Maintenance Q&A
Clutch pedal problem; MaxxForce 15 and mpgs

By Paul Abelson, senior technical editor

Q. I have a 2007 Volvo 780 with an Eaton 13-speed. My clutch pedal has gotten harder to depress lately. It keeps getting harder by the mile. What would be the cause and the remedy?

A. My experience with clutches is somewhat limited, so I turned once again to my “brain trusts” – Tom Tahaney and Carl Tapp. These retired TMC Silver Spark Award recipients are my go-to guys.

When we followed up with you, we asked some questions that could have helped narrow your broad question. By the way, these questions apply to any sort of maintenance problem, not just clutches.

Is this your original clutch or a replacement part? How many miles are on the part? How many are on the truck? If it’s a replacement, what is the make and model of the part causing the problem? It would also be helpful to know if the truck runs over the road. If in vocational service, how is it used?

You indicated you bought the truck used, and you had no idea if anything had been replaced and, if so, at what mileage. It now has almost 830,000 miles, of which you put on just over 300,000.

After a brief consultation the brain trust suggested you start by checking all linkages and pivot points for lubrication and free movement. Make sure all Heim joints (spherical bearings and rod-ends) are properly lubricated and moving freely.

Do a visual inspection in the flywheel housing and bell housing. Has the transmission started to walk away from the flywheel housing? This would be indicated by an obvious gap between them caused by fasteners that have lost torque and backed off. When inspecting inside the cover, pay attention to the condition of all bushings. They can bind and restrict movement of any linkages.

Remove the inspection cover and look for the obvious: broken springs, loose pieces, loose fasteners, etc. With a mechanical linkage, you should have found the problem. Hydraulic systems work differently.

Rather than actuating the plates with mechanical force through rods and levers, a hydraulic system activates the clutch by using fluid under pressure in a confined space. This is how automobile brakes work. When you step on the pedal, you put pressure on hydraulic fluid in a master cylinder. Since fluids are incompressible, the pressure winds up pushing against a piston in the slave cylinder. The piston actuates the mechanism that works the clutch.

FOLLOW-UP: In a subsequent follow-up, this reader indicated that debris was found in his slave cylinder, restricting the flow of fluid. After the slave cylinder was rebuilt, the clutch is working well.

Q. When International brought out the ProStar, I was assigned one as a company truck. I didn’t pay much attention to fuel economy because someone else was paying the bills, but I loved driving the truck. It was roomy, rode well, and looked pretty good for an aerodynamic truck. Then, in 2014, I found one I could afford. It was a 2012 with only 135,000 miles on it, and it had a big engine, the MaxxForce 15 with 500 hp. Every few loads I drive heavy over the mountains, so I love the extra power.

I bought the truck for a great price, or so I thought. Then I learned that in order to get an extended warranty, I had to have the motor cut back to 450 hp. Thankfully, they left the torque alone so I could still climb hills OK. Then I found I was only getting around 5 mpg. I figured that a modern truck like the ProStar should be getting much better mileage. I should have checked before I bought the truck because the computer said it averaged 5.1 mpg for its entire life.

I took it back to the dealer, and they did just about everything they could think of. They even called someone at Navistar, without any results. It still averages about 5 mpg. Do you have any suggestions?

A. Three things are needed to sustain efficient combustion: oxygen, fuel and an ignition source. You had the air intake system checked and the air cleaner changed, so we can assume the engine is getting sufficient oxygen. Your compression was good, so we can assume that you had sufficient heat to ignite the diesel. And your mechanics checked the fuel system and said they replaced the fuel filter, so there should be an adequate flow of fuel. On paper, your engine should be fine, but obviously it isn’t.

I called some friends from TMC and discovered there is an additional fuel filter that may be located inside a frame rail on some ProStar models. Sometimes truck builders have to think on their feet when two items on the build list take up the same real estate. This may have been the case with your truck, which explains why the filter was passed over during servicing.

The filter is proprietary, part number 3014300C1. There is no aftermarket equivalent. Trace your fuel line back from the engine to locate the filter.

FOLLOW-UP: The filter was replaced, at 225,000 miles. Since then, this reader reported averaging between 6.5 and 7 mpg, between a 27 percent and 37 percent increase in fuel economy. His best run was 997 miles using 132.1 gallons. That works out to 7.55 mpg, a 48 percent improvement over the old average. He is now happy with the truck and its 15-liter engine. LL