Every once in a while we come across something that we have never seen before in our 944’s. This is one that we cannot figure out – at least not for absolute certainty. So we want to put it out there and see what others may think.
We love a good mystery, especially at Halloween. There may be a goblin in this engine.
Some background – this is a Series II engine, M44/09, from a 1988 car. Prior to this failure, it was running very well with oil pressure and temps well within range. No tapping, no strange noises, no strange running issues. Just a good-running low miles ’88 engine, original to the car.
One recent afternoon making the trip from our Global Headquarters in DeLand, FL to Orlando, we cruised the Interstate at a solid 70 mph using the Interstate 4 brand-new pay-per-view toll express lane for most of the 25-mile run. The express lane didn’t have an exit for the one we wanted, so we went to the next exit, turned around to go back one exit. Easy.
After getting back on the highway and at about 50 mph, the engine quit. Stopped. Or from the German, Kaput. “Chuggada-chuggada, she no go.” DRT (Dead Right There.) You get the idea. We coasted down the exit ramp, around the corner, and found a nice tree on the side of the road to provide shelter from the hot Florida sun.
Trying to start the engine resulted in it just spinning…so the first diagnosis was “broken timing belt.” The belt service, though, was only three years and about 5,000 miles ago, which made us question it – but it was pretty obvious that the crank was spinning but the cam was not.
The Rescue Team of Son and Wife, Truck and Trailer, and about ninety minutes later we were back home.
So what the hell happened?
Normal teardown for timing belt – and taking off the plastic cover, the first clue – a 3″ hunk of the timing belt with the teeth still on it fell out. The timing belt, curiously, was not broken. The place where the teeth was sheared was around the crank gear – the belt stopped and the crankshaft peeled off a piece of the belt while not breaking it completely. Not unheard of, but definitely not normal.
You can see in the photo that the belt is not broken, but about half of its thickness is sheared off, along with the teeth.
Moving on with the teardown, the intake and header come off, along with other miscellaneous parts and pieces. Then the cam tower, and we set it aside to get to the head.
Looking at the head, we could see that visually only one exhaust valve was bent – #2. That was good news for us since we have seen other heads that have been destroyed in a cam belt failure. We could also see that exhaust valve for #1 was very slightly bent, which is again not unusual.
But then things got interesting.
Checking the cam tower more closely, we removed each of the followers/lifters to check for any damage. The exhaust lifter for #1 (slightly bent) was a little difficult to pull out – because it was broken. “Broken” as in “two or more pieces broken.” Take a look at the photos below.
You can see that part of the lifter is still in the cam tower, and the other part is completely broken off. (We have tagged this cam tower and camshaft as “do not use ever again” as we have to assume that there is damage.)
Checking the pistons, there is no damage to the pistons, including #2 that has the more seriously bent valve. The rollers, gears and balance shafts are all moving well, the water pump is not seized and turns easily.
So now you have all the facts as we know them. So the question is this – what caused this?
- Did the lifter seize in the bore, then the cam broke it? (Nothing to indicate that it seized, though)
- Did the lifter simply fail from some manufacturing defect or possibly some existing damage?
- Did the belt itself just fail on its own, and when the engine went out of time, the cam/valve broke the lifter?
- Did something else fail, causing the belt to fail and then causing the valve to break the lifter?
We don’t know, and we also have never seen anything like this before. What do you think?
Same thimg hapened to me with my 87s.belt did not break but teeth came off.i guess not unusual as one might think.
More puzzled about the broken lifter.
If the belt went..the weakest link…cam stopped…piston slammed valve… valve slammed lifter. Lifter broke.
If lifter went first, seems to me valve would have still been in time for atleast a few seconds or so but that valve would not have fully opened. And it would have made a bunch of noise.
Then again..it’s just my guess.
My guess is that only two or three teeth got stripped at first. This caused a small shift in valve timing that put the valve/piston event right at a point where there was maximum interference, like maybe right at the peak of the cam lobe. Then the cam and belt stopped dead, stripping the rest of the teeth, while the crank/piston finished coming up to TDC and broke the lifter. Just a wild guess.
I just read an old Porsche TSB (#18715) that showed and discussed several cam belt failure modes. Several examples were of missing teeth, and in all but the classic oil on the belt failure, porsche said that too little belt tension was to blame. It’s interesting that today you only hear about the danger of too tight a belt.
OK, oil pressure within range, but which oil? And how low is low miles?
20W50 Castrol GTX, 1200 miles since oil change. 5 bar at speed, 4 bar at idle. 29K on car and engine.