Hi all,Ā Iāve just been perusing the mk1 service manuals on the mellens.net website looking for some wiring diagrams and by chance came across this in the maintenance section regarding the replacement schedule for the cam belt:
MANUFACTURERāS SUGGESTED SCHEDULED MAINTENANCE
For 1992 and earlier model years, the manufacturer recommends the belt be replaced every 60,000 miles. For
1993-95 model years, the manufacturer recommends the belt be replaced at 60,000 mile intervals for vehicles
sold outside of California. For vehicles sold in California, inspect timing belt at 60,000 and 90,000 miles, and
replace timing belt at 105,000 miles.
Ā
I was just wondering, what makes California so special?Ā It canāt be weather related as it goes from burning hot desserts to frozen snow capped mountains in CA.
As I say, justĀ wondering really.Ā ( I know, too much time on my hands Ā )
The real schedule is 60,000 miles and that is the mileage / time say 6 years to reliably change the belt and prevent belt snapping during normal use.
California has a law that says the manufacturer cannot advise a belt change to say 100k miles therefore Mazda are doing nothing more than tipping their cap to the local lawmakers.
There are those who think it is ācuteā therefore not to change the belt until the car has done 100,000 miles.
There are those on here that will also say the engine will not be damaged by a cambelt breaking.
On the other hand I and some others have had cambelts breaking while on a motorway at 70 mph.
Therefore be grown up and change the item at the uk specified 54k miles 6 years.
I have had some dealings with Dave and reasonably sure that he has already done his cambelt. Americans do loads of miles in their cars so although well over the top have more chance of braving this claim out than we do in the UK. Cam belt / waterpump change is expensive if you pay a garage to do it - Ā£2-300+ on a car that has no problems and likely worth around Ā£1000 - I can understand why some would not be that bothered about rushing to get this done.Ā Ā Ā
There is no debate, the standard MX5 engine, MK1 - MK2.5, has a non interference engine. This means that when the cam and crank timing go out of sync as is the case with a cam belt snapping, the valves will not hit the pistons and render the engine scrap.
On the rare occasions a belt snaps due to fatigue rather than damage caused by external factors, it is far more likely to happen on startup or being revved at the MOT station than whilst cruising on the motorway.
What interests me more is the potential danger created when a belt snaps at 70mph as I suspect the piston compression and valves being out of sync will slow a car down way too quickly for comfort. If not very quick on the clutch I can see this being dangerousā¦so what actually happens Drum?Ā Ā Ā Ā Ā
Interesting question. I donāt pretend any expertise on this but the compression should be offset by the expansion on the downstrokes of any pistons in cylinders with closed valves.
Not having thought very deeply about it, I used to assume that it was compression that created āengine brakingā. Then I got a diesel - high compression, almost zero engine braking! But unlike the petrol, the diesel didnāt have a throttle plate (the accelerator dictates fuel input, not air intake), so it presumably it is restricting the intake that provides engine braking.
Slightly different case with a broken cam belt but I donāt see why an engine with stationary valves not in contact with the pistons should slow a car especially violently. But Iāve never tried it:)
Gentlemen, I was not suggesting I wouldnāt change my cambelt at 60000 miles, or recommend anyone else took a risk and left it longer. I was just intreaged why California was different from the rest of the world. Eddie seems to have answered the question by saying itās just a marketing ploy. Makes you wonder how they get away with it. You could just as easily say the oil only needs changing every 3 years or 50000 miles.
I donāt suppose weāll ever know, but I wonder how many can belts snapped in CA?
D
Iāve had a cambelt fail twice, once a replacement fitted an hour later and the car was fine, but most recently I needed to scrap the (different) car.
In each case the cams settled to the āall valves partly openā setting equalising the forces from the valve springs, so no hint of compression and the engine cranks rather freely.
Basically you simply lose power, slightly less abruptly so than when running out of fuel or spark. It just seems to gently die.
If you have power steering, consider leaving it in gear to be able to steer the car safely to a coasted halt. In the last instance I was able to pull into a bus lay-by with help from the starter motor to run the car the final few yards.
I was forgetting that there is no drive to the cams so they would just settle in a comfortable position with spring tension equalised as far as possible. As you say valves would tend to be part open, clearing any piston compression driven by the crank.Ā
There is no way on earth that you can say what likely situation the car will be in when and if a cambelt goes.
No one has a scooby do if it will break in a garage or at startup or on the road at low or high speed.
In my case while doing circa 70 mph going down a hill with a truck behind me it let go and the car just lost power. That was without me pushing the brakes, therefore the truck behind me had no warning of me slowing down but it was a mid summer bright evening so that may have helped.
I have no real interest if it took out the engine or not, it put me or in any other case the driver in a dangerous situation and having AA brake down membership to tow you home is of little use with a truck up your back end.
It is just plain stupid to run an important part of a car over itās advised service life when that could put a driver and the occupant of the car and other road users that may be caught up in danger.
Please stop going over the fact that it not an interference engine, it is just daft to give people a reason to reduce maintenance and āpossiblyā and āmaybe notā put the occupants of the car and other users in an unsafe situation with their car on the road.
It is more likely to break when being torqued with revving - one of the reasons why garages were scared stiff of being sued a few years back. When under constant pressure of motorway cruising, less likely to break.
