ChuckUFarley

UPDATE July 18 2021: I will post a new reply with what I found after removing the cam cover (see post #11). Spoiler: It's not the IMS sprocket that is the problem! Exhaust cam is broken.

I have this posted on the 986 Forum as a response to a thread there but thought it might be useful here too. The car is a '99 C4.

I just got done with a full rebuild on my '99 C4 over the past few months going from 3.4 to 3.8 Nickies, and last Sunday 7/9 started it up for the first time. It ran great for about 7 minutes, then suddenly ran extremely roughly. Shut it down immediately. After pulling the cam plugs on both banks and rotating the crank slowly by hand, I found that Bank 2 cams were rotating as expected but Bank 1 cams were not. During the rebuild I pinned the main sprocket but did not weld or pin via setscrew the two smaller sprockets that connect to the exhaust cams. I strongly suspect that the Bank 1 sprocket (the one right next to the large sprocket as shown in the photo below) has come loose and is not turning with the shaft. The chain is not broken, confirmed by removing the tensioner on the bottom of Bank 1 and being able to put tension on the chain.
One hell of a gut punch after the amount of work that went into this, but at this point I know my way around the engine enough that it should go much easier this time around. This of course means a complete teardown and re-do unless someone else has any ideas.
The somewhat misleading part was that the Bank 1 cams (the side that does not move) came to rest at exactly where they should be for Cyl 1 TDC, which was highly confusing for a few minutes until I watched the Bank 1 exhaust cam for rotation while turning (and they didn't move at all). I wonder if the pistons "helped" to turn the valvetrain back to TDC position for the few rotations...by pushing back against the valves. Ugh. Should be interesting to see all of the collateral damage once I start pulling it apart.
Pin or weld your sprockets, people! Learn from my mistake.

Any thoughts on re-using this IMS, if it is indeed true that the Bank 1 sprocket is loose? Meaning, weld or pin the two small sprockets on the shaft I have, vs buy a replacement and do the same? I know runout will need to be checked, I believe 0.003" is the working limit. It was ok when I built this originally.

I'm curious to hear if anyone else has experienced this first-hand.

Pin in main sprocket:

U

dougn

Oh man! I/we await the diagnosis

Porschetech3

I guess you also made sure the exhaust cam/gear bolts hadn't came loose?

ChuckUFarley

The cam sprocket bolts are definitely still tight, especially since I can turn the crank as many times as I want and the Bank 1 exhaust cam doesn't move a bit.

And by the way, many on RL already know this but I just want to give a special thank you to Skip AKA Porschetech3 for helping me out when I was still scratching my head on this, he's a gem of a guy who we should all be thankful is amongst us (and puts up with our questions with grace).

Weazer

I second your comment regarding Porschetech3, we are lucky to have him. Great advice in your post addressing IMS and cam sprockets. Definitely an opportunity to learn from another's misfortune and avoid a known failure point with these engines, much appreciated.

hatchetf15

OP - Sorry to read about your setback. If you reach out to LN Engineering, I bet they have some data on the IMS troubles they see during all their work on M96 and M97 IMS.

ChuckUFarley

@hatchetf15 Yes, I definitely plan on talking with LN, they've been great throughout the rebuild, supporting the block machining and another mishap I had along the way. I know they offer a IMS reconditioning service but am somewhat surprised that they don't appear to pin or weld the small sprockets either (maybe I'm wrong). That was one of the cues that I took as to why I didn't do mine. That, plus watching other rebuilds through the process where they didn't do theirs either. I'd sure be curious to hear how many out there did indeed stabilize the small sprockets during the rebuild process. I saw the Flat6 video talking through the issue and showing their weld fix, so they obviously do it.

dougn

I have to believe if there was destruction due to piston/valve contact, it would have made a terrible mechanical racket. I once mistimed a big dirtbike and it sounded awful but amazingly did no damage. The valves just kissed the piston. I know its unbelievable

Porschetech3

Thank you for the positive comments, I really enjoy helping someone with a big problem that they have shown they have spent considerable time and effort on. And the positive comment about grace was a very welcomed and pleasant surprise !!

