Sludge

Three months ago, a 2006 Audi came in for an oil change.  We’d never worked on it before.  We did an oil service and some other routine work.  The car left, and we didn’t hear from its owner until today.

After lunch, I got a call from a garage in Colorado who had the car in for service.  The mechanic told me the car had 54,000 miles on its odometer – 4,000 more than when we had it here.  He said the car came in with a cam adjuster fault code, and the engine oil was very dirty.  In response to that, he drained the oil, ran BG engine flush through the motor, and changed the oil again.

The mechanic said that the oil was very black, which caused him to think we must not have changed it.  He alleged this supposedly overlooked oil change caused the customer’s present problem, which he believed to be sludge buildup causing the cam adjuster to act up.

In response to his allegation I reviewed the internal shop ticket for the car.  The ticket showed the technician worked on the car a total of 5.1 hours on March 22, and the materials charged on the final bill were pulled from stock.  An oil change was indeed on the list of tasks.  There was absolutely no reason to doubt the oil was changed from the records I saw.

However, it raises a good question.  When someone comes in for a service like an oil change, and then says “you charged me for it and never did it,” what do you say?  When you put a new tires or wiper blades on a car, they are plain to see.  However, new oil looks pretty much the same as old oil, much of the time.  

You can recognize new oil right after it's installed, but in some engines it will be black the very next day.  Unfortunately, those tend to be the engines whose owners generate complaints.  To answer, I first say we are presumed to have done what we say we did, in the absence of evidence to the contrary.  When a bill says "change oil" most people accept that the service was provided as described.

The “black looking oil” cited by the Colorado mechanic is not evidence that we hadn't changed it.  It was just black oil.  No more, no less.  Since visiting our shop, the car was driven cross-country, from Massachusetts to Colorado.  If the car had sludge in the engine before we saw it, the detergents in the Mobil 1 would flush it and as a consequence the oil would be black, after 4,000 miles of cross-country driving. 

All the oil companies stress that point - you cannot judge the suitability or age of modern engine oil by looking at the dipstick.  Analysis is needed.

I’ve seen Mobil 1 oil turn black quite rapidly in other sludged motors, during my twenty-five years in this business.  For all I know, this engine is no different.   Black oil after driving to Colorado is not evidence of anything but a dirty motor. 

The only way to know what happened inside that particular engine would have been to sample the oil and send it to Mobil or another lab for analysis.  That’s not possible in this case because the shop owner drained it and flushed the engine.  For other people who have the same question in their own cars, oil analysis provides valuable insight.  If you were charged for a certain oil and you doubt you received it, analysis of a sample is the only way to resolve the question. “Looking dirty” is not the basis of an informed decision.

More important, analysis will tell how often the oil should be changed based on wear in your motor, and how it's holding up. 

Sludge buildup is a big problem in some of today’s cars; one that is widely known as the result of extended neglect, or repeated use of oil with the wrong rating for the car.  It’s not a problem that crops up all of a sudden, if the car was driven a few thousand miles past the target oil change interval.   Sludge buildup happens as a result of short duty cycle driving patterns combined with too-infrequent service intervals over a long period of time.

Mercedes, BMW, and Audi all specifiy oil change interval of 10,000 miles or more, which many mechanics feel is too long.   The only way those intervals can be met without sludge buildup is by the use of special long-life synthetic oils.  Use of inexpensive conventional oil is a recipe for disaster when change intervals get long.  Even with good oil, many prefer to change more frequently, using intervals of 7,500 miles instead. 


Many people encounter sludge when they buy a used car, only to discover it had poor care or no care earlier in its life, and they are now the recipient of whatever problems will ensue.  If you suspect your engine has sludge buildup, I encourage you to use an oil with strong cleaning properties, like Mobil 1 0-40, and change it every 4-5,000 miles until an analysis shows the engine to be cleaner.  In some cases, engine disassembly is needed to resolve the problem.

If the sludge stays in place in a motor it many not do any harm.  The risk is that a sludge clump will break loose, clog a vital oil passage, and precipitate the failure of the engine.  It's the automotive equivalent of a stroke.

The whole issue of sludge damage can be prevented with more frequent service.  Take better care of your car, and this won't happen to you.