13 replies to this topic

[berge]

Very simple question.

Two identical cars, both with 'interference' engines. One is manual, one is automatic.

If the timing belt breaks the manual transmission car is almost guaranteed engine damage due to the fact that the there is a direct connection to the drive wheels with the transmission in gear. True or False?

Same situation, only the vehicle has an automatic transmission. If the belt breaks, the engine just dies with very little chance of damage because it's an automatic transmission and there is no 'direct' relationship between the wheels and the crankshaft. True of False?

or is it none of the above. A friend asked me this question and although I'm fairly confident of the answer with a manual transmission car, I am not so certain regarding the same vehicle with an automatic transmission. He thinks that with the automatic, the engine would just die and there would be almost no chance of the valves and the pistons getting 'intimate'.

any thoughts would be appreciated.

[rgagne]

Same basic damages... 1 rotation of the crankshaft is sufficient for each piston to hit the valves that are opened. On engines with hemispheric combustion chambers, the valves "heads" will be bent, but on older engines with "vertical" valves, damage could be done to the pistons and connecting rods...

For the manual transmission car, there will be much more rotations of the crankshaft, but since the first one will bend the opened valves, it will not do much more damage...

RGagne

[Catalina Park]

Get a normal car with an automatic gearbox and drive down a hill switch off the key count to ten and turn it back on, will the motor be running or will it be stopped? It will still be running because the engine is being driven by the gearbox even though it is an auto, once it has pressure from the engine oil driven pump it keeps the pressure until the torque converter stops being spun by either the engine or the gearbox.
So yes you are going to mash your valves! with both.
I had an auto car that I switched off the key at the top of a 1 kilometer hill and gave the accelerator some pumps and at the bottom of the hill turned the key on and bang........exit one muffler!

[AS110]

Most cam belts break when cranking (starting the engine) so in this case it doesn't matter what the trans is.

As for breaking at speed,see above.

There is no rule as to why one engine will bend valves and another won't.Cam belts have been a life line to the auto repair industry,and they are my bread and butter,so I have seen them all.I have seen total engine destruction with an auto trans,so don't think that makes you safe.Valve benders just mean I can charge more money.

A lot would depend on if it was a break,or a stripping of teeth.Belts breaking is rare,they usually strip teeth.

Of course diesel valves will hit pistons,if it uses rockers,the rockers will sometimes just break,that is an easy fix.If they use buckets,we have big time damage as the valves push up through the camshaft.Why anyone would design a diesel with cam belt drive is beyond me,as for buying one - spin,click...spin ,click...spin ,click....spin....?

[TazioN]

One stupid question. I'm not very familiar with the details of valve operation, but I would think that, if the belt/chain were to break, wouldn't the hidraulic lifters and springs manage to lift the valves and seal the ports? Or are they basically locked into the point where the cam last pressed them?

[marion5drsn]

I’m not too sure that the rear wheels drive an automatic transmission. It seems like back in the 1960s or so that GM removed the rear oil pump from the automatic transmissions and one automatic transmission mechanic told me that you could push a car from L.A. to NYC and the engine wouldn’t start. I don’t know how much truth there is in that statement, but you don’t see many cars started by pushing. Any automatic transmission mechanics in the lists? M.L. Anderson

[H. Eckener]

Hello M.L.,

Yes you used to be able to push start some early automatic transmissions, but after the early 1960's GM did a way with that capability. I think this occured before the advent of the THM-300 and THM-400 series transmissions. Old trannys used to have two hydraulic pumps, now all of them to the best of my knowledge have only one hydraulic pump. This was the design change that impacted push starts. I don't believe any modern automatic car can be push started, but I could be wrong.

[Wolf]

TazioN- but in some cylinders the valves are open (and the cam has no drive), so the cam prevents the valves from closing.

[Catalina Park]

The car that I race is a 6 cyl Holden and when Holden designed the engine they used a fibre timing gear on the cam and a steel gear on the crank and also hydraulic lifters, the fibre gear is for quietness but it dosen't have a long life expectancy when the motor gets lots of revs! They also designed the engine to have clearance between the valves and the pistons in case of stripping a gear.
When modified for racing you shave the head and block to raise compression from about 8:1 to 12:1 and this means that the clearance is now gone! But we can replace the fibre gear with an aluminum gear and this keeps the opposing forces apart a bit longer! I did strip a crank gear once (steel gear) and bent every valve! The Aluminum cam gear was brand new and the steel gear was being reused and they didn't like being run in during qualifying!

[stevew]

I once had a 1984 Honda Accord 5-speed and the timing belt broke at 70mph.

At the time, I couldn't figure out what had happened; the car simply died. I was low on gas, but not on "empty" and after I coasted to a stop, I tried repeatedly to start it.

There was no valve damage at all.

$200 or so for a new belt and water pump and I was on my way.

[AS110]

The Holden motor strips the cam gear just like a cam belt,usually when cranking (but it was running fine when I stopped it).We've had then strip while just in for a lube,a bit hard to explain away.

The 4 cyl motor can't use the alloy gear,but I guess Holden don't want to be reminded that they chopped 2cyls off the six,then tryed to get rid of then to the stupid Kiwis.It was a sad car,and only the Lada saved it from being a laughing stock.

Then they had another go when they put the Camira engine in the VN, just for us.It died on the floor.

[berge]

ok then. here's the follow up to the follow up, since there seems to be so many learned folk on this board.

do timing belts deteriorate with age or do you just go by mileage. if you have a 10 year old car with 40000Km's, do you have to change the belt or can you wait till 160000kms.(which is what his manual says)

[AS110]

A lot of manufacturers are now giving a time replacment as well,usually 4 to 6 yrs.

Check out a very old,but unworn tyre,it has gone hard and cracked,the same with cam belts and condoms.Use at your own risk.

[Jaxs]

The variatiions between automatic and manual does not apply, the belt drives the camshaft, belt breaks camshaft stops turning any damage by the crank wwill only be to the valve open at the time of belt failure. Unfortunately, some small diesel engines will destroy the valve gear for no apparent reason. The Ford 1.8L diesel engine is a classic example, the Peugeot EU series engine will also go into a self destruct mode as a result of the timing belt failing.

The later European engine with hydraulic ' lifters' will reduce the amount of damage. The Renault and the VW diesel engine can escape with just damage to the hydraulic 'lifters' . The ' Renault' dealer recommends hydraulic ' lifter ' replace first before major dismantling.

The primary cause of belt failure is loss of correct tension, the later GM engines rely on an automatic tensioner to maintain correct operation. Timing chains are begining to replace the belt.
All teh manufacturers give an approved mileage and time before replacemetn is necessary. Some are as much as 4 years or 80.000 miles.

Rgds

Jaxs.