Will a Volvo AQ140 run with the timing belt one notch wrong

[Ifraser]

I have a 1976 boat with twin AQ140's. The timing belt has failed on one of them ( wore away rather than snapped ) the engine stopped while idling and I attempted to start it for a minute before realising the belt had gone. I have replaced the belt and timed it as per the book but the engine runs like a bag of nails with a lot of top end rattling which wasn't there before. Could it be one notch out of sync ( can an engine run one notch out??) or is it possible that the valves have been damaged? Anyone any knowledge / experience of this problem
Thanks very much
Iain

 

[oldharry]

Really bad news I am afraid: Usually when the timing belt snaps the rising piston smashes in to the valves that are open when the cam stops, destroying both the piston and the valves. The probability is that at least 2 pistons are badly damaged, and the corresponding valves badly bent or even broken right off. It is extremely unusual for the engine to run at all after cambelt failure as the damage is so severe.

Cambelt replacement is essential at or before the prescribed service interval, and many engineers advise replacement every 2 years regardless because of the damage that is caused when they fail. So replace the belt on the other engine now before your write that engine off too.

Head off job to sort the valves (have the head checked too, it could be warped by the impact) , check the camshaft is not bent. and there is no other damage eg to valve guides and seats. Next the pistons: the damage will probably be obvious - like holes where they hit the valves - but even if they are intact there is a good chance they are distorted, or the con rods are out of true.

Why manufacturers put a rubber band in instead of 'proper' drive chains, I never understood. Most engines are severely damaged at best, and written off altogether if it happens at speed as conrods snap and go through the side of the block....

 

[BarryH]

There will be no damage from your belt breaking while running. The 140 is a sohc lump. If it was a twin cam then things would be different.
If your getting "top end" rattle are you sure its mechanical. Remember that the belt also drives the dizzy. All three pullies need to be timed up correctly. If the belt is out one tooth then this will affect the ignition timing and cam timing.

My advise is take off the belt and time it properly. When the pullies are in the correct positions and you've checked everything by turning over by hand to see the marks line up. Refit the cooling and check the ignition timing with a strobe. If the cam and ignition timing are out you will get a metallic "tinkling" noise or in extreme case a rattle.

For a sure fire check for mechanical damage to either pistons or valves carryout a compression test. I've had a belt go on my 145 at high revs. I let it cool fitted a new belt and carried on. Theres masses of clearence between the valves and pistons on the sohc lumps. As I said before, if it was a twin cam then you'd be looking ata a total rebuild.

Oh nearly forgot. Check that the belt tentioner is free to spin and works as it should. Its a car derived part so is cheap and easy to replace if its beyond its sell by date.

 

[William_H]

Agree with Barry I have had 2 cars where timing belt or gear failed at speed ...no damage despite dire warnings. Check the compression if you have doubts and then the valve gear. good luck olewill

 

[boatmike]

Agree entirely. The belt is indeed probably out of position. Yes it could still run but will probably not have suffered internal damage. Re-time it, and try again.

 

[Strathglass]

Timing Belts slightly NB

'Why manufacturers put a rubber band in instead of 'proper' drive chains, I never understood. Most engines are severely damaged at best, and written off altogether if it happens at speed as conrods snap and go through the side of the block'

There is a very good reason why timing belts don't last.
It is the car spares aftermarket built in by the manufacturers.

Many years ago I was involved in the costings of adding data collection systems in motor vehicles.

One large rubber company in Edinburgh were developing rubber timing belts. They had quite a good setup with two rolling roads etc.

The test vehicles were supercharged MG 1100 cc saloons.(this gives some idea of how long ago it was)

They did a very good job.
The belts were found to have a life in excess of 300,000 miles.

The motor manufacturers told them to reduce the life of the belts to under 100,000 miles. Reason being to justify the stocking of spare parts.

The engineers reluctantly agreed and it has perpuated to this day.

Iain

 

[spannerman]

As Barry says this is a 'Fail Safe' engine where there is clearance between the piston and valves even when they are in the open position. Concur that the timing is out and causing 'pinking' and the valve timing is probably out one tooth which won't help.
When you have lined up all the timing marks you have to have the belt tight from the cam pulley down to the jackshaft pulley and then to the crank pulley so that all the slack is between the crank pulley and cam pulley so that the tensioner can take it up, otherwise when you adjust the tensioner it will pull on the belt if its already tight and cause usually the crank pulley to rotate away from its TDC mark and then the timing for everything will be out.

 

[Ifraser]

Thanks to all for the replies....I hope the majority are right and Harry is wrong!! but I'm still greatful for Harry's and any input. My next plan, when the missus lets me out to the boat again!!, is to re-check everything. I tried advancing one notch but ran out of time before I could try slipping the belt back one notch. I will go armed with all the timing gear next time, strobe, dwell meter etc..Thanks again for all your help, Iain

 

[oldharry]

[ QUOTE ]
Thanks to all for the replies....I hope the majority are right and Harry is wrong!!

[/ QUOTE ]

So do I! I have yet to see an engine that survived a broken belt... including Volvos, but perhaps thats because they have not come my way unless they ARE damaged! /forums/images/graemlins/crazy.gif

So lets hope when it is timed properly all is well.

But I have seen these belts fail in automotive applications as low as 38K miles - wrecking engines that were meticulously maintained and not abused. So my comment re the stupidity of using 'rubber bands' remains, as does the risk of using the wretched things.

 

[boatmike]

Depends on the shape of the combustion chamber and the position of the valves Harry. Some dont hit the piston crown when stuck open, some do. We think this is a "safe" engine.

 

[oldharry]

Well, glad to hear Volvo got something right for once... and it will be interesting to see if this one has survived. Lets hope so! As I say, engines that have come my way tended to be the ones that have not - sadly. And what a mess some of them were in! All for the want of new rubber band...