I'm about to do just the secondary tensioners on my 99 XJR (75K miles).

No noise or roughness yet -- I just want to have peace of mind as the mileage continues to climb.

If I understand the many postings on this subject in the forum, the primary tensioners typically get noisy when they begin to fail. And I believe they are less likely to cause engine damage when they fail (as compared to the secondary tensioners, which are said to make the engine go BOOM when they fail!).

So conventional wisdom seems to be that the primaries can wait until they tell you it's time for attention (by getting noisy), whereas the secondaries should be replaced ASAP because when they show symptoms it might be too late.

Two questions:

1. Anybody do just the secondaries, then regret not having done the primaries at the same time?

2. Why don't the primaries tend to cause the same degree of damage as the secondaries when they fail? It seems to me that a skipped tooth is a skipped tooth, whether on the primary chain or the secondary chain -- what am I missing?

Cheers,

 

That's down the individual, job time/down time, cost, specialist tools, secondaries go far more commonly.

If you want the job done right, then it's bonnet off, rad out, timing cover off (replace pulley bearings..) and replace every god forsaken plastic part in there. New seals and loctite ultra grey.

Service the engine inc. belts and gearbox, all good for another 100k.

 

The primary tensioners apply pressure to a long curved chain guide and when the tensioner fails, the chain rattles for a long time and eventually damages the tensioner guide.

The secondary ones have a small plastic 'shoe' that becomes disconnected (or the whole tensioner falls into several pieces). The 'shoe' breaks and travels along the chain.
When the 'shoe' gets to the sprocket, it gets between the chain and sprocket. The sprocket is now quite a bit larger in diameter but the chain is still the same length. The chain is stretched and breaks which causes the exhaust cam to stop rotating.
If the cam stops with a valve in the open position, the piston hits the valve and bends the living @#&% out of it.

You now have piston to valve contact with resultant engine damage.

If the chain winds around the intake sprocket, the chain 'wads up' and the sprocket essentially gets larger in diameter. The chain contacts the head and pushes out a large section of aluminum causing engine damage resulting in head replacement.

There are a few more scenarios that can occur, but I have outline the most common that I have seen.

bob gauff

 

Thanks for the clear explanation, motorcarman! Much obliged.

Cheers,

 

Perhaps others in the forum would be interested in a couple of parts sourcing stories related to the tensioner project...

1. The Ford part for the left secondary tensioner (Ford p/n 2W9Z6K254CA) shows as "Part Discontinued" on the TradeMotion sites. But when I called Tousley Ford directly, they said it was a good part. I'm not sure how to read these tea leaves -- but I jumped on ordering the part in case it was indeed going out of style :-)

2. Gasket pricing craziness. My local dealer, San Jose British Motors, wanted **$13.57** each for the spark plug seals. After finding a tip about Engler Imports elsewhere in this site, I contacted them and was quoted $5.53 net each. Since eight are required, that's more than $60 difference right there.

I don't really understand why the local dealer would put more than 100% markup on top of the list price. Maybe so they can sell to local shops at a discount and still get their full revenue? Just seems odd.

Bottom line: I got the two VC gaskets, the eight spark plug seals, and the four VC seal washers from Engler for a reasonable sum. The local dealer wanted 1.9x the money. Yikes.

Caveat emptor, I suppose.