I would expect it to lose power completely as there was none but we have already worked out that it would not lock up in the same way that an interference engine would likely do. At least you were able to coast your way out of trouble without the engine stopping you before you could bang your foot on the clutch. Not a nice experience I can agree having gone through similar on the M4 when my Renault 16 carb froze up in the outside lane.
Itās all bad news but an interference engine could lock up completely creating more danger and whether it is of interest to you or not most of us would rather replace a belt than an engine!
We can at least agree on that, so who including the OP is suggesting otherwise?
We are all adults and should be treated as such. Having a car with a non interference engine has its benefits as already discussed. I sell these cam belt / waterpump kits and adhere to the service intervals as well as doing this job myself, so this is another point completely lost on me.
California market Miata cambelts are the same as for other States.
Mazda in California recommends inspection at 60k miles. It is logical that an inspection may fail, and the belt is replaced.
California State consumer protection law mandates that a manufacturer cannot specify major work before a major service interval. In California, the major service interval is taken as 105,000 miles. Timing belt replacement without failure is considered to be part of a major service. A belt that is inspected and failed before this interval can be replaced because it is considered a repair, and not a service. If it has failed within a warranty period, I assume this is treated as a warranty item. In practice, if Mazda have set realistic expectations as to the service life of the belt, many Californians will be finding themselves footing the bill for an unexpected timing belt replacement during the 60k service, due to visible damage on the belt.
My understanding is that ānon-interferenceā is not a design feature, in that the B6 designers were not sitting around wondering whether they should go for interference or non-interference design. The non-interference specification is more a reflection of the relative age and inefficiency of the engine. In order to meet mandated fuel efficiency targets, compression rate of engines has increased, and hence engines are more likely now to be interference. When Mazda tuned the B6 to give out a proper amount of power (130hp) in the 1001, the engine ended up becoming an interference engine. Newer engines also have extended oil change intervals. I have seen one estimate (not verified) that the cost of a belt/tensioner change is Ā£0.002 per mile.
I would add my personal experience of a belt letting go at speed was not āoh my timing belt has snapped, I better do thisā, but more of a WTF moment of confusion; at 70mph, hood down, you do not hear the engine stopping, you just get a telltale from the gauges (why have I got no revs?). And in those seconds before you regain composure, that White Astravan is suddenly very close. And when safely stopped, I would guarantee the first thing every owner would do is to try turning the engine over, just like when that window cable snaps, you bang mercilessly on the switch in the expectation that the window will heal itself.
Visually inspect belt regularly for contamination and damage
Change cam belt and always change waterpump at the prescribed mileage interval
Change cam belt / waterpump if purchased with dubious history of it being done before
Split pulley on short nose crank engine to avoid chance of wobbly crank issue
Stick to cam belt / waterpump + associated pulleys and gaskets only unless other problems encountered. Adding oil seals, etc to the job at the start make it onerous and a lot more time consuming/expensive particularly if paying a garage. Do the job but try to keep it as simple as possible.
I recommend the cheap but quality parts I supply on the basis of quality, proven reliabilty and cost. I would not criticise the use of Mazda parts but the extra cost is another barrier to getting this job done.
My best advice is to try and do this job yourself. It is a challenge but achievable. When I have done these, spent a lot of time cleaning and repairing previous damage that a garage could not entertain. At the end of the job you will have thoroughly bonded with your car and feel a sense of achievement. If you set out to do this job allow plenty of time(at least 3 days) and have a plan B in mind just in case the job gets too much or something unexpected occurs.
Cam belts should be replaced following two principles.
Mileage and time.
Even if the mileage is low rubber/composite belts degrade over time and cars that sit idle stress the belt where curved more tightly in the same places as a piston compression stroke will be where the engine stops. Which is why starter ring gears wear in specific places also. If left for long periods (especially outside) the bright steel cogs get a film of rust and this contaminates the belt at those places too.Ā
When starting an engine a massive shock load is taken by cam belts which is why many snap at that time and for reasons mentioned previously they will snap at specific points. Once running its a linear load.
Low mileage, rarely used cars should take a year offĀ any timescale for replacement and I would change them every three years at least and would always recommend Gates (they invented the serpentine belt)
My Roadster is in for a belts change today (all 3).Ā Fingers crossed there are no surprisesā¦
The garage owner didnāt recommend replacing the water pump however.Ā He said that heās not aware of many Japanese water pumps failing, and if it aināt broke etc.
that i dont understand you have everything off and it makes sense to replace the water pump, if the water pump goes you have strip everything off again to replace
Change the waterpump, especially if you know it has done 60000 miles. Waterpumps can last 120000 but a lot fail well before that, I know, the one on our Eunos did forcing me to do the cambelt/waterpump job.
If your garage are so sure about this, get them to guarantee the waterpump will last until changed with the next cambelt service interval.
I recently changed the cam belt on my NB. The car had done 42,000 miles. After careful inspection of the water pump and the general condition of the cooling system internals I decided that I would not change the pump after all. The two ways that pumps fail is either from general internal corrosion or bearing wear, the latter causing the gland seal to leak. The cooling system of my car was pristine, as new metal surfaces everywhere and there was absolutely no play in the shaft bearings.
If I need to do the job again in future, then so be it, my risk but it will only cost me my time.