Doctor Diesel

What beats a running flush?

dd4Hi Doc,

I have had a subscription for Diesel Car for about 20 years and over these years your advice has influenced my choice of vehicle. In the days when DIY maintenance was possible, you also saved me fair bit of cash, and I always find your section in the magazine interesting and informative. So thank you, and keep up the good work! 

In your reply to Sheena in Issue 347, “Suspicious Minds,” (I’m a wee bit behind on my reading) you mentioned that using flushing oil was not necessarily a good thing. Could you expand on that? My local independent garage suggest using flushing oil at every oil service. 

The other point was that you thought that 11,000 miles was not too bad a service life for front discs and pads, and I was a bit surprised by that. That’s every year for the average motorist, which seems excessive, although Sheena does not say how old her car is. My current car (a Nissan Qashqai 1.5 dCi) has 46,000 miles on the clock and is still on its original discs and pads. My previous six cars, all diesels, all achieved mileages in excess of 35,000 miles before the front brakes needed replacing. Regards, 

Andy Munro, Erskine

Great to hear from you then, after all these years Andy! I’m glad that we have been of service to you over that time.

Regarding flushing oil, I’m afraid that I’m sometimes a bit cynical (wot me?) but nowadays I think using flushing oil at oil changes is just “a nice little earner” for the garage. If you change your oil at the advised intervals (or preferably earlier, when computer advised servicing that can take you well over 15,000 miles), then it simply should not be necessary. These days good oils are more than adequately dosed with active detergents that keep the sludge and general junk in suspension, and it should not build up anywhere in the engine, particularly the sump itself, and in the oil passages. Many people tend to think, because their oil looks “dirty” within a few miles of an oil change, that the engine is dirty and the oil is lifting muck that has been left in the engine, or that the oil hasn’t been changed. But that is just the detergent doing its job.

The negative aspect of using highly detergent flushing oil is that it might loosen up long-standing and harmless deposits that don’t come out with the flushing oil drainage, but maybe later on, causing problems like maybe filter blockage. It’s all a delicate balance and not dissimilar to the human bloodstream. If you watch medical TV programmes, you’ll see that there’s a very fine line when somebody has a stroke or a cardiac event, as to whether to use a blood clot dispersant, (like Warfarin or aspirin) or whether that might risk creating some other critical internal bleeding in the brain, causing even greater damage.

You may think my parallels are somewhat theatrical, but it really is like that. Some of the deposits that build up in engines are best left well alone, whilst others do need clearing, or can lead to later problems – like blockages in oil passages. But if you don’t do high annual mileages, and your oil services are nearer to every two years than yearly, then you would be better off making the oil service an annual event.

Brake pad and disc wear is these days often more a function of corrosion than wear. When the asbestos pads and linings were outlawed, the cast iron used in discs and drums were reformulated to maintain stopping efficiency with the new, harder, asbestos-free pads and linings. So the discs tend to wear somewhat faster, and the cast iron now used seems to rust faster as well. Then the pitted rusty discs can wear the pads faster… etc. So discs are often changed because of pitting of the surfaces with rust, rather than frictional wear.

Also, with servicing intervals much longer than in past times, garages cannot afford to let fairly worn discs and pads hopefully go until the next service which might be 15 to 18 months away, and most owners would be unhappy to have a brake warning light come on 8 to 9 months after a service, or hear that nasty screeching noise of steel pad backing scraping on the disc surface, when it’s too late for new pads alone.

But again (cynical me!) many garages do take advantage of some owners, particularly the ladies, and change discs and pads before time as another nice little earner. Good garages will save the old worn/corroded discs and worn pads to show the customer, either as standard practice (probably rare!) or at least on request by the owner. Regarding your own car, with 35,000 mile discs and pads, and your previous cars, it’s probably all a tribute to your good driving (little braking, plenty of anticipation) and possibly that your car is rarely parked up wet, with water around the brakes, and/or regularly garaged?

I bet you’ve had very few new clutches fitted to your cars over the years! Maybe Nissan brake discs are better quality than many others (different cast irons are used) and, if yours get replaced with something other than OEM quality parts, the next lot might not last as long. That’s another false economy – buying cheap, but poor quality parts. Watch out for that. Hope that covers things Andy – don’t leave it so long before you write again!

Doctor Diesel

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