My buddy was so jazzed by my new car (87) that I bought a few weeks ago, that he went out and bought an '88 that I had passed up in my search because it had an auto and I really prefer a manual. Anyway, We decided to put them through their paces together out in a rural spot just to see.... After a little fun we pulled over and got out to chit chat a bit and when I got near his car it had the sound my old 633CSi made when the timing chain tensioner went south coming from under the hood. After lifting the hood we pinpointed the sound to the passenger side cams cover about in the center. I have never seen the guts in person but I thought the exhaust cam is driven by a chain. I could be wrong. Is there a tensioner in their too?
Are they rebuildable? Scared the crap out of us
both. I because I came darn close to buying the car and my buddy because
he did. Is it possible for the chain to skip a tooth and cause severe
problems if ran when the chain is loose? Is this another weak point? He
said he had been trying to keep up with me (which he was doing) and I was
running like a scalded cat at about 6k in 4th. He said he was in third
(auto)at close to redline for a brief time before we slowed and stopped.
After we shut it down and debated for about ten minutes on what to do, he started it up and it sounded fine. Kept to low revs and now is scared to drive it. Any advice that I could pass along as he has no computer? Thanks in advance.
The timing belt drives the exhaust cams, and the timing chains drive the intake cams. There are spring-loaded hydraulic tensioners for the cam chains. While anything is possible, the timing chains and tensioners don't usually give problems on a 928. Another possibility is air in the oil circuits causing collapse of a couple of lifters, one or more lifters sticking because the engine hadn't been run hard in a long time, etc., etc.
Only you and your buddy can diagnose from the sound.
Only other choices are buying the manuals and starting disassembly, or just changing the oil and driving it.
Had this problem with my 85. The chain tensioner has a pressure valve
which, on mine, consisted of a spring loaded ball in a hollow bolt in
the chain tensioner oil line. Commonly, the spring gets weak and
doesn't hold pressure. The hotter it gets, the thinner the oil and less
pressure is held in the tensioner--this causes it to allow the chain
tensioner piston to slap up and down causing a horrible racket at idle.
I replaced the pressure valve bolts--I think they were $12 each and that fixed the noise on my 85. If this is the problem the sound will disappear when you rev the engine because the pressure builds up, unless your tensioner pressure valves are completely shot.
Earl Gillstrom reminded me of another possibility. The camshafts were
changed, eliminating the rear journal, leaving open oiling holes in the
rear bearing saddles. As a result, some '87s and '88s had plastic plugs
blocking those holes. If the plugs are forced out by high oil pressure on a
hot engine, or excessive pressure on a cold engine, the open oiling holes
will drastically lower the idle oil pressure.
The fix is to install later metal plugs to block the holes more reliably.
While getting the cam covers off isn't easy, it is easier than replacing the chain tensioners.
See http://www.nichols.nu/tip578.htm for more details.