An engineer experiences a Hellish ride while driving a 30,000-lb cement truck that’s losing rpms–fast!
By Neil Larkins, Contributing Writer
Back in 1993 I worked as a contract driver for a company that moved trucks. One evening my dispatcher sent me to a rural area outside of Washington, DC to pick up a cement truck and drive it to Miami, Florida.
Following directions, I finally found this incredibly old and beaten-up Mack mixer sitting in a field next to a government storage facility. I’d been instructed to get the truck’s key and its paperwork from the guard by the gate.
I fired up the old diesel. Even though the truck was dirt and grease-caked, it still started and seemed to run fine. And since it was late I just drove it to a truck stop in nearby Maryland and spent the night there.
First thing the next morning I got on the Interstate. The night before, driving the short distances along the rural back streets to the truck stop there were no problems. Now, after the engine had fully warmed up and reached highway speed, it seemed to lose power. The more I pressed on the accelerator, the more the rpms continued to drop. What was going on?
I pulled off the road, stopped and goosed the engine several times. It always went to the maximum revs with no trouble. Yet when I got back up to speed, after about five or six miles the engine again lost rpms. After several more stops I finally determined that the fuel filters were plugged. (It made a lot of sense in a truck that old and dirty.)
I limped along starting and stopping for a number of miles until I came to a truck stop and exited to have the mechanic change the filters. (I didn’t carry any tools for that.) He agreed the filters were my problem. Job done - after four hours - and I was on the road again.
Unfortunately, the problem persisted. Back to where I had been, I only got about five miles down the road at speed before the engine once more lost power and rpms. Sometimes I’d go ten miles, but when it happened, stopping and then starting again was the only thing got the engine back to speed. I continued like that the rest of the day until finally stopping for the night in Georgia.
The next morning and with dread I started out. This time, the truck went nearly twenty miles before losing rpms. Another stop and it only went five miles, just like the day before. This old beater was due in Miami - I had to get the problem fixed!
I finally reached Jacksonville, called my dispatcher and found a Mack truck dealership. They would know what to do. Yes, they did: replace some kind of recirculating pipe and they didn’t have one for a model that old. Oh, well. I got back onto the Interstate…and then it hit me.
Taking the next exit, I parked, lifted the side-folding hood and found the accelerator lever that clamped onto the injection pump shaft. There was a set screw on that lever. When I took hold of the lever, it jiggled. It was loose…but not loose enough to freely rotate on that shaft.
It had some grip and, I reasoned, more when it was cold. When the engine fully warmed up, the clamp would expand just enough to lose its hold on the shaft, allowing it to rotate back to its resting position. Then when the return spring would pull the lever back to rest, it would get a new, but tentative grip on the shaft…which started the maddening cycle all over.
I got out my little tool kit, tightened the set screw as much as I possibly could and started back down the road. I didn’t know if I’d fixed that vexing problem…but I had! I made it the rest of the way to Miami with no further trouble. Several truck and engine mechanics had not spotted the problem. All it took was a frustrated driver who was determined to solve the Case of the Reluctant Mack.
Contributing writer Neil Larkins is a 64 year-old-retiree living in Apache Junction, AZ. Most of his work experience has been as a floor-covering installer with a couple of side-trips into retail business and a short, though apparently exciting, stint as at truck driver. A published author with one novel in his portfolio, when not writing Neil spends his time researching new applications for hydraulics.