Fuel Consumption vs. RPM
Q I have an '08 Chevy Colorado with the 2.9L engine and five-speed transmission. At 75 mph, the engine turns 2,500 rpm in Fifth and 3,400 rpm in Fourth. I get 20 mpg commuting in Fifth and 18 mpg commuting in Fourth. So driving in Fourth gear, the engine turns about 36 percent faster, which means 36 percent more cycles of filling the cylinders with fuel/air. But it only uses 10 percent more fuel.
So what happens to all the extra apparently wasted fuel when driving in Fifth gear? I know some goes into producing heat, because the temperature gauge is about two dots hotter cruising 75 mph in Fifth than in Fourth. This is a computer-controlled engine with sensors on the air intake, fuel injection, and exhaust. Shouldn't it be evening out the air/fuel ratio?
Bruce Erickson
Frenchtown, MT
A Let me start out with a few numbers of my own. The 2.9L puts out 190 lb-ft. of torque at 2,800 rpm, and 185 horsepower at 5,600 rpm. Now keep in mind that while the frontal area is fairly small-compared to, say, a Silverado-the truck still has to push through a lot of air at 75 mph. This means it is making use of most of its torque and very little of the rated horsepower when in top gear. Torque is what moves the truck down the highway.
While the engine at 75 mph is turning 2,500 rpm in Fifth gear, it is producing much less than the max hp it can achieve, and a bit below the rated torque. Comparing its fuel mileage in Fourth gear at the same vehicle speed, the engine is now turning almost 1,000 rpm faster, which also causes it to make quite a bit more horsepower. Just for giggles, let's say at 2500 rpm the motor makes 75 horsepower and at 3,400 rpm about 102 horsepower. Actually, the latter figure would be a bit higher as the horsepower gain curve rises as the rpm increase. Yes, rpm are quite a bit over the rated torque rpm when in Fourth gear, but the torque curve doesn't descend quite as quickly over the rpm rating as it does when under the rated rpm. This is especially true with a small engine like the Colorado has.
So now, in Fourth gear, the engine is turning higher rpm but making considerably more horsepower and about the same amount of torque than it did at the lower rpm in Fifth gear. This makes it "easier" on the engine to push the truck down the road at the same given vehicle speed, even though the engine is spinning faster. The drawback is that you are burning 10 percent more fuel and producing higher piston and crankshaft speeds that are wearing out the engine faster, hence the Fifth-overdrive gear that the design engineers added. If you had the proper scan tool, you would find that the percentage of throttle opening is higher in Fifth gear than in Fourth. However, it seems to make one wonder why, if the throttle is held open wider, and the fact that the engine is basically an air pump, then it should have more air flowing through it in Fifth gear than in Fourth gear and hence use more fuel. In reality, it is actually flowing less due to fact that the velocity of the airflow is much slower at lower rpm than it is at higher rpm, and the engine's computer compensates part of its fuelflow based on airflow, as well as on engine load or vacuum.
The bottom line? The engine is not working as hard to move the vehicle down the road in Fourth gear as it is in Fifth, but the higher rpm will cause it to consume a bit more fuel.
Best Mild-Lift Setup for Old Chevys?
Q I am starting to build a '77 Chevy Stepside pickup and am trying to figure out how to fit at least 35-inch-plus tires without affecting turning, and avoiding as much lift as possible. What modifications would be the best, with a low center of gravity being the focus?
Theo Hemrick
Quantico, VA
A Chevy suspensions, with their reversed-arch leaf springs, don't have a great amount of compression travel, so a spring lift up front not only gains more tire clearance but also more suspension travel for better ride quality and articulation. At the back end, I would really suggest a shackle flip kit that changes your spring shackle from a tension shackle into a compression shackle (that is, the spring eye is now below the shackle mount instead of above it). This way, you don't have to use lift blocks or new springs to gain the necessary amount of lift. In reality, your truck's rear springs are most likely pretty fatigued after some 33 years of service, so you might consider some replacements.
