carrera tensioner failure
 

Hello all, I’m new to the board but not to the P-cars. I rebuilt the 76 Turbo Carrera in 1998 and installed the Carrera tensioner upgrade. I took the entire engine apart and added all things that were needed and put it all back together. The car is totally fast. All has gone well other than I was backing out of my garage a couple of days ago and heard the dread chain on case noise while starting the 930. The engine did not start and I figured that I would not push the issue; about two hours later I had the timing covers removed and found the left tensioner piston was at the dead bottom with a bunch of slop in the chain. I pried up the pad of the idler arm that floats on top of the piston that pokes out of the top of the tensioner and a small amount of oil came out the bleed hole on tensioner. A compression test of all cyls gave me 90 pounds ( the low compression is normal ?? because of 7:5:1 compression ratio or am I dumbed out here?). I pulled the right bank intake and exhaust covers and found ½ inch lash ( .5 ) on no. 5 intake value which had obviously tanked. The question/dilemma: did the valve cause timing to jump and that somehow could have caused the chain to jerk? Tensioner failed and caused no. 5 intake to kiss piston? This in my mind is a real-life case of tensioner failure even though they are hydraulic. Any feedback would be appreciated as I will pull engine and fix all the issues but don’t like the idea of a rerun of this problem.

Thanks,
Munchovie.

P.S. I left off the fact that no. 5 had zero lbs compression.

If the left tensioner failed, I cannot imagine it would have anything to do with an issue on #5. Left equals holes 1, 2, and 3, and right equals holes 4, 5, and 6. Since the right bank runs on its own chain and sprocket on the intermediate shaft, it shouldn't be affected by any kind of timing 'event' on the left bank.

I haven't done a compression test on a turbo in forever so I'll defer to those that have, but 90 lbs, especially a consistent 90 lbs, doesn't sound that bad on a turbo.

Any further info on what is wrong with #5? Why zero compression?

Also, what kind of idler arms are you running? If you are running the original idlers, pull off the left side idler and inspect the shaft bore. Any galling?

Good luck, and welcome to the board! It is tradition that we see pics of your 930...

I've had a hydraulic tensioner fail. It happens.
-Chris

They can all fail, the carrera ones just fail less often. I like to add the safety collars on ALL tensioners as a precaution.

sorry to but in, however, Chris, can you tell ius all which collar to use as th Pelican site offers a collar but it says NOT TO BE USED ON HYDRAULIC TENSIONERS... the p/n is 99-0458-053-m230.
Is there another p/n to use for these hydro tensioners?
Thanks alot as I needa an answer here myself on completing an engine and want as much insurance built-in as possible.
Bob

I'll preface this by saying that I have no direct experience with it. That being said, it's just math, so I shouldn't be too far off unless I'm doing something completely boneheaded here, and if I am, I'm sure someone will correct me.

One atmosphere of pressure @ sea level is 14.7 PSI. 14.7 * 7.5 = 110 or so, so ideally, assuming you're at sea level, you should have 110 PSI from a 7.5:1 compression ratio. 90/14.7 = 6.1:1. Obviously on an older engine you might not see compression right near the ideal value, but 6.1:1 where you should theoretically have 7.5:1 seems like a fairly significant difference to me (down almost 20% from ideal).

Olstyn I don't think compression can be calculated exactly without considering cam profiles i.e. overlap but I am sure someone on this board can set us both straight on that issue.

911 tweaks, for the safety collars on the pressure fed tensioners, you have to make them and they fit inside the tensioner piston bore. Do a search for "Jerry Woods Modification" and you will discover a nice writeup with pics on this site. Here is another link on the net:
http://www.911t.org/tech_tips/limiting_travel_of_the_pressure_.htm

BTW, It cost next to nothing to do it just a little attention to details.

