Tom. M

Hi all,

I posted last week about my belt slipping at the crank (before last weeks track attack)...I had the chance to dig into it last night and here is what I've found so far..

................................

I got to the shop last night and was able to pull the front covers and access the belt on my GT. The belt was stripped of its teeth right near the crank (about 4 inches on both sides of the crank gear) with random missing teeth overall.


My hypothesis on this is that the belt was tension was likely ok, however, the worn cam gears and oil pump gears had significantly shortened the life of the belt.

My recent retension before the TA had probably just pushed it over the edge and it was time for the teeth to go. Cranking at the store, I bet the crank gear was positioned by one of the stripped teeth and just shredded for about 6 inches of belt before it just spun.

OK ..on to the compression testing...
Started on the drivers side front..and worked back to the firewall..
Not exact numbers...since that wasn't the point..but hear 190, 185s and 180's..as the battery lost juice....

Drivers side Compression..OK...

passenger side compression..front to back...
Not as high numbers..but still fine...175's etc..

get back to the last one (number 4 I believe) thread inthe compression tester...and...

dang.not a good look on Thurstons face...he says...hold on..lets try again....hmm...not good....so he removes the tester and just puts the tube in to feel for compression....crank again..not good...so..

cut to the chase...one cylinder with bad compression. Have to pull the passenger side head, and likely replace two valves. Will know which ones when its off. Of course that means the intake is going to come off again..but needed to look for an air leak anyway...

Still planning on trying to make the May 8 Heckowee tour..we will see..more a function of getting down to do the work.

Couple of lessons learned...

When your car stumbles and dies when starting....think many times before just blindly cranking.

Check your t-belt tension

When checking said tension, make sure you look at the cam gears..feel along the edges of each gear....if they are sharp....think twice before buttoning her back up.

Thats all for now..will update as i dig in..


Special thanks to all who helped out late last night....would have been much harder to do alone..

I think I dodged a pretty big bullet..as this could have happened while running...on the street or on the track...with much more dire consequenses...


Later,
Tom
midlman@rennlist.net
89GT (on its way to recovery)

Gregg K

Oh no!!!!!!!!!!!!!

Thank you for your advice. I will heed.
In my opinion, this is the reason why these cars are not higher priced. Is that not correct thinking? If it weren't for this timing belt potential problem, I'd be a lot more confident about the 928. So it appears that regular, and stringent inspection are part of owning an interference engine.

Hope your wrenching goes well.

Gregg

zmandan

I think I ask before can not remember if you responded:

How old was the belt / miles and how many miles on cam gears?

and glad to hear it wasn't as bad as it could have been

LOL

dan z

Tom. M

Yep...thought I answered..but here it is again...

Belt was replaced in 2001....so about 3 years...and mileage was 200K kms..(not 220 like I thought)..but still about 45K kms which is just shy of 30,000 miles.

I stated before that the tbelt tensioner was bypassed buy the PO mechanic..(first bad sign), and likely not filled with oil..(should have checked..I know...)..

The cam gears are likely original as I don't have receipts for their replacement. The oil pump gear was worn the worst and was also likely original. So...245,000kms or 152,000 miles on the gears.

Probably a word to the wise....replace cam and oil pump gears around 120,000..or the second t-belt change.... I think miles are likely a bigger factor than age..but don't know..

I think my recent tensioning probably just pushed the belt over the edge so to speak.

Later,
Tom
midlman@rennlist.net
89GT (on the road to recovery)

Tom. M

Hey Gregg,

I don't think my case is a good example as to a belt failure. The T-belt is a pretty robust system when installed correctly. If the previous owners mechanics had any clue, they would have investigated the tbelt warning that was documented rather than just bypass and tighten the belt. Also, they could have looked at the gears and noted obvious wear patterns (that I believe were there all along) and recommended replacement. It wasn't as if they couldn't get the PO to pay....he payed through the nose for many many things that were just not done correctly. When I got the car, they had done a bunch of stuff under the intake ..new seals, idle stabilizer etc...and guess what...they didn't replace the flappy actuator, and actually went as far as to remove the vacuum line to the front that feeds the flappy and the line under there to the flappy.

I suppose I could be partly to blame since I also didn't notice the bad wear patterns on the cam gears...only pic i ever saw of bad cams was where they were worn to the point of being concave on the tooth surface. Mine were just shiny..not terribly..but shiny..and the key point...the edge of the tooth was sharp.....yes..run your finger on the edge of the tooth..if you get a "paper cut" then the gears are gonzo.... That is one of the reasons i am posting my story...and pics...later..for all of us to learn and see what bad really is....

