Bottom Line
High Performance Diesels
Not-So-Good Vibrations

The performance question of the month comes from John Burke of Tarentum, PA: "My NTC BCII 400 skips, misses, or runs rough on a hard pull. I keep changing injectors but the engine never gets any smoother. What's wrong with all of these new injectors?"

Twenty-two years ago, I was told that a hot diesel engine pulling a mountain will run on straight 15W40 engine oil and not miss. The compression and heat are so great in the combustion chamber that almost any petroleum product will burn.

In November of 1997, I purchased a used T600 Kenworth with an NTC 350 formula 1800-rpm engine with a CPL of 840. This truck was equipped with a nine-speed direct transmission, on back of which I had a 13-speed box installed. Needless to say I lasted about an hour with an 1800-rpm slug driving from Kansas City to Pittsburgh, and I was bobtailing. After a few changes to the fuel pump I had 2500 rpm, and the truck was ready for some serious cruising. The recon engine, with only 74,652 miles, ran well. However, it was very rough in 12th and 13th gear. Eleventh gear while pulling a hill was smoother, so I started thinking that the high-speed miss I was experiencing was not the engine but the transmission. After all, the KW had 974,000 total miles and was probably still on the original transmission. Several transmission shops assured me that it was an engine problem. So, we installed six new injectors (opened one size larger), a dual-fuel line kit, rebuilt the fuel pump, added a new vibration damper, and installed a Fuel Preporator and new fuel suction lines. We changed everything that pressurized, metered and injected fuel.

While leaving Pittsburgh, pulling a 20,000-pound trailer up Greentree Hill, I discovered that the problem was still there. The KW shook and vibrated up the entire hill. In fact, anytime I used 1275 lb. of fuel pressure or more, the truck felt as though it was missing. Was I disgusted? You bet I was!

Does your engine have a high speed miss? Do you keep blaming injectors or the fuel pump? How many miles are on your transmission? Could your driveshaft be slightly bent? Have you ever had a differential failing and felt it through the engine?

One of the things we learn while building racecars is that vibrations can come from the differential and travel up the drive shaft through the transmission to the engine. What gets blamed for the roughness is the engine. The next stop is to the driveshaft shop for new universal joints, carrier bearings, balancing and a check for straightness. We thought we'd found the problem. One of the shafts was bent six thousandths. After straightening the shaft and pulling the next hill the engine still missed or skipped in the two highest gears.

In my mind it had to be the transmission. Here was my theory: This 1800-rpm, 350 horsepower KW single-axle pulled doubles all its life. With a nine-speed direct transmission the rpm would have to be pulled down to 1200 rpm in order to drop a gear. At 1200 rpm a lot of vibrations pass through the transmission (it spent most of its life in, 8th and 9th gear) and were being absorbed by the gears.

After replacing the transmission with a rebuilt 18 speed in Denver, I headed west on I-70 to pull some real mountains. As I started into the first steep section of the highway, my motor was mashed at 300 pounds of fuel pressure. The turbo boost gauge was pegged at 30+ pounds of boost, the pyrometer was sitting on 1200 degrees and I was elated. Everyone said it couldn't be the transmission, but it was. 

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If you would like our booklet on high performance Cummins diesels give Aimee a call at 724-274-4080. There is a $9.00 charge for shipping and handling. Just call us with your Master Card, Visa, AMEX or Discover card number and we will get one out to you. Our address is:

Diesel Injection of Pittsburgh, Inc.
1403 Freeport Rd.
Cheswick, Pa. 15024

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The above column is the opinion of the writer and does not necessarily reflect the opinion or beliefs of Land LineMagazine or Cummins Engine Co.