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The Car Doctor

High idling, Northstar coolant leak, troublesome turbo

Email|Print|Single Page| Text size + By John Paul, AAA Car Doctor
May 1, 2008

Q. My 2000 Oldsmobile Alero has recently been idling very high, sometimes as high as 3,000 rpm. This does not happen all the time, but when it does I notice that the temperature rises to the halfway mark on the gauge. To cool it down, I turn on the air conditioner, which runs the fans and does cool it down eventually. Could you please tell me what could be causing this problem and what would remedy it?

A. Considering the extremely high idle speed, I would want to take a look at the throttle cable. It is possible that binding is causing this very high idle speed. If the idle speed is 1,200 to 1,500 rpm, then I would look for vacuum leaks and problems with the throttle position sensor, throttle body and idle air control motor.

Q. What car gets the best miles per gallon?

A. This is a simple question with a not-as-simple answer. Currently, the Toyota Prius gets about the best fuel economy, followed by the Honda Civic hybrid. Starting in the fall, Massachusetts will be selling passenger cars with diesel again. Today's clean diesels are quiet and very fuel efficient. Volkswagen's Jetta diesel can easily return 50 miles per gallon on the highway. For a listing and comparison of vehicles and fuel economy, go to www.fueleconomy.gov.

Q. My 1998 Cadillac Deville has 91,000 miles on the Northstar engine. The problem I have is the engine is losing coolant. I have replaced the thermostat and checked the water pump, and I don't see any coolant on the ground. The car is in great condition, except for this problem. What do you think is wrong, and do I need to take the car to the Cadillac dealer for repair?

A. It is possible that one or both cylinder head gaskets has started to fail. When a head gasket fails, coolant leaks into the combustion chamber of the engine and is burned as part of normal combustion. In severe cases, you may see water vapor coming out of the tailpipe. There are several tests that any garage can do to determine if the cylinder heads gaskets are leaking coolant internally. If this is the case, it is an expensive job. The typical repair time to replace the head gaskets is about 21 hours of labor, plus parts. Although this is a difficult and time-consuming job, any competent repair shop should be able to perform the repair.

Q. My 2001 Infiniti QX4 normally rides fairly smoothly in the city. However, on a trip from Chicago to Springfield down I-55, once I reached a speed higher than around 60 mph, the vehicle appeared to jerk forward every few revolutions; the ride was no longer smooth. I pulled into a gas station and looked at the tires, but the inflation was OK. When I returned to the road it started again, so I turned back to Chicago, traveling no greater than 60 mph. I still could feel the jerking motion; however, it was not as bad as going at 70 mph. Any thoughts?

A. From your description of the problem, it sounds like the truck is suffering from a surging condition. The first thing I would do is have a technician check for correct fuel pressure and any trouble codes. A computer trouble code will help determine the problem. If there are no trouble codes, have the technician look for a contaminated mass-air-flow sensor. I have seen cases where some dust and lint on the sensor can cause a pretty dramatic surge at highway speeds.

Q. I am quite confused by what my local Saab dealer is telling me about the turbocharger in my 2001 9-3, which just reached the 100,000 mark. The dealer has maintained the car for four years. On one hand, I'm told the car is burning oil because the turbo is not working. On the other hand, I'm told that a "seal" is leaking into the exhaust, preventing the oil from getting into the turbo and causing smoke. They don't seem to recommend the turbo be changed; however, they're not recommending the seal be fixed. I don't understand what I'm being told, and I'm still waiting for a written description of the problem with an estimate to do the repair. From the reading I have done on turbo care, I see that the oil has to be changed regularly; this, according to my service records, has been done. In that I am not being told the turbo itself is defective, I wonder why it has failed or is about to fail when it has been maintained by a Saab certified service department. Could you please help me understand or tell me the type of questions I should be asking?

A. It is possible the turbocharger is leaking internally and this is causing the oil consumption and oil smoke. A turbocharger's life is determined by oil quality and how often the oil is changed. Driving style can make a pretty dramatic difference too. The dealer may not be recommending the turbocharger replacement due to the overall cost. A quick check shows that replacing the turbocharger would easily cost $1,000. If you decide to replace the turbocharger, I would also switch to synthetic oil. It has superior lubricating properties and is better suited to turbo-charged engines.

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