So IMHO....bad reputation comes from people who just plain don't know how to work on the 928...

later,
Tom
midlman@rennlist.net
89GT

fst951

Hi Tom,

I have seen a number of 1987 and later cars with 200k mileage with good drive gears. That said, I have also seen a number of 80k miles with much gear wear to the point of causing timing belt wear. I think what most often happens is the belt is replaced with a new belt, but the tensioner is not rebuilt. The tensioner is then dry and it cannot remove tension from the belt as the engine heats up and expands. This may make the belt tension almost double. If you catch it early enough, you can put the gears on a lathe and cut about 0.020" off the diameter and have a good gear for another 100k miles if tensioned properly. I suspect that bad maintenance previously and a dry tensioner might be the culprit originally.

Sorry for your problems.

Good luck.

Gregg K

I'd have never guessed that the sprockets got sharper. That info is invaluable. Pictures will be helpful too.

Tom, I have to thank you again for putting this info out there for us. I totally agree on the quality of maintenance. I have a German milling machine that ecclipses the quality of the Porsche, believe it or not. It's easy to keep it up, and easy to kill it.

I think the key to this kind of engineering is that maintenance must mirror the engineering. That is, time is not the most important factor. I believe the Europeans value quality over time. And that is just plain foreign to Americans. Damn you Henry Ford!!! (shakes fist at sky, while looking up)

Gregg


fst951,
That is brilliant. Good thinking. Both those are good ideas. It would suggest running the car and rechecking tension several times just after work has been done.

Tom. M

Hi Fst 951,

Good info for sure. I am almost positive that the tensioner was dry, (it was replaced with the tbelt), but they probably did not fill it. I was suspect and should have acted sooner, but as always..the car was running fine so...deferred maintenance.

So..my actions of retensioning the belt probably added up...and no oil in the tensioner stressed the belt to the limit.

So..folks....another tip of the day...get in there and add oil to your tensioner...pretty easy...and worthwhile...

Later,
Tom

jon928se

Guys

go and search for the early posts of Paul Stephens (UkKid35) story of cambelt failure - stripped belt teeth due to worn cam pulleys and the subsequent rebuild outside over the winter. I think it tells the story well enough. If I could figure out how to do searches I'ld post the URL's, but I saw most of it first hand.

Tom what puzzles me is that you reckon the oil pump gear is worn as well - they are steel ? CAm gears wear once the hard anodising wears off - why worn belt, crap flaoting around, too much tension .....

Jon
getting paranoid

Garth S

Tom,
Sorry for the pain, but great observations. A solid 'heads up' for all!
Jon,
Absolutely correct - almost a carbon copy of Paul's misfortune, although Tom appears to be more fortunate re. damage. I have also seen the oil pump gear worn - basically a 'dip' across the flat of each tooth: I'm not certain, but the gear seemed to be of the same alloy as the cam gears - anyway, I believe that problem had resulted from excess belt tension.
fst951,
very interesting comment re. machining the gears to restore their profile. The only counter to that is the loss of the hard anodized coating Jon noted - but if they're already worn, coating is academic. The thought of an additional 100KKm is certainly worth a 0.020 shave.
Regards the function of the tensioner in all of this - it seems to cut both ways. Recently (Post within the last week), Jeff [944Boyee 'misspelled'] had his tension alarm come in within 150Km of a new TB install - suggested he have the tensioner oil checked, as it can easily be 'forgotten': he reported back that with oil added, no other changes, all is now OK. The opposed view is that a 'no oil' condition significantly increases tension: this also seems supportable, as tensioners are too routinely neglected, run dry - and cars end up with worn drive gears while the static cold tension measurer is fine. Anyone a definitive answer on this aspect?
Anyway, best of luck Tom.

fst951

I see a lot of 928's from all years with aluminium oil pump drive gears. I also see a number with steel or ductile iron gears. I don't know the rhyme or reason to the material and year. We must realize that a number of these gears have probably already been replaced and could have been replaced with an earlier or later gear made of different material. I think they all last rather nicely.

With regard to the machining I will say that a new gear is always better, but if it is only slightly worn it is worth the 10 minues it take to machine and deburr each gear to get them reshaped. If you really wanted you could probably take the aluminium gears and get them re hard anodized at an anodizing or plating facility for a $10 if you wern't in a hurry. You loose a little contact area, but on a stock or slightly modified engine, I can't imagine that it hurts too much.

Make sure that you don't get suckered into Factory Valves for your head if you need them. We have some really nice TRW's that we use and you can get them for less than $30 each. Only replace the bent onces most likely, but ask the machinist how the stems measure and if you can get away with polishing them. You will probably need all new guides so make certain to get the newer bronze phosphorus as they are a little more pricey, but they keep the valve train cooler.

Good luck

BC

When you say "cooler" do you mean that they do not convey as much tempature to the surrounding head FROM the hot valve?

Thanks

fst951

NOOOOOOO. I mean that the new valve guide materials do convey more heat away from the hot valve to the relatively cooler head.

Hope that is more clear.

BC

I was thinking that that is what you meant. Thanks.

fst951

I appears that you might also be in a position to benefit from better guides! Are you rebuilding a broken or worn out engine? I assume that is the reason there is no engine in your S4 engine bay? Lots of work but worth the wait.

Good luck!