The 928 Timing Belt - a Rite of Passage

Tech and Talk about the Porsche 928
dr bob
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The timing belt saga that started almost a year ago was finally fixed in mid-August. I noted the dribble of gear oil from the tensioner after a couple months just sitting on tall hibernation stands. As the car was brought out in the spring, A careful look conformed that there was oil from the tensioner housing. So it all came apart again far enough to replace the gasket where the oil seemed to be seeping.

By mid-July there was oil again, plus I got a timing-belt warning on trailing throttle, after a hard uphill pull. Checked the belt tension when I got home, all was still perfect as far as the belt tension. This time the whole tensioner came out for another rehab. The problems turned out to be a very fine crack in the tensioner boot, a scuffed o-ring on the tensioner piston likely due to careless installation, and possibly some seepage through the threads on the adjuster bolt. A new boot, a new o-ring, and a more comprehensive sealing effort on the adjuster bolt itself has the whole thing functioning correctly again. I've added a few hundred miles to it since then, and as I prep the car for an early-onset winter hibernation period this time, the timing belt and related bits are no longer a concern.

The seepage from the gearbox turned out to be a tired o-ring at the TV (English translation: Throttle Valve) cable that detects/transmits throttle movement from the front end of the car. After sitting for a few months, the fluid level in the sump slowly rises as upper fluid becomes lower fluid. The leaking o-ring is apparently a problem noted by MB owners who enjoy the same gearbox, enough so that there's a better material and ever-so-slightly larger section o-ring available for the duty. The local MB dealer had them on the shelf, so in one went and so far no more drips on the floor. More and regular driving would do the job as well I suspect.

With the timing belt fun and some more serious 2022 health-related issues, I've managed to add less than 500 miles to the 928 odometer this driving season. More wrenching than driving time this year for the first time in my 25 year stewardship period with this car. It's been dusted a few times but sadly not needed any real cleaning or washing. Missed the Treffen event down the street at Sunriver, didn't even enter the car in the Festival of Cars event, haven't been able to attend any of the region tours or even meetings. By next spring I should be able to sit in it for longer periods, and hopefully add some tour events to its log book without adding any greasy fingerprints.

Happy and safe holiday wishes to all who are following along!


dr bob

1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Bend Yacht Club Commodore Emeritus

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Tom
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First and foremost, I hope your '22 health issues are behind you and that you enjoy a healthy and happy '23! We've had way too many family health issues in '22 as well, so here's to a better '23! Sorry you missed out on those events this year. :(

Interesting to hear about the MB cross-over parts. My dad's old 450SL has so much in common with the 928. If only they had a 928 on the Rockford Files, my dad might have gotten one of those instead. The 450 is still a fun car to fix up, and I'm getting proficient at the CIS injection, but by no stretch of any imagination is it, nor has it ever been, a performance car. Wallowing 1970's-era luxury at it's finest. :)

But I digress -- happy to hear your car is all fixed up and ready for the spring. When you said timing belt warning, is that an actual thing on the 928? If so, what condition triggers a warning?


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dr bob
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Thanks Tom!

In the 1970's, Porsche was doing some engineering and assembly work for M-B, IIRC the 500E coupe. This was well before M-B swallowed up the AMG operations, so that car was their low-volume "muscle car". Depending on who you listen to, the automatic box in the 928 was designed by Porsche and manufactured by M-B, .or. Designed and manufactured by M-B and just adapted by Porsche for the 928. Lots of shared engineering talent that migrated between the two neighboring companies as project loads changed. A conversation a couple years ago with the son of Tony Lapine points to the gearbox design coming from Porsche, based on the collection of engineering and design documents that he'd accumulated from his now-deceased dad. Apparently he has a pretty massive collection of that stuff. Tony Lapine led the design effort for the 928 for Porsche, after being poached from GM's Opel operation. His history includes time at GM US during the Bill Mitchell years, and a stint with Zora and the Corvette design team. It's easy to find a lot of design commonality, with features in the 928 that appeared later in the Corvette like the rear-mounted gearbox.

The good news on the gearbox front is that there's a large mass of installed 722.x auto gearboxes in M-B cars, enough so that there's a huge amount of support available in the form of parts and upgrades, solutions to problems, great documentation, and thankfully a lot of talent available when it comes to actually working on them. For the 928, the tailshaft housing is replaced by a bolt-on differential case, while the front cover has a bolt-on bearing cover and drive hub where a bellhousing would sit. There are very few other adaptations needed really. I can carry my spare gearbox to virtually any M-B-qualified transmission house, and except for the differential setup, could get a complete refurb with updates for less than an arm and a leg.

I suspect there was a lot of cross-pollenation between the V8 in your dad's Merc and the one that appeared later in the 928. For sure the engine management and CIS fuel system are very similar. M-B had purchased the early Kugelfischer mechanical injection system used in some of their 1960's cars, and the CIS system was a logical successor to that. CIS appeared on a lot of euro cars, from Porsche/Audi/Volkswagen, Saab, Volvo, BMW, plus many more that aren't so common here the US.. It's a pretty impressive system, especially considering it was all mechanical until pretty late in its lifecycle. It's a wonderful and quite reliable system, so long as all the internals are kept clean and wet. Dirt or varnish build-up is its Achilles Heel, matched closely with user tinkering on the scale of most popular causes of problems.

On the timing belt warning: The tensioner is a spring-loaded hydraulically-dampened pisto9n tensioner. On the top where the piston engages the tensioner arm and roller, there's a coil spring that has a plastic sleeve and a a metal contact ring that together form a spring-loaded switch. If the tensioner load reduces to a point where that coil spring can open the switch contact, it sets a persistent warning in the instrument display. Because the aluminum engine dimensions grow a lot between cold and operating temp, a simple position switch setup wouldn't work. The solution that they came up with is quite elegant in it's simplicity really. Of course having that warning come up causes a minor increase in heart rate, perhaps a good thing so you remember to give the belt some attention.

Meanwhile there's an aftermarket adaptation of an Audi tensioner that's a lot better at maintaining a constant belt tension as the engine gets bigger and smaller. The factory tensioner uses a stack of Belleville coned washers as the primary "spring". The stack is very compact, but offers an amazingly non-linear pressure curve. The Audi package has no provision for the factory tension warning system, at least as it's been developed so far. The factory system is very sensitive to correct adjustment, and obviously has some sensitivity to me scuffing an o-ring during regular service with the belt. It's a devil I know better now than I did the first and second and third times I replaced the belt. I suppose I could figure a way to add a low belt tension warning contact to the aftermarket package, but it's also quite likely I have maybe one more belt replacement on my car before I get recycled, so not a whole lot of incentive to re-reinvent the 928 belt tensioner system.

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We have deicer and cinders on the roads here already, so the 928 driving season came to an unexpected early end. My favorite local Cascade Lakes Scenic Byway tour route has been closed early for the winter, thanks to some early snowfall there. The car got a new battery this summer, after 10 years of service from the last one. The new tires I'd planned for this year didn't get purchased, probably a good thing in the big picture. The current PS2's are old but still have way too much tread left, meaning I don't drive it enough. Front seats will come out for some leather treatment, likely in spring when it's a little warmer. In the meanwhile, it goes back up on tall storage stands for its hibernation season.


dr bob

1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Bend Yacht Club Commodore Emeritus

Free Advice and Commentary. Use At Your Own Risk!

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