Dharn55

As I remember from my discussion with LN they only pin the large sprocket that has the chain from the IMS to the crankshaft.

ChuckUFarley

Something wasn't sitting right about the idea of the IMS sprocket slipping...153k miles running fine, now suddenly slipping after a rebuild? And slipping completely, like the cams not budging one bit, and much less resistance when turning the crank by hand. So I thought I didn't have much to lose by taking a deeper look inside the Bank 1 valvetrain, instead of removing the engine and tearing everything down, and pulled off the valve cover this morning. Glad I did.

It appears that the exhaust cam has broken at the flange where the cam sprocket bolts to. Check the video out below.

So I ate lunch and thought about things for awhile. What would cause a cam to break? Too much load, either torsional or bending. It's designed for torsional loads and being tubular in cross section it's pretty strong this way. Bending...not so much. So what could cause a high bending load?

Me, that's what. So I started looking through assembly photos and remembered something that happened. I replaced the exhaust cam sprockets with the latest revision, which is one piece instead of the two piece. Unbeknownst to me at the time of assembly, the old bolts are 3mm longer than the new bolts (M6x15 old, M6x12 new) because the sprocket is different, one piece instead of two piece and therefore thinner. When I installed the very first bolt on the Bank 1 cam sprocket, I clearly recall things not feeling right, and seeing looking at the back side of the cam flange seeing the bolt protruding through the flange and touching the head casting. It was one of the big pucker moments, but I thought at the time "good thing I didn't torque that too much." I bought the correct M6x12 bolts for the new sprockets and moved on.

What I am 99% sure happened is that by imparting a side load on the cam flange (bending load) I cracked the exhaust cam right at the root of the flange. I didn't break it completely, just an incipient crack. I got through timing and probably 10-15 rotations by hand of the engine without it breaking. The cyclic loads for the first 7 minutes of running fatigued the crack until complete failure.

Pictures below. I can even see the mark left on the head casting by the sprocket bolt, so I must have put a decent amount of torque on it.

Lessons learned:
1. Being a first-timer, part revisions can be tricky to make sure all of the associated parts are accounted for. Looking back, I see several people (Mr. Raby for example in the video set...which is indispensable for this project BTW) saying "get the new sprockets AND THE NEW BOLTS" (emphasis mine) but I didn't listen.
2. When this happened during assembly, I should have thought about the brittle nature of the cam and stopped and inspected the cam for cracks. Magnaflux, dye pen etc. I was too excited and didn't want to slow down or stop.
2. I'm glad I didn't just start dropping the engine and tearing into things (split the case etc). Stopping and thinking a bit paid off.

So in general I'm satisfied. Yes, a stupid mistake but I found root cause and it makes sense. This was one of those "hard to detect, high likelihood of failure with catastrophic results" risks that we all hate.
But at least it wasn't the IMS and I don't need to go there!

So now the search is on for a new or used good cam. 222.55/3.2/3.4EX1-3, full P/N 99610522255. Anyone got one of those laying around that they'd be willing to part with?

Also, has anyone successfully installed and timed Bank 1 with the engine in the car? I got everything out, but the oil separator pump is very close to the subframe. If I have to remove the engine, so be it but it sure would be better if I didn't have to.

Thanks for listening and being a good resource for this amateur Porsche engine builder. Sorry if I misled anyone with my initial diagnosis but it was a process of discovery for me and I dragged you all along for the ride.

Dharn55

I think I have one of these cams. Let me take a look and I will PM you.

dougn

what position are all the valves in? I mean was there any interference?

ChuckUFarley

@dougn , the valves on Bank 1 were exactly in the Cyl 1 TDC position, but I am sure they were 'helped' along to that point as the chances of that naturally happening when the cam broke are pretty low. I plan on doing a leak-down tonight to see if I have any bent valves.

In fact, the cams are out at the moment so all of the valves are at rest (closed).

Dharn55

Here are some pictures of the cam I have.