[Rick Thomson]

Problem: The top strand of the timing belt tightens and loosens approximately every 180 degrees of crankshaft rotation.

Information: I am the original owner of a 1986 944 Turbo with 32,000 miles (long story). I am in the process of replacing a large number of parts due to age, not due to failure.
The original timing and balance shaft belts were replaced for maintenance at 20,000 miles in 1998 by a Porsche dealer (Autohaus Lancaster, as in PA, not CA).
The belt I installed is a Gates T107, replacing a Gates PowerGrip HTD. I compared the belts for length, tooth profile, and tooth spacing, and they appeared identical.
The only part of the engine replaced prior to installing the belt was the water pump. I upgraded to the latest version, modified the rear cover appropriately, replaced the 32mm stationary idler with the 46mm idler, and replaced the original tensioner with a new INA one.
I removed the spark plugs to make it easier to turn the engine.

The engine was set to TDC by the timing mark at the rear of the engine, the cam gear timing marks, and a probe in contact with the #1 piston. The rear-engine marks on this engine include a stationary cast pointer and a line on the flywheel. With the cam gear marks aligned, the flywheel line is at the center of the pointer.
When installing the new belt, I kept the bottom strand tight while installing it on the cam gear, leaving the top strand slack.
I removed the belt, advanced the cam gear approximately 1/2 tooth, reinstalled the belt, and retarded the cam gear to its timing mark, removing the belt slack. I rechecked the rear engine timing mark to confirm it had not been changed.
I set the belt tension so it required substantial effort to rotate the water pump by hand.
When turning the crank, its initial movement was easy and smooth, but it gradually required more effort. The belt teeth started climbing the bottom teeth of the cam gear, and the top strand tightened. Eventually, you could see the top strand “jump” slightly on the crank gear, the top strand loosened, and the turning effort became easy and smooth again. This scenario occurred 4 times during one rotation of the cam gear.

I repeated the belt installation process several times, but nothing changed the outcome.

The increase in crank turning effort is due to the camshaft. As the belt teeth climb the cam gear, the tension on the top belt strand increases, and when the cam turns a bit further, you can see the belt relax and the top strand returns to normal tension.

Is it possible for the timing between the cam and the crank to be off just enough to experience slight valve interference? The reason I ask is that when I first started the belt installation, I thought I wasn't getting the belt wrapped properly around the crank gear, so I removed the pulleys and the gear from the crankshaft to facilitate wrapping the belt. At some point, I moved either the cam or the crank independently of the other. To reset the timing, I started by moving the cam back to its timing marks. I ran into some resistance, so I moved the crank a small amount until the cam could match its marks. I then set the crank to its mark at the rear of the engine. After reinstalling the belt, the timing marks remained spot on.

I welcome any and all comments from forum members.

[N0tt0N]

Welcome to Dorki, Rick! Make sure to introduce yourself here: https://dorkiphus.net/porsche/showth...ouche&page=255

I know less than half what the average Dorki knows about FoFos so take what I say as that of a generalist. There are some experts here!

What are the chances you didn't reinstalled the cam sprocket completely flat? That is, if the cam sprocket had some foreign object behind the flat seating surface or there was a friction fit that is slightly unparallel, it would be another source of potential tightening/loosing at the same frequency as the cam shaft. The others wheels would have a different frequency so are unlikely to be an issue. Low probability but just in case you experienced any issue reinstalling the camshaft sprocket!

Good luck!

[cmartin]

I'm getting confused by strand. But, whenever I've had a similar issue the tensioner is bad, if automatic, or I've set the tension incorrectly.

To tension the timing belt on a 1986 944 Turbo, first set the engine to Top Dead Center (TDC), install the new belt, then use the eccentric nut on the tensioning roller to create the initial tension, allowing the belt to be twisted about 90 degrees. Next, tighten the tensioning roller's locknut while applying counter-torque to the eccentric nut, then rotate the engine twice, re-check the tension using Porsche special tool 9201 to ensure it meets the 4.0 +/- 0.3 spec, and make final adjustments as needed.

[TurboPooch]

The belt is popping because it is too loose. Hmmmmm.....popping 4 times on one revolution on a four stroke engine is not coincidence. Remember the cam has to overcome valve spring pressure on at least 4 of the eight valves at any given time.You don't mention anything about properly tensioning the belt with a belt tensioning tool, either the Porsche factory tool or one of the clicky type before turning the motor over. By setting the tension based on how hard it is to turn the water pump you are not even close and bent valves are in your future. 86 models have manual tensioners and not the spring loaded one of the later cars and MUST be tensioned with a gauge. The old guys like me will tell you that belt tension can be set by the 90 degree twist method which gets close but if you have never done that I wouldn't suggest that. I could go on, but Clarks Garage has a written out procedure that will help you. https://clarks-garage.com/ Lots of videos out there as well but you need the right tools.