Timing Chain Stretch Question
 

This is not really a Porsche or 986 question per se.

My question is what is the biggest cause for stretching timing chains that makes them need to be replaced. What causes the most stress on the chain to make it stretch? Specifically is the most stress at start up ignition? Think of when you are on a bicycle and how from a start the strain on the chain is likely most when you first push down on the pedal to get the wheels rolling....

The reason I ask is modern cars have start/stop systems that turn off the engine at a stop light to save fuel. But then a car will have probably 10x more starts for the same number of miles. Does that then lead to quicker failure of the timing chain?

All of this started when I got a diesel Mercedes SUV with the hope of getting it to last a long time. So the other day I started to research how long diesels are supposed to last - and saw lots of stories about how they last up to 500K miles. But then I researched the specific diesel engine in my car and I saw some reports about the timing chain in these new cars wearing out at 70,000 miles.

Some people posted that it could be due to the start/stop in new cars.

So I know there are a lot of great mechanically inclined minds on this board who know a ton more than me and I thought it might be of interest to everyone who has bought a new car because if true - it would apply to everyone who has these new start/stop cars.

What are people’s thoughts?

Start/stop is really not much of an issue, however other factors are:

1. Design of chain, not all chain styles are equal and some are simply better than others.

2. Type of oil used and change intervals. You can use the best design chain and still get failures if the oil is low in ZDDP, dirty, and not changed when needed.

3. Relative chain lengths; the longer the chain, the more "whip" it sees which factors into stretch over time.

All that said, chain stretch in the M96/97 is relatively low, even on high mileage examples.

Chains don't actually stretch - the pins and rollers wear and the slop in them makes the chain get longer.

My 01 bent all its valves while cranking it at 89,000 miles, Jake later bought the motor and reported it was due to stretched timing chains.

My replacement motor now has timing off by 6 degrees. Replacing the pads on the variocam solenoids only bought it down less than a full degree.

Modern cars with auto start/stop are designed to withstand the rigors of thousands of additional starts as compared to their non-start/stop counterparts.

For example, the starter motor itself will see thousands of additional operations but has been designed with this in mind so the overall reliability stays about the same between start/stop and non-start/stop cars. The same is true of the other components that will see higher wear because of the start/stop system - they are designed to take the additional wear and tear.

Here is some good reading on the subject: https://www.greencarreports.com/news/1109687_dont-start-stop-systems-wear-out-your-cars-starter