Garth S

Quite understandable - same root cause as in Rick's post to start this thread.
Excessive wear of these bushings with the TB under tension will naturally pivot the carrier arm and specifically the tensioner roller forward of the vertical plane defining the operating centerline of the TB and all associated rotating components - driven or not : phew , I need a breath of air after that one ..
In an extreme case of bushing wear, the tensioner moves ahead adequately to 'kiss the backside' of the harmonic balancer .... which is just a few steps ahead of the kiss of death for the valve train.
As Big Dave questioned earlier, a more durable bushing would be nice - or if bearings would work, are there 10mm id bearings that could be fitted to machined recesses in the carrier arm?

JEC_31

I wholeheatedly agree with that.

Every time I describe the 928 Timing Belt Replacement & Tensioner Rebuild to a gearhead (both import & domestic types) they shake their heads. When I get to the part about the gear oil warming up in the block, which heats the bi-metallic washers, which extends the tensioner - some actually laugh.

The whole system is a classic case of over-engineering, a theoretically perfect but actually way over-complicated system that suffers catastrophic real-world failures when one or more of it's smaller "support" components fails.

Some would say it's simple statistics, that the number of mating and interworking parts increases the number of possible failures, but that doesn't take into account the design decision of placing a plastic bushing in a stress/wear point.

16V non-interference 928s sound better everytime I read one of these posts...

jetrainor

Not much to add here except to comment that while doing the TB/WP this summer on my 87, the mechanic noticed the tensioner arm was badly bent. ITs a very tough piece of cast iron and hard to imagine how the belt could bend it without failing itself. Also, the driver side timing cover was pretty chewed up....we replaced both and i notice since the fix that the driver side pulley is wearing into the new plastic cover. Meantime, the motor is out anyway for a winter rebuild.

87 euro auto

Rick Carter

I need 8 exhaust valves is there an OEM eqivalent available? I also need a belt, 32 valve guides and the following:
92810556701 roller/bracket
92810554003 tensioner carrier
99992400340 tensioner carrier bushing
92810551212 tensioner roller
92810557100 roller
92810557200 washer
92810557003 pin
90007413902 hexnut
99902506807 washer
90002500702 washer

There may be more but that's all for now. Maybe someone could compose this to the tune of 12 days of Christmas.

Rick Carter

I forgot to include that I have a complete S4 gasket set. Do I need gaskets not included for my 85, if so what are they?

Rick Carter

The 8 exhaust valves are on their way. Much thanks go to DR at 928 Spec. and Garrity at Motorsport for their help. Also thanks to those who offered parts, including 85/86 heads. Since only 8 exhaust valves need to be replaced I get to keep my ported heads. I've been assured the new valves will be radiused the same as the others. I posted earlier I needed new guides, that was a mistake (would have been costly) I need new seals.

heinrich

excellent

Rick Carter

Sorry to hear about what happened. Bent valves from piston contact seems to
be way too common, but even one instance of it is more than I'd want to
hear about.

There are a few questions about the incident that I was kind of wondering
about. 1) Had the cam timing ever been checked on the engine? If so, was it
checked using the dial indicator on the lifter method that's shown in the
shop manual? 2) Did the timing belt actually break when this occurred, or did it just slip? 3) Did it happen at high throttle/RPM, or during just normal operation? 4) Was the engine at normal operating temperature when it happened, or was the engine still cold or warming up ?5) Did the engine just stop when it happened, or did it run for a while after the initial piston to valve contact?

Mike Schmidt
'88 928S4

1) Cam timing had been checked and set 2 years ago with a dial indicator per the shop manuals.
2) TB did not break but there was ground rubber from where it contacted the tensioner arm, and the tensioner arm had a shiny groove cut in by the belt where if had been rubbing for a while.
3)Happened at low RPM, probably why only 8 exhaust valves (US driver's side) were bent. Complete loss of compression on the bent valve side, 50 pounds compression each cylinder other side, probably from the belt having slipped on the cam gear.
4) Engine had been running a while and was fully warmed up.
5) Immediate shut down by driver (see 3).

I think the problem started with over tightening of the belt in 2002 when I had some work done at a shop in NC, Stuttgart Motorwerks - Pete Martin, that included TB service. A couple of years ago the belt had slipped a little on each side and there was ground rubber. The warning light never came on. I took the car to MAG in Dublin Ohio and they put on another new belt, they told me the previous belt was over tightened. MAG rebuilt the tensioner. 2-3 months ago I had a blip of a TB warning light and also noticed the temp gauge was reading higher then usual. I took the car back to MAG and asked them to check it as I thought the WP might be going bad but there was no loss of coolant or overheating. I told them about the warning blip. They checked and said the WP was OK, checked the TB tension and said all was OK. They did put in a new thermostat. I think the temp gauge has a characteristic 928 malfunction that unfortunately coincided with the TB warning blip. As I said before if you have an interference engine, PM should include pulling the cam covers. It is probably a good idea to replace the 12+ year old bushings in the tensioner arms. Along with replacing all non metal fuel lines etc.

BC

I have to get back in there and adjust the belt, or at least check its tension. The light has come on a few times in the heat of SoCal traffic, and it doesn't come on when temps are down towards 65degrees. It probably just needs a quick turn of the bolt, but also a refill of the tensioner and the new boot you mentioned, and that I have. I want to go in there for motormounts, and at the same time do the X-pipe and some other stuff. I need another car and about a week to do that though.

Every time i look at that reciept, I laugh. Not in shadenfruede pissyness at that PO, but at the abject misunderstanding that these engines are not that complex and that job could have been done for about 1k or less with a used cam.

heinrich

Well yes and here's the truly ironic thing.... Had they left the old tensioner in there just as it was ..... the tbelt would not have failed. Now remember, scratch this ontothe tensioner or something ... it is an S4 setup now. We don't want it to be mixed with other prt someday in future