Top-end rebuild - now I have a knock
 

Over the weekend I finally managed to get the engine back in the car. During the past several months I have replaced the P/C and had the heads done at a reputable shop. All of the parts were OEM. I did not split to case as the engine *only* has 125,000 miles on it.

On Sunday I started the car for the first time and broke in the engine for 25 minutes of moderate medium to medium high revs (2,000 - 3,500 RPM). Everything went like clockwork. The engine fired right up, ran smoothly, and sounded normal.

After changing the oil and filter, I took the car out for its first drive. After about a block of driving, I noticed a knock in the engine. No smoke was visible and the engine was still running smoothly with plenty of power. I parked the car overnight and had nightmares of what could possibly be wrong.

Today I started the car again and there was no knock. I carefully drove around my neighborhood while the engine warmed up. Sure enough, after 2-3 minutes of driving the knock was back. After lifting the car on jack stands, I crawled under the engine to see if I could tell where the noise was coming from. I believe it's originating on the passenger side. Could this be a valve out of adjustment? Please tell me everything is going to be OK. Right now I'm too scared to look.

Randy

uh o

valve out of adjustment is usually a loud tapping sound. Cut off a broom handle to about 2-3 ft and move it around the engine while holding other end against your ear. you should be able to isolate the loacation of the knock that way.

It didn't knock with the origingal filter, and it started after you changed the oil and filter (that's what is sounds like) 99.9 percent of the time when a trouble arises it's from what was most recently changed.

Does anybody else care to weigh in?

Randy:

Tapping/clicking sounds come from loose valves, worn guides, and worn rockers/shafts.

Knocking sounds come from the bottom end; rod bearings, wrist-pin bushings, wrist pins, intermediate shaft gears and bearings.

I would recommend using a stethoscope (either a broom handle or a long screwdriver) and listen all around the engine; top and bottom to isolate where the noise is coming from and what it sounds like.

Subscribing for information purposes.

While under the car, I used a piece of hose to try to isolate where the noise is coming from. Maybe I should try the broom handle as well. I'm pretty sure the noise is from the #5 or #6 cylinder near the head. No unusual noises were detected in or around the crank case. What's alarming is that it sounds more like a knock than a rocker arm tap.

Is there any way I could have messed up the cam timing so bad that I have piston to cylinder contact? The engine runs very smoothly and the noise only becomes pronounced when the engine warms up. This weekend I'll pull the valve covers and check the clearances.

Thanks
Randy

You said new P/C is the piston hitting the head??

If it was the other way around and would quiet up when hot I would say you had some piston slap. I have had exhaust leaks that made some very strange sounds before too.

Randy,

I'll second what Steve said above. To add a little detail from my own experience, I've seen the result of what can happen when a wrist pin circlip is not fully engaged in its groove. Don't ask me how I know. :)

The wrist pin can wander out and contact the cylinder wall. It definitely makes a knocking sound. It did not do so immediately; the knocking sound occurred after a few minutes of warm up. And the sound was not clearly from the top end, nor from the bottom end. That lesson, learned 25 years ago, still sticks with me.

It sounds like you have some diagnosis to do. I'd consider draining the oil and looking at it very carefully. Maybe open the filter and look inside. And as you had planned, you should remove the valve covers and scrutinize the valve train, valve timing, rocker shafts/arms and whatever else can be seen from there.

A knock is a very bad sound, though. I'm afraid you're going to be doing some disassembly.

Good luck and please keep us posted as to your findings.

Rob

Hose is only useful for tracking vacuum leaks,..........:)

If you didn't get the cam timing correct, you might get piston-to-valve contact. If the deck heights are off, one can have piston-to-head contact. Generally speaking, neither condition makes a "knocking" sound.

After you confirm valve clearances, use a long-handled screwdriver and place it all over the engine (top & bottom) while its running to listen. Have someone rev the engine a bit while you are doing that.

Rob's suggestion is excellent and I'd strongly recommend that you pull the oil filter and cut it open with the special tool (keeps the steel casing material from contaminating your efforts) and unfold the paper all the way out on a clean bench to look for bearing material. You may need someone who is well-versed in this to help perform the forensics.

Great question, but I don't think so. The P/C are stock Porsche and nothing hit when the engine was breaking in. In addition, when I turned the engine over on the stand, everything moved freely.

I'm going to try to further diagnose the problem this afternoon with the screwdriver as suggested.

Thanks to everybody for the advice.

Randy

I checked the valves this afternoon and they are all set correctly. Using the "long-handled screwdriver" method I've isolated the sound to the #5 cylinder. Looks like the engine is going to have to come out. :(

Randy

Bad news, sorry to hear. It is every home mechanics nightmare. But, look at it this way, you already have the experience and tools. It will go quicker than you think the second time around. Take some pictures and let us know.

Rob, you called it. Either the circlip came out or I never put one in. I haven't tried to fish around in the crankcase with a magnet to find it. Now I need to evaluate the rod, piston and cylinder and see what damage has been done.

I really can't imagine how this happened. When I assembled the engine, everything was triple checked. Somehow this one got by me. I reckon, I'll remember this for the next 25 years. Thanks for the good advice.

Randy

It is always nice to find and fix a problem before it turns into another project, huh?

Any thoughts on what will be re-used and what will have to be replaced with new parts?

Randy,

Bummer. Well, at least you know what it is. Actually, for a knocking sound, it's probably the least bad of all possible bad reasons for a knock. The big question, as you have already asked, is where-o-where did that circlip go. If it's in the engine somewhere, then you might feel obligated to fish it out. You might worry that it may do more damage as it goes through the engine's digestive tract. Can you tell if it was ever there? If it went between the piston and cylinder and into the crankcase (the worst case), then it may have left telltale swirlies. Likewise, if it went up into the combustion chamber, it may have left little hammering marks on the head or piston crown before it got spit into the exhaust (a better case).

The next question is whether there's significant damage to the piston and cylinder. In my case, it damaged the cylinder, though not as badly as I expected, and as I recall, the piston looked okay. Back then, when I was on a stricter budget, I replaced the P/C pair with a used one from the same cylinder height group. And it worked fine.

Good luck. My guess is that you won't have to split the case.

Rob

The circlip did not make it into the combustion chamber. If in fact it was installed, it's somewhere in the crankcase. I'll be fishing for it with a magnet tomorrow.

The irony is, I had marked this piston as suspect because although it was in spec, there was a gouge in the wrist pin journal. On the top of the piston was my black magic-marker "x" right above where the wrist pin slid out. It really makes me wonder if I ever installed the circlip.

I'll keep everyone posted whether or not I find the clip.

Randy

Most likely I will replace the piston because the wrist pin journal is a bit messed up. The Cylinder will clean up with a quick hone job. Other than the $50 in gaskets I'll have to replace, I plan to reuse everything else.

Randy

Well, the engine is back together and in the car. I never found the circlip after fishing around the engine case with three different magnets for hours. I really don't think I ever installed it. I'm not sure what makes my ego feel worse - not installing it at all or just installing it wrong.

After consulting with the local Porsche shop, they suggested I reuse the piston, cylinder, wrist pin and rings. Aside from a small grove in the cylinder from the wrist pin, everything looked OK. So far so good. The engine runs fine and there is no smoke or expensive knocking sounds. I guess the real test will be after 200 miles or so when everything finally breaks in.

My thanks to everyone who offered up suggestions and shared past experiences.

Randy

Well done to you and the posters that helped. If it makes you feel better, I just had to remove and reinstall my left side P&C's after I figured out I had installed them upside down. :eek: