Hai gebissen

I haven't posted here in a long time. And the reason is simple...my 928 died over a year ago, and I was mad. For those who do not remember me or this car, it is an '87 S4 5-speed that I acquired three years ago, in May 2013. The hood, left fender, radiator, and both bumpers were missing. The car had been cannibalized to fix another wrecked S4, and was a parts car on a trailer. My brother Mike (Courtshark) bought the trailer, and I bought the 928. I am a Saab mechanic and shop owner, but I always wanted a manual trans 928...so I decided to put it back on the road.

The odometer showed 200K+ (and I later learned it didn't work). The lower intake was full of water from the hood being off. The injectors were out. It was a mess. Over the summer of 2013, I stripped the engine down to the long block, sparing no expense in replacing every sensor, gasket, injector, plug wires, and anything else questionable. I replaced the water pump, timing belt (Gates racing), cam sprockets, oil pump gear, and installed a Porkensioner. I tracked down all of the missing body parts, dealt with a collapsing fuel tank, had the speedo rebuilt, put the interior back together...and it all worked! The car ran strong, and I took it to the Frenzy that September. All it needed was paint.

I drove it occasionally through 2014 without incident. It sat through that winter and in March 2015 I drove it as soon as the weather was suitable. I remember the last time I drove it...it was running especially well. (I should have known it was about to end...my old 9000 turbo did the same thing before it spun a rod bearing.) I parked it in my driveway, and because of weather I didn't try to drive it again for a few weeks. When I went to start it the battery was very weak, barely turning the engine over. I tried my jumper box with no luck. I took the battery to work and charged it up. Reinstalled the battery, turned it over...and heard the distinct sound of little to no compression. Pulled the plugs, checked compression, and found none on one bank, little to none on the other. I hoped the cylinders were washed down with fuel, but I knew what had likely happened...the timing had jumped. I shut the hood and walked away, beyond frustrated after all the time and money I had spent to get the car on the road. I have plenty of other cars, so the 928 just sat. And sat. Until now...14 months later.

I finally had it towed to my shop on Monday, and today I pulled the timing covers...both cams are off by a good 8 teeth. The Porkensioner looks to be extended a normal amount...it has been three years since I installed it (maybe 1500 miles) so I don't remember what the spec is. Everything looks right...except the timing. Has anyone had the timing change like this after trying to start/jump with a weak battery? I was meticulous in torquing everything to spec back in 2013, but I am guessing the cam sprockets moved.

Really disappointed, but beyond the point of being mad and just want to drive it again. I really need to figure out what happened so it doesn't happen again. I don't see a part failure so I have to assume I did something wrong. Not sure why it took so long to fail though.

GregBBRD

Skipping cam timing with a Porkensioner is not supposed to be possible....that's why he designed it to replace the stock tensioner. And the Gates racing belt is supposed to be virtually indestructible.

For both cams to be off the same amount, the problem most likely occurred at the crank gear.

Something else must have broken....or ran between the timing belt and the crank gear, so the belt could skip over the gear.

It will be interesting to see what failed.

Hai gebissen

They aren't off exactly the same...one may be off 9 teeth or so. I will count again when I get to the shop in the morning.

I did delete the small pulley next to the crank gear when I installed the belt. I seem to recall being under the impression that it was not necessary.

Ducman82

many people do that delete. it would have to be some thing crazy for both to skip that much..... interesting to see what happened.

Hai gebissen

Just drives me crazy that it happened in my driveway. The car drove perfectly until I parked it in that spot and shut it off.

dr bob

The challenge with belt teeth "hopping" on a sprocket is that it can take a while before the belt teeth can drop back into the gear teeth. If the cam timing is advanced, that is the likely cause.

Very slow cranking/trying to start on a low battery can easily result in a backfire through the intake, causing the crank to spin backwards a little, and that change-of-direction can be all it takes for the belt to have enough slack to walk up on the crank sprocket. From that point you are doomed, as you crank and wait for the belt to find it's way back into the crank sprocket teeth.

Would the little console/rollers under the crank prevent this? It might be just enough to keep the belt on the gear. Mine has two rollers under the crank gear, and I'm of the opinion that Porsche put them there for a reason.

Is the PKensioner to blame? Most likely not. It probably saved a camshaft, as broken cam noses often result from the belt tracking up on top of the crank gear teeth.

Regardless, the path to recovery has the heads off, bent valves replaced, reassemble. You --might-- have that most excellent luck that I very seldom enjoy. Carefully move the crank to the closest 45º point for any cylinder. That will have the all the pistons part way down in the barrels. Count cam gear teeth to find the correct indexing point for the cams for the crank location you've chosen. Reinstall the belt, then rotate the crank to 45º BTDC on #1, confirm the cam timing. Then do a quick leak-down on each cylinder, listening for air through the valves.

PorKen

Ouch. 8 cam is four crank teeth. 2-3 cam teeth is about the limit, IIRC.

Was the crank gear changed along with the other gears? With that kind of mileage I'd expect wear there, too.

Check that the PKT pulley rotates freely on it's axis (pin not binding). Normal pin extension is about 5-7mm.



FWIW, I have been running the Racing belt with a PKT and no crank pulleys on my '88 S4 manual for a good 20K or so. (1000s of WOT runs and 10s of dyno sessions.)
It has had quite a few jump starts recently when it was in the body shop for a few weeks because the battery kept dying.

the flyin' scotsman

I too was thinking of the crank gear.......did the woodruff key fail?

john gill

Just go back to basics and reset the timing as per the workshop manual .
I would rotate it carefully after first making sure the belt is close to the required tension . to see if you can get to tdc (0 ), if the belt is broken or damaged , replace the damaged or broken parts .carefully get the crank to the marked 45 on the damper and then go through the procedure in the manual.
I dont think it is particularly clever to remove the little slave pulley , if you have seen how much slack there appears in these belts at changing engine rpm, the factory placed it there for good reason, particularly on sudden overrun . you would quickly replace it . because it backs up there instantly prior to the tensioner.
Oh yes and you will need some of the specialist tools to perform the wok correctly.

GregBBRD

I once had a brand new engine "sneeze", kickback on startup, and jump cam timing....but that was with one of those "rubber band" aftermarket belts that I assumed stretched (which I refused to install on anyone's engine, after that).....certainly not a Gates racing belt.

Hai gebissen

Passenger bank, crank at 0<br/>

Hai gebissen

Driver bank

Hai gebissen

Tensioner, extended 10mm

Hai gebissen

Crank (key intact)

dr bob

I'll stand by my diagnosis from post 6 above. The cam gears are advanced, caused by the belt "riding up" on the teeth of the crank gear. With the gear's effective diameter larger, it pulls more belt than it would with the belt engaged in the teeth of the gear.

For those playing at home who might wonder why you NEVER rotate the crank counter-clockwise with the belt covers installed, this is exactly the reason.