docmirror

Pretty simple really. I was discussing the prospect of changing to the Porkensioner and I know it's an advanced design like we find on modern cars. I had one on the 968 and it worked flawlessly.

My recent experience with the seized water pump and quick notification of the 'toothed belt' warning helped me avoid expensive repairs. I'm on the fence about changing, notwithstanding the cost either way.

What I would like to find if there is ANY evidence of a standard Porsche mechanical tensioner failure leading to a catastrophic cam mis-timing event.

What I'm not interested in are evidence of timing BELT failures, or water pump failures, or oil pump, cam gear failures. Specifically interested in anything related to the tensioner system failing.

GregBBRD

I've never seen one, never heard of one.

I guarantee that Anderson, for all the cars and engines he's taken apart has also never seen one, either.

However, I've had many engines saved from catastrophic damage by the belt tension light.

That fact has always been one of my main questions about Ken's "thing".

"Why?"

RKD in OKC

Feel like I HAVE to tell my story when anyone asks about the stock tensioner. My toothed belt warning light came on, basically right after lifting to decelerate from speeds over 140. Had everything checked out by a couple of very good 928 mechanics and they found no reason for the tensioner light to come on. The belt and tension was always well within spec. The warning wasn't RPM related as much as speed related. To make me feel better the tensioner was rebuilt and all the wiring redone. But the light would still occasionally come on, but only just after lifting to decelerate from speeds over 140.

Then that one fateful trip the toothed belt warning light started coming on when lifting off the throttle at high speed. Because of the previous warnings I babied it heading for the nearest mechanic. The light came on a couple more times after lifting from speeds that were not so high. Then bang and the engine died. Everything looked fine and the belt as still in tact. Was headed for Sean's so he came to check it out. Removed the belt covers and everything still looked okay. The belts was NOT broken. Towed it to Sean's.

Closer inspection showed the cam had broken off where the key way is that aligns the hub the 3 bolts go into to align the cam gear. AND one of the 3 arms of that hub was gone. There was a stain on the cam gear showing the arm that had broken off had been cracked for some time. Of course a borescope showed the tell tale bent valve marks on the tops of the pistons.

So the short of it. The cam gear had broken off and crashed the valves. The toothed belt warning light warned of the upcoming disaster but no one thought to take the cam gears off to inspect further as to why. If they had, I'm sure they would have found the hub arm was cracked.

When re-assembling the motor I was asked if I wanted to go to Porken's tensioner. I did not BECAUSE the stock tensioner had warned me, even if no one checked deep enough to avoid the valve crash. The Porken tensioiner would not have warned me, and the valves would have crashed anyway, but now I know to heed the toothed belt warning light even if it is not the actual belt tension that is setting off the warning.

The Forgotten On

My mechanic who was one of the first US Porsche mechanics trained on the 928 says the only improvement is the superseded design, after(correct me if I am wrong) 1982.

Sure they leak fluid if you just look at them; but they never fail to function correctly if maintained and adjusted correctly, just like the rest of the car.

Vilhuer

Running tensioner dry long enough will kill its internals. Even then warning works. Only reason I can see to use something else is money. Which would mean no more usable used parts around and only factory option would be to buy new from Porsche. For now its cheaper to buy dead engines and part them out.

Captain_Slow

This is interesting. I also see the warning light when mashing the pedal and the auto trans shifts around 4000 rpm. My imagination may be overacting, but I suspect the Conti belt is slightly elastic and stretches as rpms build, then during the shift there is an instant of reduced tension as the belt contracts as rpms are reduced after the shift to the next gear. This very short period of lowered tension is what I imagine is triggering the tensioner light. KLVA installed the timing belt about 2 years ago, and we did the 1500 mile tension check together. I have about 5000 miles on the belt. I had tightened the adjuster a quarter turn after the first time I observed the light. When we checked the tension it was just a millimeter past the higher tension side of the window on the Kempf tool. We made no adjustment as the Conti belt are known to stretch.

My car has the flat copper ribbon sensor design. These ribbons are known to crack, with the two parts remaining in contact until accelerating and aggressive shifting causes a momentary gap and sets off the light. I purchased the wire to replace the ribbon.

For now I'm driving it less aggressively. Haven't seen the light and don't expect to see it with current driving style (If I do...will pull over immediately). When I get around to replacing the timing belt in a few more years I'll put in a Gates belt, replace the copper ribbon with the wire, and hope the light stays off.

I think the Porkensioner makes a lot of sense. But I like the warning light feature, and since my car has made it this far on the original tensioner, I have decided to keep it. But I could change my mind.

bureau13

My "no personal experience, just reading" perspective: I think the problem with the stock system is more that it requires more constant maintenance than it should (or appears to? I'm not actually sure what you're supposed to do with it, everyone always just says "if it's maintained it works great") and that it gives a lot (relatively speaking) of false positives (full disclosure: I haven't had one, but I've only had the car a short while). The more false positives you get, the more likely you are to ignore it when the real warning comes in.

I think the problem described above with the cam gear is pretty rare. Personally, I've been thinking I would go with the Porkensioner when it's belt time, but the one part that bothers me is the lack of warning in the event of a water pump failure. I would say that's the one failure mode that seems common enough to make me think twice about changing. That's actually why I'm seriously considering going with a Porkensioner and that Engine Guardian pump. There was some talk about it on here in some old thread, but nothing recently...however it's still being made, and just seems like a great idea. I love supporting continued innovation with these cars too, and the two devices together just seem made for each other.

