SharkSkin

Rob, how about a pic from further back, showing more of the mark? It looks a bit skewed. If the roller was skewed relative to the belt it would create lots of friction. This might be evident on the belt itself... If the mark is skewed on the belt then you have the cause too.

Garth S

Very easy to confirm if the tensioner roller is the bad actor - release the tension, and it must spin as 'smooth as silk'. With the tensioner arm still attached on the pivot bolt, you can test for any sideplay in the bushings. That is unlikely : it is also unlikely the carrier arm is distorted. Check the big roller first. ( the other idler rollers only make intermittant contact with the belt - and are never under tension).

SharkSkin

The bearing may still turn fine; they can take a lot of abuse. I'd replace it anyway if it got hot enough to melt rubber. If the roller spin axis is not parallel with the other axes in the system(cam sprockets, crank) then it will build up heat which can erode the back surface of the belt and over a period of time cause the bearing to fail prematurely. I doubt it would necessarily fail after a couple of minutes' running.

My money's on this being a roller alignment issue.

geekapalooza

Dave - Here's a pic from a little further back.

Garth - I'll also be sure and do those checks once I get the front of the engine exposed again.

Aaaargh - more work ahead - the joys of 928 ownership!

SharkSkin

Hmmm.... now it's starting to look like a trick of perspective and optics. It seemed to me that the roller mark looked skewed on the belt. Besides, if the roller were frozen I would expect it to heat two groooves, not the entire belt. Very interesting...

So the water pump pulley has no groove in the middle then, right?

Don't worry... adversity makes the enjoyment so much sweeter...

H2

I changed by Water Pump (with new pump) even though it only had 20K miles on the previous 5-year old belt, I just did it to be safe. Just yesterday I had the water pump bearing seize on my Camry (90K miles) with 30K on a new belt. It trashed the TB. Had this happened on my interferance-engine Shark, I'd be a very sad guy today. Only $500 to fix the damage....Shark could have been 10 times that.

Harvey
'85S

Nathan Valles

Now the $5,000 question...

Since the tensioner got hot enough to "make an impression" on your timing belt, do you think you should you change your timing belt again just to be sure it doesn't have any internal damage from all that heat?

Hate to see it go 20k miles from now.

Gretch

He already said he was going to replace the belt again. I also agree with Garth.....the tensioner roller alignment was not correct. This would cause the belt to slew back and fourth across the cam gears when the engine was running........and would also account for the burn mark not being centered on the belt.

Bill Ball

Also, it is easy to route the belt incorrectly around the idler roller that is on the tenioner arm. The idler roller is supposed to be between the belt runs and not touch the belt. Just throwing that out here while we ponder why the tensioner roller is doing this.

Garth S

Bill, an interesting thought: I don't think you could get the belt installed on the wrong side of the idler - and still squeeze the tensioner roller in as a final step, for it is a tight fit at the best of times: then, I've never tried .....
Rob, How far around the length of the belt are these two channels carved out? Are they in one spot only, more or less continuous, or scalloped out on regular intervals? ( intuition says the latter??)
When you hand turned the engine to review the belt condition, were there any chirps, noises from any 'bearings'? - or any spots of unusual resistance? That likely would be tough to determine if the plugs were still in?

Gregg K

I've avoided looking at this post, just out of sheer fear. This is an area I only want to know about while doing preventative maintenance.

Rob, if I might butt in here, and if you haven't already pulled things apart, I would suggest putting a straight edge on that suspected idler. See if it is indeed skewed out of parallel.

If it helps, I know how you feel right now. I suppose that's what this forum is about. Support- even if it's virtual. Haha.

Jim bailey - 928 International

Yes Auto-Pac rebuilds the water pumps for Porsche they have used Auto- Pac for maybe 6 -7 years , as I recall . WE also have Auto- Pac rebuild our water pumps and have used them for about 14 YEARS ! Base on my conversations with thousands of 928 owners and shops , it is my opinion that the rebuild by Auto- Pac is at least as reliable as a NEW pump and not replacing an old water pump ( especially an original new one ) is false economy . Over the last decade that I have been talking to 928 owners .....I have had many , many people who when ordering a water pump when I ask if they need a timing belt they reply " NO that was just done .... weeks ago ". I firmly believe that there is far less risk using the Auto- Pac rebuilt from us or Porsche than using the "good" used one , especially true for those who pay several hundred dollars for labor each time the timing belt has to come off . One important point .....not all " rebuilt " parts are done to the same standards and perhaps that is one reason why some people have doubts about rebuilt parts . It makes a big difference who you do business with . Our rebuilts are $119 and from the same rebuilder that Porsche now uses . As far as what happened here on the Tensioner roller , I suspect that the Lock Ring item #4 on 103-10 was not installed on the new roller and when the bolt item #5 was tightened it pressed the edge on the roller into the tensioner arm and locked the roller . Also note that the oil filled tensioner does NOT use the oil to create tension , is not oil fed and does not require bleeding ....you fill it with oil it acts like a shock absorber on later cars to help control flutter .

Bill Ball

"As far as what happened here on the Tensioner roller , I suspect that the Lock Ring item #4 on 103-10 was not installed on the new roller and when the bolt item #5 was tightened it pressed the edge on the roller into the tensioner arm and locked the roller . "

I know he does this all the time, but this is exceptional deductive reasoning. Good job, Jim.

geekapalooza

The Lock Ring item #4 on 103-10 WAS installed on the new roller. Dang good deductive reasoning though!


Yup - The belt was routed correctly.


Right - there is no groove on the water pump pulley. The only groove is on the tensioner roller itself.


I noticed on the PO's previous TB change receipt that the tensioner arm was replaced, but apparently not the rollers. Hmmmm... that seems strange - huh? Even if the arm was bent, why did the other old roller hold up, and this one fried instantly? I'm guessing it was just a bad bearing from the factory?


They are carved out about 3 inches, in one spot only, which should just about translate to the length of belt that rested on the actual (very hot) tensioner roller.


Not that I could tell, but not sure since I left the plugs in.


So here's what I'm buying / ordering:

1. Complete new tensioner arm
2. Tensioner arm bushings - again
3. Tensioner pivot stud (the shaft that the arm pivots on, mounts on the WP)
4. Belt - again
5. Tensioner roller - again
6. Tensioner roller mounting bolt and nut

Anything else I should add to this order (besides a 6-pack)?

Once I get in there (probably this weekend), I'll be sure to post a pic or 2 of the "damaged goods".

Thanks again guys. It's great to be a part of an excellent community that's always there when needed.

geekapalooza

Well, I couldn't stand the suspense, so I jumped back into it tonight and remove the roller assy and the TB.

Now I'm just absolutely totally stymied. The tensioner roller looks, feels, sounds good! It shows no discoloration from getting too hot (i.e. - turning blue). It has a tad bit of free play if you try to wobble it from side to side, but so does the old one. Also the pic made it look like half of the TB was running off of the tensioner, when actually it appears to be fitting over the roller just fine. The front edge of the roller just meets up with the front edge of the belt. I did notice that the new roller may have a tad more free play than the old roller, and it does not "coast" near as well as the old roller. Not sure if that is anything though.

Ideas? I'm lost at this point. Now I'm wondering about that water pump again....

Anybody in the Houston area care to swing by and lend a second set of eyes on the situation, you're more than welcome to drop by. I'll supply food and beverages!