Knock knock, who is there? It’s the conrod and bearing, if you keep on starting the car or driving it’ll pop out to say hello.
If your engine is rattling or knocking try this test:
When the engine is idling if you rev the engine gently and it makes a rhythmic knock on the overrun then 9 times out of 10 it’s been run low on oil, the bearings have made contact with the crank (metal on metal) and this has worn part of the bearing metal away. There are many different degrees of severity (the louder the knock, the more damage has been done) but that’s the basics of it. If you want to be 100% sure then take the oil filter off, cut the threaded end off, pull the paper element out and look in the pleats around the outside for non-ferrous bearing material, usually in the form of bright aluminium flakes.
If it’s really knocking hard or constantly DON’T try to run the engine any more to show your Facebook fans and don’t drive it. It will not get better until it’s been rebuilt, don’t waste your time and money changing the oil or any of that rubbish, but it will get worse and cost a lot more if you carry on running it. You probably know very little about engines, but I think you’ll agree that this is not how it’s supposed to look in there:
If the engine is run under load and/or without oil what eventually happens is the bearing and the crank journal get so hot they fuse together with the friction, this grabs the thin shell bearing and spins it round in the rod (hence the term ‘spun a bearing’) so now it’s damaged the crank, bearing and connecting rod. The next stage is where it really kicks off and things start breaking apart, sometimes the rod can seize, snap off and punch its way out of the block.
NCs and Duratecs are known for rod bearing failure, but that is no fault of the bearing.
Why are so many MZR/Duratec engined cars (including the MX-5 Mk3) running out of oil?
Let’s look at an MZR piston:
Piston ring tension (its strength in being able to expand to fit to the bore) is very finely balanced, too much tension and friction will go up, mpg and power will therefore go down. Too little tension and you won’t get a good seal against the bore.
As you can see in the picture the top two rings are loose in the grooves and able to expand to seal against the bore, the bore is only fractionally bigger than the piston, it’s a surprisingly tight fit. Now note how the oil control rings (which have much less tension in them) have actually become stuck in the groove. They can no longer expand to fit against the bore and oil is getting past them. You can see the soft black deposits of burned oil between the rings.
So there it is in a nutshell, oil passes by the rings, gets into the combustion chamber and is burned off until there is barely any left in the sump, the bearings then run out of oil and contact the crank.
If your engine is using oil there is nothing you can do about it apart from strip it and rebuild it with some new parts.
Forget anything else you may have read on the internet such as:
Not letting the oil level get to the halfway point, overfilling it, changing the oil, revving the engine, putting solvents in, faulty PCV valve, replacing the valve stem oil seals etc, it’s all tosh. There is no other way around it whatsoever apart from filling it up with oil every few hours!
But why specifically does this happen?
Various reasons and theories:
Oil and in particular oil changes: Mazda (and any other manufacturer) are selling cars in a competitive market, what sells cars is low running costs so they often spec long service intervals and semi-synthetic oils. These are on the whole ok, but for the uninformed or unwary sometimes too long for this engine with its Achilles heel in the form of weak oil control rings.
What happens with the Duratec is an oil goes in which is usually not fully synthetic (Mazda dealers use semi as that is what is specified), it’s left in for Mazdas prescribed interval (12k miles), often longer and the car is used by some middle aged or old person to crawl to work and back, maybe to the Supermarket once a week. The engine never sees more than 4000rpm, has old lesser quality oil in it so the oil control rings on the pistons stick together in the groove. This of course makes a mockery of the perceived increased value of a ‘Full dealer service history’ car.
It is thought that the later engines were equipped with either rings with more tension on them or perhaps a modified piston to avoid this issue. There does also seem to be a lot of 1.8 engined cars up for sale with these issues. Whether it is actually more common to the 1.8 or not is not known, it could be that the lower value of them means that when serious problems arise they are just sold on rather than repaired.
The 2.5 is certainly not immune to this problem.
Oil change intervals are a guide, not a rule. Some car manufacturers issue guidelines on oil change intervals dependent on use as things are seldom black and white, sadly Mazda don’t.
If your car is used for a lot of stop/start journeys, short trips, trackdays etc then change your oil change schedule to suit.
If your engine is healthy and you want it to stay that way then do the opposite of all that makes it break; Oil and filter changes at 6 – 8000 miles, use fully synthetic oil and when safe to do so (and the engine is fully up to temp, after at least 20 mins of driving) redline it now and again.
Straight and uphill motorway slip roads private roads and tracks are good for this, get it in the lowest gear you safely can and nail it all the way up to 70mph, just be careful and don’t have an accident. Mazda engines love to be revved, they were built for it and this will help keep the piston rings free.
The problem is if you’ve bought an early car already with a certain amount of miles on it then the damage could have been already done. You can’t rewind the clock, but you can repair it by opening it up.