The Car Doc talks rattling diesels, conventional vs. synthetic oil and more

John Paul, aka “the Car Doctor,’’ answers readers’ car-related questions.
John Paul, aka “the Car Doctor,’’ answers readers’ car-related questions.

Q. I’ve been following your fuel quality messages and wonder if they also apply to diesel. I have a 2012 Passat diesel that rattles and runs rough for about 30 seconds to a minute when first driving away on the first use of the day when the temperature is below 45 degrees. The dealer recognizes the condition but doesn’t know what is causing it. He suggested draining and cleaning the tank and the filter. This is going to be quite costly and there is no guarantee it will work. Would using a better grade of diesel, if it exists, help to cure the issue?

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A. Diesel fuel just like gasoline can vary in quality. It is possible that your Volkswagen may have some deposits that have built up in the fuel system. At this point I would start with a diesel fuel treatment. Just like gasoline treatments, these additives will help remove accumulated deposits in the fuel injectors. You should be able to find diesel fuel additives at any auto parts store. At the same time a technician using a scan tool may find that one of the sensors are outside of its limits.

Q. My college-age daughter’s car is a 1998 Honda Accord with 130,000 miles. I removed a front wheel to check on the brakes and noted part of the sub-frame that the sway bar attaches to was badly rusted. I’m concerned about it failing. Is it possible to replace the sub-frame components in this car? The question is, is it worth it and how much would it cost? The issue is confined to the right side of the car; the left side frame is pristine. The car is in excellent condition otherwise and has been largely trouble free. I’m hoping the car would last another two or three years, and see many of these cars easily exceeding 200,000 miles. What do you think?

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A. A quick check online finds this part is readily available used and prices of $250-$350 and I would estimate about one half day worth of labor to replace the sub-frame. If the car is truly in good condition spending this amount of money on an 18 year old can still be worth it.

Q. I have a 2009 Scion XB and it has 40,000 original miles on it. Due to health issues, I drive less than ever now, no more than 2000 miles a year. The dealer has serviced the car every 5000 miles using “conventional’’ oil. Would it be better to change over to synthetic oil considering the amount the car is driven? If so, would there be an problems switching to synthetic considering it`s used “regular’’ oil since new?

A. Synthetic oil is superior in just about every way compared to conventional motor oil. It flows quicker in cold weather, protects better against heat and offers better protection against wear. There are no problem switching to synthetic oil in your Scion and it may help protect against engine wear. Driving as little as you do can actually be hard on an engine since it never really gets to fully warm up and eliminate contaminates. No matter what kind of oil you use, even driving very little it is in my opinion a good idea to change the oil once per year.

Q. I have a 2008 F150 with a small V8 engine and automatic transmission with about 160,000 miles on it. Over the last year I have noticed the idle become more and more erratic. After warm up at a red light it almost seems like it could stall. Yet the truck runs great off idle and gets about 17 MPG on the highway. The truck sets no codes and the spark plugs were replaced 50,000 miles ago. Note that at times in the cab I can smell a hint of exhaust, very slight and intermittent. I’m waiting for the check engine light and codes that don’t seem to be coming, any ideas?

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A. It sounds as if the truck is running slightly “lean’’ this would give you a slight stumble, exhaust smell but still behave acceptably on the road. I would look for vacuum leaks around the intake manifold and the exhaust gas recirculation valve.

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