Timing Belt Oddity

[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 putting it on the cam gear, which left 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.

[t36]

Rick, are you able to post a video of this?

[Rick Thomson]

I could give it a try. What specifically do you want to see?

[blueline]

Hi Rick and welcome to Carpokes!

I can't help with your query but I thought I'd say hello and share the photo posting link from Tom and also mention that videos need to be from a hosting site (such as YouTube). It works and displays wonderfully as you can see in the example below.

As for photos, the more the merrier! Carpokes does host photos and is a very pic-friendly site, Here's Tom's PSA guide in case needed:
viewtopic.php?t=410

And here's an example of how your video will likely look if pasting a YouTube link:

[Rick Thomson]

Tim, thank you for the welcome and for the information on photos and videos. I will put it to good use.

[ReidMcT]

A couple general remarks:
1. The tension of various segments of belt will vary as the engine rotates. This is because of the varying forces of the valve springs against the camshaft as it rotates. Maybe what you are seeing is normal.

2. Pulleys wear and become out of round. That will cause tension variations.

[Tom]

I do think a video would be helpful. I'd also put the plugs in to tension the belt. The tension spec is an arbitrary number (not referenced to any known scale), and is only accurate/relevant when you follow the measuring procedure outlined in the manual -- i.e., rotate the motor twice, then back up 10 degrees (1.5 cam teeth), then measure. Doing that distributes the tension and slack on the belt as contemplated by the manual for measuring. If you do not have the factory tensioning tool, we have a 3D-Printed tool you can download and print for free (or purchase) that will allow you to tension it to the factory spec.

viewtopic.php?t=777

For the record, I'm not a believer in using any of the calibrated-thumb methods to tension the belt, or basing it on how hard it is to turn the water pump pulley, especially on an original-owner, 32k mile example. People get away with it, yes, but that mostly just confirms the motor can survive an ill-tensioned belt (until it can't). That's not just a random internet opinion -- see my "Tensions Run High" article in the May 2018 Panorama Magazine for details, where we made a study of that very point.

https://www.pca.org/panorama/2018

I do think a video would help, because I'm not sure what you are describing is just normal engine tension or something else. With plugs installed, you'll get resistance from compression twice per revolution, and you'll also get recurring resistance from the valve springs. When you say the belt jumps, do you mean the belt jumps gear teeth -- or just that you feel turning resistance that suddenly frees up? If the former, that's a problem of course. If the latter, that sounds pretty normal. A video would help us confirm that. Posting on Youtube and linking here is the easiest, but if you have problems, feel free to just email it to me at admin@carpokes.com

[Rick Thomson]

Tom, thank you very much for the information you provided.
My description of the belt "jump" referred to the reduction in tension on the top strand, resulting in slight but noticeable movement of the belt on the crank gear. The degree of tension increase and decrease while rotating the crank seems abnormal to me from a strictly mechanical design perspective.
I will install the spark plugs and proceed as you suggested. To that end, please let me know how I can purchase your tensioning tool, as I do not have access to a 3D printer.
If the issue persists after doing the above, I will definitely submit a video.

[Tom]

Tom, thank you very much for the information you provided.
My description of the belt "jump" referred to the reduction in tension on the top strand, resulting in slight but noticeable movement of the belt on the crank gear. The degree of tension increase and decrease while rotating the crank seems abnormal to me from a strictly mechanical design perspective.
I will install the spark plugs and proceed as you suggested. To that end, please let me know how I can purchase your tensioning tool, as I do not have access to a 3D printer.
If the issue persists after doing the above, I will definitely submit a video.

The crank gear has teeth, so I'm not following when you say 'slight but noticeable movement of the belt on the crank gear.' The only way for the belt to move on the crank gear is to jump teeth? If that is happening, then something's very wrong. On a healthy motor, the amount of resistance from compression and valve springs can be pretty strong. That said, are you sure the transmission is out of gear? That's about the only thing I can think that would create excess resistance, other than some kind of mechanical problem that would have been apparent before starting the job. If you mistakenly made contact between a piston and valve, I'd have to think you would have noticed the mechanical contact -- and I kind of doubt anyone would/could crank hard enough to bend a valve. For the tension tool, just send me an email at admin@carpokes.com and I'll be happy to get that going. You can see it and ordering details below:

app.php/carpokes-store

[t36]

I think Tom has nailed it.
Definitely crank 2 times, and then go back 1.5 teeth. Check your tension with Tom’s tool.

A video would really help, but it sounds like what is being described is the compression and feeling of the cycles in the motor. The belt will tighten and loosen at various points. A properly tensioned belt should not get enough slack to jump a tooth or hop within the teeth of either the crank or cam gear though.