jcorenman

Martin,

Everything on your report is right in line with what I've seen, except the silicon. Your lead (zero) is suspiciously low, but with 2.4K on the oil I think that just means it is below measurable limits. We usually see around 1 ppm per 1K miles. Iron is typical, which means there are no major steel parts being ground into oblivion (assuming the oil didn't have time to settle when the sample was taken). Aluminum (cylinder bores, cam bearings) is right in line, we see 3-6 ppm. Sodium likely is an additive, it is not common but we did see 380 in a Brad Penn sample.

OK, silicon: There are three possible sources as far as I know: poor air filtration (sand is silicon), the Alusil engine block (e.g. cylinder bores), and silicone-based sealing products (RTV, etc). I think air filtration can be ruled out absent any unusual aluminum from the cylinder walls, and the same with silicon from any block wear-- Alusil is mostly aluminum which would be showing up in large quantities if there was any wear of the cylinder bores.

We saw a silicon spike of 39 ppm once, years ago, after I resealed an oil-separator under the oil-filler with RTV and then ran the engine before it had a chance to cure. That did no harm, just showed up on the next Blackstone report.

So your 53 ppm for silicon is significant. It could be the new oil-pan gasket, I didn't see that but maybe we missed that oil sample, or wiped the gasket down with something different. I think the tensioner pads are nylon, but I don't know what else might be in the plastic mix.

Bottom line, this looks completely healthy except for the silicon. It may be completely innocent (e.g. OP gasket), or might be pointing to something. Maybe what we need to do is grind up some tensioner-pad into an oil sample and send it to Blackstone

So does an 86.5 have the test pipes? I thought that was a '87-88 thing, but a corroded test-pipe would be a good candidate for an exhaust leak. They lead from the top of the engine (either side of the intake on an '87-88) down to a fitting just upstream of the flange on the exhaust manifold. The simplest way to plug them is with a suitable bearing-ball and the original flare nut.

Jfrahm

PET shows a sample tube on the 1986 but I do not know for sure if one is fitted. Part 92811114903 , looks to be only on the RH bank (coolant tank side). I had an 86.0 but that part did not make itself known during my tenure.

M. Requin

Yes, both test tubes are installed on my 86.5. I had to spend the morning getting a new wifi router, and now farm chores are up, but I'll get on these leads ASAP.

Thanks again, gentlemen!

M. Requin

The problems I am having have resolved into, I think, two separate issues- noise and low compression.

I have spent some time going over the exhaust system, with no joy. But going over the belt drives, it was immediately apparent that the air pump is totally trashed. It's out of the car and I'll do a PM this weekend. And thanks to this post https://rennlist.com/forums/928-foru...p-failure.html by khalloudy way back in '09, I am pretty sure it has been the source of the noise. I also think this is related to the TB tracking forward as the bearings are very loose.

The other issue also involves the TB, and I think relates to the low compression on the PS: the belt is one half a tooth off. Considering how well it has been running until very recently, this is pretty surprising. As noted in my first post the belt doesn't have that many miles on it, and the sprockets are in fair condition (although I plan on replacing them). Unfortunately, to me it seems that this points to the timing chain tensioner.

Any thoughts?

jcorenman

The noise can certainly be the air pump. Have you run the engine without it, to confirm that the noise is gone?

I don't see a connection between the air pump and the belt tracking, however. Belt-tracking issues are usually water-pump bearings, bent shoulder-bolts (pre-87) or worn bushings on the tensioner arm.

Half a tooth off, how?? Are you judging by the marks on the cam sprockets? Are the covers off, or are you peeking into the vent-holes? Do you think this is recent, or could it have been off since the belt was last changed?

Your compression difference side-to-side is consistent with the cam timing being half a tooth off-- and not enough to be particularly noticeable. I am not seeing anything that points specifically to timing-chain tensioner issues.

Certainly diving into the TB make sense, at least to check everything (and change sprockets if you want). You mentioned getting rid of the shoulder-bolt, I assume that means new WP and S4 tensioner parts? Be sure to replace the tensioner-arm bushings and check everything else carefully.

M. Requin

I have not run the engine since removing all the accessory belts and TB covers, but it would probably be a good idea.

That was a brain f*rt - please ignore.

Yes, the covers are off, and I am going by the marks on the sprockets. And I think this is recent. Every time I have the air tubes out I check belt tracking, and that has been a number of times in the past two months.

I have not yet decided to go the S4 mods route, my Laso WP seems to be fine, but everything else necessary to rebuild the tensioner sytem will be done. The next WP will see the accompanying mods, certainly.

Thanks for your comments- it is really helpful to be able to bounce ideas back and forth with knowledgeable types like yourself - my tendency is to get too caught up in worst-case scenarios and this helps prevent that.