Remember, I don't know crap compared to most everyone on this list, this is just my perspective on the topic so far.

docmirror

So far lots of what I expected, plenty of failure modes not related to the tensioner. Having now experienced a water pump failure, and knowing that our water pumps have a somewhat sketchy history, it seems awfully important to have a warning system. My first belt job, I inspected the bearing and week area of my original pump, and left it on. I think we have a good source of pumps now, but nothing is going to last forever. The damage caused by a timing failure is too great to let it run on a modern style tensioner which typically only runs the timing belt, and no other accessories.

MFranke

About 15 yrs ago I had the tension light warning me of impending water pump bearing failure. Repeated trips to the Porsche shop and $ checking the belt and tensioner and still no one diagnosed it or had the thought to inspect the pump or listen to it. Finally the water pump bearing failed, the tension light came on and I saw engine temp rising so I just turned off the engine. In the end, no damage to anything except the water pump. After this was when I took more of an interest in doing more of the maintenance and inspections myself vs relying on the shop.

I installed a Porken tensioner and new Laso pump from Roger during my last belt replacement. I inspect it periodically and have taken to using a stethoscope fairly routinely to better keep a pulse on the health of the various bearings and engine components.

JPTL

This is a very interesting subject you bring up, Doc.
Subscribed to this discussion.

docmirror

Did you leave the center cover off? Or drill a hole in it? Where do you put the mechanics stethoscope to evaluate the bearing?

My bearing failed without so much as a whimper. I was just driving along at 75-80 and it seized with no warning at all. Tension warning, then the temp climbing, and I just shut it off as well. Didn't have the radio on, and could hear just fine but no audible warning of any kind.

mdkelly1

+1 to RDK's experience.

These cars almost always talk to you in one fashion or another to let you know when something is wrong. I've learned to listen to them closely when they are talking.

I purchased the 928 sixteen years ago. It had a broken/stripped timing belt in its past service history. Prior to that event, the timing belt had been replaced several times on a reasonable maintenance schedule by a reputable Porsche repair shop. You could also see several/many visits to "adjust" the tension on the belt as well (in my mind, more than would normally be expected).

Going through the car to check what needed attention, I noticed that ! light would illuminate (and could be reset) without a corresponding error warning light illuminating in the POD.

As you can guess, the "belt tension" warning light bulb in the POD was burned out and had probably been so for a very long time.

Once I looked at the cam gears and saw the extreme wear on the gears (grooves in the center of the teeth were worn so deep that there was almost no surface left for the belt teeth to pull against), it seemed probable that the tension had continually been adjusted tighter and tighter just to make that "darned light" go off, with little or no effect until the bulb finally burned out on its own.

The records also indicated no attention to the tensioner (no rebuilding or replacement) each time the timing belt had been replaced.

The car had been talking but no one had been listening!

That certainly made me think about how I should approach maintenance and ownership going forward.

Replaced all the gears, rebuilt the tensioner with correct parts , new belt (running gates currently), new correct water pump, replaced tensioner warning bulb, set correct tension, checked operation of tensioner warning, etc.

There have been some other "car talking to me" experiences with the bulb controller trying to tell me something (bad connection in the tail light harness socket pins), intermittent passenger headlight low beam (bad connection in the wire connector at the bulb), moaning sound under load when going through tight lefthand turns (wheel bearing), and others.

I've learned to listen to the 928 and try to understand and trust what it is telling me.

Only sharing my experience.

SteveG

.02. It would seem that although RKD's tensioner did not malfunction, the related problems were attributable to improper (overtension) which broke the cam gear or spider. Capt. Slow may have overtensioned epending on which tool he tested it, but most of us use the tool that test torsion, not tension (I forgot the name). The gears on my GTS were abnormally worn for 65K and I suspect that was the cause. Note, I said possibly. Apologies to Capt and Kevin, but this a problem with the system, i.e., knowing (or not knowing) whether the cam gear and cam snout is under high pressure.

Note: I like having a warning light, but it didn't save RKD. No blame is assigned to either user above. Only the warning that OE is a multiheaded problem.

Second Note: everyone should make sure they don't have the flat blade connector on their system. I had a new one shear. If your supplier gives you one, send it back and demand the braided wire connector. Porsche should have paid to replace all of those.

PorKen

Objection: asked and answered.

The stock tensioner system's strength and it's primary weakness is that FAIAP the stock tensioner is a bolt and nut. The tensioner itself cannot fail, conversely, it cannot do a satisfactory job of managing free belt length and tension because it has little to no range of motion (or damping).

I would consider ANY gap in tensioning or free belt management a failure. The presence of a warning light (w/3 min cold start delay), belt guiding pulley(s), the need to pre-stretch the belt with overly high static tension, and that tension needs to be checked periodically -or at all- belies the factory tensioner's limitations.

If the 928 had continued production, a relic from the early '70s would have been replaced with a modern tensioner/damper system, as the 968 did in '92.


Use whatever you feel comfortable with, which suits your driving and maintenance style. Personally, I do hundreds/thousands of WOT runs to redline a year testing my chip programming. I never think about the timing belt. Why? Because I know the timing belt is being managed properly, with a PKT.


I challenged GB many years ago to demonstrate his faith in the factory system: start a 32V from dead cold and rev it to 5000 rpm. He whined...and declined.

ROG100

The old tensioning system works but a lot of other 60's technology does as well - my 8 track system still works but times move on.
Porsche converted the tensioning system in 1992 on the 968 to the automated system with no timing belt warning light. I guarantee if the 928 had continued in production after 95 it would have been fitted with an updated system.
As I tell my customers prior to 99% of them buying a PKT system - the original tensioning system works but is becoming very expensive - especially now that the main tensioner roller is out of production. What would you like an 8 track system or play music straight from your smart phone. I think it is great that we have a choice.
All my later cars have the PKT.