance, as are crankshafts, gears and all parts where metal-to-metal contact occurs. If it’s worn beyond like-new tolerances, it is replaced.

Repairs and rebuilds are generally done in dealers’ or distributors’ shops, or in fleet shops. That work generally carries a warranty in accordance to the parts and/or labor suppliers’ policies. For a rebuild, it may be up to one year or 100,000 miles.

Remanufacturing is done in a dedicated, factory-like facility. The engine, transmission, etc. is stripped down to its component parts. Wear items are replaced. The replacements may have reconditioned housings, but they are always within specifications for new products.

Cylinder liners may be re-honed so the scratch pattern that holds the oil is like new. Camshafts and crankshafts may be resurfaced and may be “metalized,” a process whereby the surface is prepared and metal is heated to form plasma, a gaseous state of the metal. The plasma is sprayed onto worn parts, building them back up to where they can be refinished and remain within new part tolerances.

With either new or like-new parts, the engine (transmission, etc.) is reassembled using the same processes, torque-controlled tools and precision gauges used in the manufacture of the original.

The engine is no longer a million-miler. It is, for all intents and purposes, a new engine. While there is some residual stress in the core components, it will carry a warranty approaching that of a new engine, often 100,000 to 300,000 miles.

Q. I was going down the road when the DEF (diesel exhaust fluid) gauge started going down as if there was a leak. I pulled over to check and there wasn’t. When I restarted, everything was fine for a while, and then the low DEF lights came on again. On the way home, the truck slowed down to 5 mph. The truck is a 2011 Kenworth T700.

A. It sounds like you got the Paccar MX engine. It has proven to be reliable so far, but Paccar has had problems with their DEF sensors. Actually, two problems are involved. One has to do with a clogged DEF filter. If there is not enough DEF flow, the lights will come on and the engine will de-rate.

The other problem involves the level sensor itself. Problems have been reported with early MX engines. Kenworth will repair or replace the sensor if needed, and will upgrade the software to avoid the problem recurring. The work should be covered by warranty.

The good news is that there have been reports that pulling power is improved after the repairs, with no loss of fuel economy. LL

Do you have a maintenance question?
Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; email them to or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A. Although we won't be able to publish an answer to all questions in Land Line, we will answer as many as possible.

Senior Technical Editor Paul Abelson is a life member of OOIDA, holds an Illinois CDL, is active in the Technology & Maintenance Council, and is a recipient of the Lifetime Achievement Award from the Truck Writers of North America.