'Persuasion' could bring tach back to life

  • Article by: PAUL BRAND
  • Updated: October 20, 2010 - 11:17 PM

Q The tachometer in my 1997 Dodge Stratus operates intermittently. Also, the odometer display goes out when the tachometer doesn't work, even thought the odometer still accumulates mileage. After driving a while, they both resume working -- but sometimes go out again. Any recommendations before taking the car to a mechanic?

A Pound firmly on the top of the dash with the heel of your hand. I've had more than one vehicle with intermittent dash lights or instrument function that responded to this form of persuasion. The problem is likely a poor-quality harness connection or ground to the instrument cluster. If you're automotively inclined -- and with all due respect for the driver's side air bag -- you might be able to remove the six cover screws and the nine instrument cluster screws in order to disconnect, clean and reconnect all of the harness connectors. Who knows? You might get lucky and bring the instrument and light back to life.

Because of the age, mileage and value of your older car, I suggest you get a solid labor estimate before committing to any repair. You could easily spend hundreds of dollars in troubleshooting or repairs.

Q Won't the new 15 percent ethanol blend lower miles per gallon in all autos?

A With the Environmental Protection Agency's recent announcement that it will allow a 15 percent ethanol blend for use in 2007 and newer motor vehicles, the question of benefits of ethanol has once again come into focus. Most concerns revolve around corn prices and land use, cellulosic vs. corn-based ethanol, potential damage to engines and vehicle emissions.

But focusing specifically on fuel economy and based on the fact that ethyl alcohol contains just over one-half the energy per gallon compared with gasoline, you would expect a reduction in fuel mileage from increasing the percentage of alcohol in the fuel. Modern engine management systems do a remarkable job of optimizing combustion and fuel efficiency, so a late-model car owner may not notice much change in mileage when the percentage of ethanol is increased from 10 to 15 percent. But I suspect a back-to-back comparison of fuel mileage between pure gasoline and a 15 percent ethanol blend in the same vehicle would identify a measurable reduction in mileage.

Q My '03 6-liter Chevy truck starts well when cold, but after warming up I have to crank it for 30 seconds with the gas pedal half-depressed to get it to start. It has a rough idle, and if left idling, the check-engine light comes on until I drive a short distance. What's going on?

A You're describing a flooded condition on hot restarts. The most likely cause is a faulty coolant temperature sensor that's telling the powertrain control module (PCM) that the engine is cold even though it's warmed up. The PCM enriches the fuel mixture during cranking by increasing the length of the injector pulses to supply more fuel. If left idling, the rich mixture eventually triggers the check-engine light. Once you begin driving again, the added demand for fuel with the engine under load brings the mixture back into a normal range.

Here's an easy do-it-yourself test. Unplug the coolant temperature sensor just before a hot restart. It will turn on the check-engine light, but if the engine starts easier, idles better and drives away cleanly, you've probably found the issue. Obviously, a scan tool can quickly identify this problem as well.

The other possibility is the thermostat being stuck open, keeping the engine from ever fully warming up. The rich mixture from driving in the warm-up mode can cause the symptoms you're experiencing.

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