[rs911t]

On the 2nd to last lap of Sunday's last session. Coming up the straight to the roller coaster, the car suddenly began running rough and making a clattering noise. I was probably running 5500 rpm.

My first thought was an exhaust gasket - that's how it sounded. I slowed and immediately pulled into the paddock. With the engine opened, it was clearly a sporatic, crsip, metallic sound accompanied by some backfiring, like a missing cylinder. Tony noted to the missing cylinder and that the noise sounded "high", like metal-to-metal contact on a valve cover or the like.

I left VIR not feeling so good ...

Here's what I done to date:

(1) pulled the all covers and inspected the valves. No missing nuts or loose pieces. All the rockers are sound and it place.

(2) drained and inspected the engine oil. No pieces of metal and nothing visible in the sump screen.

(3) removed and inspected the plugs. All look just fine: no damage or oil.

(4) removed and inspected the distributor cap. Cap and rotor are intact.

(5) performed a leakdown test this morning. All cylinders between 1% and 4% leakage after sitting cold for two-weeks.

So, my question is this: what do I check next ?!?

[APKhaos]

Originally Posted by rs911t

All cylinders between 1% and 4% leakage after sitting cold for two-weeks.

Now thats the best news so far!

I'm still thinking that it was something around the valve train. Are all the rocker adjusters tight [inlets on the pass side bank seemed to be where it was coming from]?? I wonder if an adjuster spun loose enough to prevent the valve from opening fully, although its hard to figure how this could cause the noise we heard. Worth a check all the same.

So, for the engine gurus reading this, the noise was a high, crisp metal to metal at a rate related to 1/2 engine revs, which is why the first thought was valve train. Engine was missing on one cylinder, but otherwise sounded fine.

[Vicegrip]

Valve clearance. Leak down will be good as the valve is closed during testing.

Start with a valve adjust. If you find one way out of adjustment strongly suspect the rocker! Also. If you fond one out but not loose just from a loose adjuster nut let me know I just got a bore scope and it would be a good idea to see if Mr. Valve met Mr. Piston. The good leak down means it would not be bent but if it hit you might want to lower your rev limit and or test the clearance. Wont hurt to check the rocker arm pivot as well.

911s have also been known to eat the cam lobes up bit this is normally a pitting over time kind of fail. I did see a cam that shed the entire top of a lobe but it was one that had been (badly) welded up for more lift.

[OldTee]

Ummmmm. Put a magnet down each clyinder, gently. Bore scoping is good idea too. If this was a different type of engine I might suggest a lifter problem. A broken ring can, in rare instances, give a good differential compression reading. Note rings may be steel or chrome I don't know what Porsche uses. Then pull the chain covers and peak around. Look again at the plugs, with a magnifying glass. Keep us posted.
John

[Marc A]

You probably have already looked carefully, but check and make sure all of the elephant feet are where they belong. I have heard of them failing, which of course has an impact on valve clearance.

Good news on the leak down, now the fun begins.

[rs911t]

but check and make sure all of the elephant feet are where they belong.

At the end of their legs, opposite the body?? But you're talking about something in my engine, yes? Exactly what, I have no idea ...

[Trak Ratt]

In for a penny in for a pound. While tearing down a 2.2 last month I found a broken cam chain ramp. It broke right through one mounting stud & would “wobble” changing the tension on the chain. The broken peace bouncing around inside in the case sounded odd at best.

[Vicegrip]

The feet are the little pivots on the end of the valve ajusters.

[William Miller]

If you find marks on your pistons, another area to suspect is the guides and valve springs. The cam could relieve the pressure on the valve, but if a guide is bad, valve sticking or spring weak, the piston will hit the valve as the valve is trying to close. (At that point in the cycle the piston is chasing the valve. If the valve is too slow or sticks they will make contact.)

I rebuilt my engine last summer. I found marks on 2 pistons, one minor and one notable. Even though I herd no noise I believe there was intermittant contact with the intake valve. This resulted in the top ring on #4 wearing excessivly and ultimately breaking. I went from a great leak down results to having an engine miss. Matt Demaria identified the weak cylinder with his ocilliscope. I confirmed it with the leak down tester.
50% on that cylinder while others were in the upper 90%.

This engine was supposedly rebuilt a few years earlier. I know the PO took care of it. When I tore it apart I found the valve guides were toast.
I ended up buying new valves, guides, springs and rockers. A machine shop in Kensington did a great job on installing the guides and cutting the seats. Very reasonable price and Quality work.

A few observations:
Upon disassembly, I noticed that the valve seats had been replaced prior and that they seemed to be very high in comparison to the dome on the head. The valves protruded from the head more than I would guess would be normal. When the heads returned from the machine shop they looked normal as the seats were machined for the new valves.

Upon dry assembly and doing the valve clearance checks I noticed the valve clearance was a little tight. (Everything still stock including cams.)
This was based on the procedure in Wayne Dempsy's book. I was able to resolve this questionable clearance by timing the cams to the Euro specifications which is slightly retarded from the US SC specifications. (I understand the cam timing on the 84+ carerra is midway between these two.)

This process taught me how close the tollerances are.


A sticking valve and or weak spring at higher rpms could result in the noise you are hearing. Just hasn't caused any noticable dammage yet.
It may be more likely on the intake because of the way the cycle goes and there is less clearance than on the exhaust valves.

I wonder if you could isolate the noise with a stethescope. Certainly the borescope would tell the tale.

If you find a mark in one cylinder. You can hold the valve closed with compressed air and remove that spring. Then that valve guide could be checked by rocking the end of the valve stem. I guess somewhere there is a tool to check the spring tension.

I hope you find something simple. You may be very lucky that you caught it in time. I did sort of. I just had a broken ring. It could have been much worse.

I also have wayne's book if you want to barrow it.

[matt de maria]

Bill has some very good points. You are in a peculiar position on not being able to do more dynamic tests to diagnose the problem for fear of doing more damage to the engine. Consequently any testing will have to be static.
You mention backfiring coming from the engine. The car ran rough. I am assuming from this that there is a problem from combustion with one or more cylinders. (Did the idle decrease?) Also described was a sporadic tapping noise accompaning the backfire. Did this noise always precede the backfire. (Is there a coupling factor between the noise and backfire?)
I do not know the specs of the engine other than it is a 2.8. I assume we are talking about the 2.8 RS pistons with 11.0 CR? What is your induction system?
As everyone else mentioned, do a valve gap check. I have had (on an 86 carrera) valves (5) slightly bent to have only given a 5% leakdown. The only way I found out was that the valve gap opened up to .010" and borescoping found the indentations on the pistons. #6 was very bent.
Check for shifting rocker shafts. I had a few that the shaft drifted sideways and the rocker was partially supported. It failed on opening fully.
Check on broken valve springs. Normal opening pressure on stock springs is 95#. You can stick a screwdriver between retainer and rocker and pry gently to get a sense of valve spring tension.
Check for operation of chain tensioner by wiggling the crankshaft back and forth and note movement of camshaft. Bad tensioners will exhibit a dead band of movement from the camshaft.
Not knowing the specifics of the engine, could you have a partially clogged injector(s) or venturi(es) causing detonation on one or more cylinders?