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Timing belt myths?

I have been talking to people and have heard some stories about what are likely myths about timing belts. I was wondering how true some of these were.

  1. A timing belt is more likely to break at idle rather than going down the road at a higher RPM. People say they always seem to break at stoplights and such when they are just sitting. Is this true? I guess the spring pressure on the lifter could essentially cause the camshaft to outrun the crankshaft in certain conditions. This sounds like it is the issue in the Chrysler V6 3.9L used in some RAM and Dakota trucks. I can see this being at least partly true.

  2. Less or no damage will occur if it breaks at idle. I personally don’t think this is right. I have heard about people trashing valves by rotating the camshaft during a timing belt change. Maybe a little less damage and carnage but the cost of repair is likely the same as the same parts will be damage. Any opinions?

  3. Some interference engines are a little more forgiving than others. Some Honda/Acura engines that were supposedly interference came through a timing belt failure without any issues. Are some engines more interference than others? I could see this being the case if an engine is interference for only a small degree of rotation of the cam/crank. I could see some getting lucky and not trashing an engine in certain cases. It sounds like any Daewoo product is not at all forgiving to a timing belt failure. These include the Chevy Aveo, Suzuki Forenza, Suzuki Reno, etc. It sounds like there is no going back on one of these if the belt fails in operation.

Any opinion on these?

“A timing belt is more likely to break at idle rather than going down the road at a higher RPM. People say they always seem to break at stoplights and such when they are just sitting.”

Never heard that one and don’t know why that would be the case. Belt will be least stressed at idle, most stressed at high RPM and/or sudden acceleration. Smells like “myth” to me.

  1. myth. Once it’s ready to break, it will break when it breaks.

  2. Well, it depends on the motor and other things, but sometimes this is true. If the motor is high enough compression and is idling, it may stop before the pistons can hit the valve, but you have to be very freaking lucky to have this happen. A lot of times it doesn’t matter though, because what’s the first thing you do when your engine quits? You try to start it, which means you slam the pistons into the valves anyway.

  3. Yeah, back in the days of D/B/H series motors, people who broke timing belts more often escaped damage if it was at idle and they didn’t do anything dumb after it broke. A friend of mine actually snapped his belt in his CRX, and managed not to do any damage to the motor because he was at a stoplight when it broke. But the bottom line is that timing belt replacement should be done on-schedule, no exceptions. Sure, it’s an expensive job (as much as a grand if you’re doing it on, say, an Acura V6), but that’s a lot cheaper than a new motor.

I could see the stress being different, not greater, at idle. If the cam tried to outrun the crank due to the force of the valve springs, the belt could be stressed in a direction that it wouldn’t under load. I understand this is part of the issue with the timing chain stretch/clatter of the Chrysler 3.9. Affected engines sound like hell at idle but normal under load.

Funny. Another one breaking at a stoplight. There might just be to #1.

On the other hand, I have experienced a belt failure firsthand and it happened at an easy cruising speed, not idle. It wasn’t my car but I fixed it on the side of the road as I knew it wasn’t an interference engine. That is the joy of working on a 3 cylinder Geo Metro. Changing the belt takes like an hour and can be done with minimal tools on the side of the road.

I know that 3 is sometimes true…at least as far as the Gates guide goes. I got differing opinions (model-specific car web site vs. mechanic) as to whether the 2.0L Zetec in my '98 Contour was interference or not. Turns out, the Zetec is N.I. in the 'Tour, but was used in other applications (ZX2 Escort maybe?) with a fatter cam, and thus was interference in that application. (Listed as an “interference engine,” without mention of application.)

(I also have heard the 4-cyl Contour is just barely non-interference, and can become interference with sufficient carbon build up on the pistons.)

There may be some truth to these. #1 My only broken belt occurred at idle (sample size of 1, I know). I imagine the stresses on the belt are more uneven at low rpms, the rotational inertia of the cam/pulley is less. #2, I had no damage on an interference engine (GTI), but who knows? #3 I’m sure is true, there’s a complete range of interference between designs, from ‘a little’ to ‘a lot’.

I was driving the car that broke the timing belt. My sample size is only 1 and mine didn’t happen at idle but all others I have heard about have broken at idle so I think there is something to this.

@cwatkin, there are two types of interference to consider. The most common is valve-piston interference, and it happens when the timing valves hit the pistons. The other type of interference is valve-valve interference, which happens on V6 and V8 engines that have dual overhead cams. Both types of interference can happen when a timing belt or timing chain breaks, but most people don’t consider an engine with possible valve-valve interference an “interference engine.”

That is interesting but makes sense. Valve-valve interference would likely not be nearly as big of a deal as valve-piston interference. Only the valves/head would need work.

On a side note, is the Mazda 2.5L V6 used in the Millenia and some other cars interference in any way? I know it is DOHC but I have been told it is not interference. I know of a 12 year old one with 50k miles on the original belt and was getting concerned about time and not mileage on this one.

Whomever told you it is a non-interference engine was likely only speaking about valve-piston interference. If it is a DOHC “V” engine, it’s safe to assume a broken timing belt would lead to valve-valve interference.

Why does it make a difference if the DOHC is a ‘V’ or an ‘I’? Valve to valve interference would seem to depend on one cylinder’s arrangement of valves, no the engine layout.

Just a guess, but I think a timing belt would be more likely to break when it is being pulled on the most, and it seems like this would occur during rapid changes in rpm. The reason you hear of them breaking at other situations such as idle or just going down the road at a constant speed is b/c it’s more or less a random event when it actually snaps, and those conditions are more common than conditions of rapid changes of rpm.

I don’t know if there’s any data to suggest one way or the other that a belt is more prone to breaking at idle as compared to breaking at speed.

As to less damage happening at idle I can’t buy into that either. I’ve done several repairs on cars that had every intake valve in the head bent due to the starter motor cranking the engine over and with the engine never even having fired up. Someone would go in and swap a water pump or what have you and pay no attention to timing belt marks. They would then hit the key and whap-whap-whap-whap; all 4 intakes now junk.
If only one valve is affected the entire head needs to be repaired anyway.

I think some engines are more interference than others. Sometimes the lesser ones will barely have the pistons nicked and valves may be bent so slightly as to not be noticable to the eye.
With others the valve head can dig deep into the piston top and severely bend the valve.
I would say that the angle of the valve stem in relation to the piston top would have some effect on if and how much it bends.

My chain on my 3800 Buick went at a stop light. No other damage. That’s all I know.

My first and only timing belt failure occurred while I was on the freeway cruising along at about 70mph, but it did happen during a winter storm and it was very cold out. It also seems that most of the people I know who have suffered a broken belt, it broke on one of the coldest days of the year.

I helped a friend change a timing belt on the median of a divided highway, a Mustang II, basically a restyled Pinto.
The belt didn’t fail by breaking, what happened was that it started shedding teeth until there was a big enough toothless gap to span the drive pulley and then it just quit turning the cam.
Fortunately, that old 2 liter four was not an interference engine.

My recollection is that most timing belt breaks happen on start up, shut down or if the water pump goes.

@Bing, the 3800 is a pushrods engine and isn’t prone to damage when the chain breaks.

The engine in my 2004 PT Cruiser was listed as interference on some parts of Gates website and non-interference on another part. I spoke with someone at Gates and they said there was possible valve to valve interference but only at high rpm.

Since I had a 1992 Voyager that went 180000 miles in 13 years without changing the timing belt I decided the PT didn’t need the belt changed at 7 years. Some idiot came out of a gas station with his foot flat on the gas, looking right and turning left across 3 lanes of traffic and T-boned me totaling the car less than 6 months later.