Captain_Slow

Martin -
OK..your post #21 brought this closer to home for me (I never had the noise issues you describe...so I've just been following along). I had an 86.5 throw the TB warning light moments after starting the engine. I shut it down immediately. Then stupidly started it again...heard a scraping noise...shut it down and called USAA and had it flat bedded to Chapman Autowerks. I looked in the inspection holes and saw the forward edge of the belt was right on the forward edges of the cam pulleys. I saw some belt shavings. Later, at Taylor Chapman's shop, I would see a LOT of belt shavings. The belt did not break. The water pump was fine (but it got replaced anyway). But Taylor is one of the few mechanics who knows 928s very well. He noticed the belt had jumped a tooth. He did a compression check on the right side (where the belt skipped) and found no problems.

He found the shoulder bolt had backed out and then bent due to belt tension. He said the shoulder bolt is a weak design and suggested buying the S4 parts, replacing the arm bushings and rebuilding the tensioner (mainly to make sure it was assembled correctly).

Is it possible the noise was the belt intermittently rubbing on the belt cover(s)? Have you noticed any belt shavings or evidence of rubbing on the forward edge of the belt?

M. Requin

I was very concerned about that, even though it appeared that the TB was close to, but not on, the edge of the sprocket, and when I removed the covers, I found that to be the case, as you can see here:




But definitely too close for comfort, and I'm sure it means the shoulder bolt is bent. I haven't gotten to this yet - my next step is to take apart the air pump. I am almost 100% positive one or both vanes are broken, and the bearings are toast. I'll post pics of the carnage shortly.

Mrmerlin

FWIW where you want to look for poor alignment is directly in back of the damper.

If the tensioner pivot bolt or bushings are worn then this will let the tensioner arm tilt,

and then the bearing wont be in position,thus it will tilt, and the belt will begin to ride into the back of the damper .

This condition will slowly grind away the belt edge and make lots of little rubber bits.

as a check to this condition remove the aircleaner and on a white surface tap out the dirty side see if you find lots of rubber bits if you dont then your belt run is most probably working as it should

the flyin' scotsman

For the 'chuffing' noise I'd check the air pump diverter valve and the pipe from the exhaust system. I removed the air pump off the S4 but not the diverter and the exhaust pulses flowed upward. Possible source of increased air into the system causing the over-rich condition? I resolved my 'chuff' by replacing the OEM cats with an X pipe and removing all air pump plumbing. I also removed the test pipes and plugged the exhaust manifold with suitable cap nuts.

For the cam timing, buy or borrow the Porken 32v and bump stick tools........you'll the get the timing perfect..........I do favour the Porken t.belt tensioner.

I have replaced the cam chain tensioners; not that my car has as many miles as yours but I believe it is a wear item through time also......27 years for my S4.

good luck

Cosmo Kramer

With the belt that far forward and the noise have you checked the water pump?

M. Requin

Well, the post mortem revealed two problems- one was the air pump, and the other belt related.

The air pump:




to the left of the vanes you can see two spring loaded graphite guides, one of which is trashed. That problem solved, although I will follow up on Malcolm's note about the diverter valve (thanks for chiming in!).

The forward displacement of the TB on the sprockets was caused, as it often is, by a bent tensioner shoulder bolt. The WP looks and feels fine, the belt still has somewhat legible markings, and the sprockets are fine as well:




Not bad for a used sprocket from 928 International, fitted some years and thousands of miles ago!

I'm replacing the air pump with used, fitting new shoulder bolt and bushings, and taking care of a few WYAIT items as well. When I am absolutely sure I am back to 100% I will change the title of the thread to reflect that. Until then, thanks to you all - an absolutely magnificent and indispensable crew - and although I cannot send each and every one of you a good Islay malt, I'll be sure to drink at least one bottle myself in your names over the holiday season, so I just hope a Very Merry Christmas from me will suffice!

Martin

mark kibort

i just ran mine hard after the belt change for 30 mins. the belt is 1/4" off the front of the sprockets. same shoulder bolt, albeit a little over tensioned.

you will know if your shoulder bolt is bad... but if it hasnt been changed, i would change it.... mine was new 8 years ago and felt perfect and was not bent. but, you never know......
unless there is an easy way to swap it out, its staying in there. hoping it wont break!

the flyin' scotsman

gr8 to read you found the issues and the repairs wont be that much of an issue.

Glenfiddich is my choice so shall hoist one back in your direction......cheers

M. Requin

Not a bad choice at all!

Man, 1/4" !? You must be running without belt covers In my limited experience, it only takes a bit for a flat belt to go off center. I haven't done the trig on the shoulder bolt (good subject for that other thread of yours?) but it's not much of an angle. Gonna replace anyway!