Oh, as to lift, I would suggest a 4-inch lift up front, and the rear shackle lift will also gain you about four inches. Yes, you will have to do some fender trimming at both the front and rear of the front fenders and relocate the front fender brace. The steering will be a bit off, so consider an adjustable drag link and/or a dropped pitman arm. Early Chevy trucks are notorious for cracking the frame around the steering box, so be sure to check that area out. There are several companies that offer a reinforcement plate and even braces for that area.
7.3L Diesel Stalls When Hot, Won't Start When Cold
Q I have an '01 Ford F-250, with a 7.3L diesel engine, which was purchased used. In July the engine began to shut off when warm. My mechanic traced the problem to the powertrain control module (PCM). We went on vacation to the mountains, where the temperature dropped to 45 degrees in the morning, and we began to have trouble starting the truck. (I would cycle the "wait to start" three times to get it to start the first time.) We have never had this trouble before, and we have camped when it has been down to 32 degrees.
I have changed the oil temperature sender and checked the glow-plug relay, as indicated in my repair manual, and I believe it is the PCM. My mechanic says no-he is checking the glow-plugs.
Could it be the PCM? And how do you test it when the temperature doesn't get below 60 degrees?
George Walker
Simi Valley, CA
A It sounds as if we have two unrelated problems: (1) the engine stalls when it gets warm, and (2) the engine will not start when it is cold outside. To get an accurate answer to both problems, I went to the best Ford diesel troubleshooter around, Ford Master Tech Mike Kelly. I have known Mike for over 20 years now, and he has never failed to come up with a precise and clear answer. If there was an answer to be had, Mike is the man. Here's what he had to say:
"As to problem (1), I have only seen a couple of things that would cause a 7.3L that's running to stall. The most common is a cam sensor that is going bad. This is a temporary problem, and usually the engine will restart after it cools off. A code scan of the system having this fault will result in a code "P0340" and/or a code "P0344." I believe that this model year is included in a recall where the cam sensor will be replaced free of charge if it is the older black one with a gold hold-down bracket.
"The other common failure on this model that can cause the engine to stall is the fuel heater element coming loose inside the fuel filter canister and grounding itself on the canister body. Usually, this will blow a large 30-amp fuse in the underhood fuse box, requiring the element and the fuse to be replaced.
"Problem (2) sounds suspiciously like a glow-plug system concern since it only occurs with a cold engine. The first thing to do is to note the tailpipe while the engine is cranking. If the engine is smoking out the tailpipe and will not start, this indicates that all the fuel delivery components are supplying fuel just fine but the glow plug system is not heating up the air in the combustion chambers. Conversely, if the engine is not smoking out the pipe during cranking, there is a problem in the fuel delivery system.
"To check the glow-plug system, just unplug the nine-pin connector on each valve cover and measure the resistance to ground of each of the two outer pins in the connectors going into the engine. Optimum resistance will be 2.0 ohm or less. Any higher resistance will indicate a failing or failed glow plug or a melted wire or connector under the valve cover. If the glow plugs and wiring pass this check, then the most likely culprit is the glow-plug relay.
"The best check for this item is to put an inductive ammeter on the wire going to the glow plugs and turn the key on. The desired reading is 200 amps since a healthy glow plug will draw 25 amps. It is also very important to note that this relay looks like a starter solenoid, but has twin gold-plated internal contacts, and using one designed for a starter will cause it to fail in a short time.
"Also the two large terminals on the wiring to the glow-plug relay, that receive power from the battery and send the power to the glow plugs, have to be very clean and very tight to carry this amount of current. I always polish the eyelets with a file or a Scotchbrite pad.
"To test the system when it isn't cold out, you must insert a resistor in the plug going to the engine oil temperature (EOT) sensor to trick it into thinking the oil is cold. I have never seen this issue caused by a defective PCM, but nothing is impossible."
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