More info on tensioner issue and pic
 

I don't know why intake valve no. 5 on right bank got tagged and is stuck down a bit from closed position; the valve lash has about 1/2 inch of up and down lash which is obviously way too much! I am in the process of dropping the engine and tranny ( I needed a good stout clutch anyway ) so I will update as I can - the engine is balanced to -0- grams so it will rev quickly but it is irritating to have to drop and repair because of a stuck or failed tensioner. I suppose it jumped time and tagged no. 5 valve.
Anyway, thanks for the feedback and give me more info if you have it!
thanks!
munchovie.
http://0waldo.com/images/76930.jpg

So as I understand it, the way the pressure fed tensioner works is:

-oil pressure is fed into the tensioner body via the banjo bolt fitting
-pressurized oil pushes upon the bottom of the piston to extend the piston outward and push on the idler arm, thus putting tension on the chain
-excess pressure is bled out of the additional hole in the tensioner body?
-what is the purpose of the hole in the piston? remove air pockets and also overpressure?
-the circlip is only needed to keep the assembly together, but can be done away with with the addition of the new spacer collar?
-it's amazingly simple how these pressure fed tensioners work??????

so what's the typical cause of failure? broken spring? otherwise, as long as there's oil pressure coming under the piston, they shouldn't fail. I guess the bore of the tensioner body or the piston itself could wear and let oil pressure escape? that doesn't seem very likely.....

The oil pressure doesn't really produce the tension. The tensioner extends due to a light spring, the oil flows in, and check valve keeps the oil from coming out so the tensioner can't collapse. I suppose eventually the tensioner would leak down without the oil line connected but it would take a while.

In recent years the quality of the new tensioners that have been available was not that great. I and others have had rand new ones that just wouldn't pump up. The manufactured said that people weren't bench priming them or some such but we never had to before... That was a few years ago, so things may have changed.

Re: The cause of the failure in failed tensioners: I think it is probably the check valve.

-Chris

Please review - thanks
 

Hello all and thanks for the info. on the tensioner issue.

Please go to http://930turbos.com/tensioner.htm and see the page I created there. I put some pictures and text concerning the dynamics of the tensioner and I need some feedback if it is correct. I woud post here but I'm not sure of just how to place the pictures and text in a tabular format.

I plan to drop the engine today and take apart the left tensioner and try to determine why it failed and will post pictures and results.

Thanks!
wunchovie.

It is my understanding you don't need to completly drop the engine to access the tensionsers, partilal drops wil do it.

On the subject of tensioners...... the aluminum spacer that goes on after the tensioner seemed to me to still allow a significant gap between the spacer and the tensioner cover. The gap apperaed to me to still be at least a quarter inch between the spacer and the cover. Is that correct? Even though it's a large gap which would allow the tensioner to walk around on the shaft is it still a small enough gap to keep the tensioner piston on the idler arm. It just seemed to me to be a pretty large gap which made me wonder why the spacer was not thicker.

The aluminum spacer is not required if you are using the later idler arms, which are highly recommended (and highly expensive).

Can someone please verify that the left cam turns clockwise? thanks! munchovie.

Would still like to hear about the idler arms. If they bound up on your shaft momentarily, they could have caused the symptoms you heard. May not be a tensioner issue. Hard to tell from your pics, but it looks like you have the early (narrow) arms and spacers.

I have my 2.7 down for a rebuild right now. I inspected the idler arm bores and they were badly galled - every time they bind on the shaft, that's a Very Bad Thing. Not sure if this is related or not, but my cam and layshaft sprockets were both worn worse than any I've ever seen.

to help the not very well experienced, "ME", can anyone here post what the "early" and "later" idler arms look like so I and I am sure, many others, can check what they have & may need updating.
Thanks! Bob

The cams do NOT turn clockwise.
The cams turn COUNTER-CLOCKWISE.
The crank turns clockwise. The fan turns clockwise.
The cams are driven through an intermediate gear driven by the crank shaft.
We all know when a gear on a shaft drives another gear on a driven shaft, the shafts turn opposite of each other.

Yep. Doug's right on. Hence the reason the cam chain shaft is called the intermediate shaft or sometimes the countershaft

Thanks for the verification of the direction of the cams; this changes the way I envision the idler wheel and tensioner action.

munchovie.

Me thinks someone spoke out of turn. No pun intended.;)