996.1 Misfire Cylinder 5, Could be just Carbon Buildup

A few weeks ago, @Puddles posted in this thread that he lost compression in cylinder #5. https://rennlist.com/forums/996-foru...l#post20414767 I contacted him via PM and he came down to my shop when he was in town for the Rolex 24. After bringing it to two shops in his neck of the woods, he decided to bring it down to me for a look over. Jeff and his son brought the car down today, wow, what a beauty! I didn't think a non-aero car could look so good. Silver with black interior in FANTASTIC shape. I think the car has about 160K miles on it. Battery is now disconnected so I can't check. Anyways, I engage Grady, his son, as my new helper, we pulled the rear bumper, mufflers, coils packs and plugs out to scope the cylinders. He has a misfire (basically it does not fire at all) on cylinder 5. The compression on cylinder 5 is just about zero and all other cylinders have a nominal 150 PSI. Here they come! Put it up on the lift for the work, it's nice that this is what I see as I type away this post at my desk. Cylinder 5 is the first one we looked at. I recently bought a borescope that has an 180 degree articulating head, so in through the spark plug hole and then rotate is so it can look at the valves, how is that for a trick? As you can see, all four valves have A LOT of carbon build up. This was the first cylinder we looked at, so I did not know what to expect: We looked at cylinder 6 next, which was typical of all other cylinders, see how two of the four valves are clean and two are gunked up. I do not know if the clean ones are intake or exhaust, I'm sure somebody knows. The shop that he brought it to previously said that cylinder 2 was scored, so they condemned the motor. The valves looked to the same as cylinder 6, and after looking at the top and bottom of the cylinder walls from the spark plug hole (that's where the previous shop looked at) I see absolutely no bore scoring. The pictures they provided were terrible, they took a picture of a screen on an old borescope. You couldn't make anything out. My new borescope is really impressive. At first look, it appears that the valves have so much coke or carbon on them that they are no longer seating. It is a high mileage car. I'm going to do some more inspecting to look at the other cylinders. Jeff already spoke to Len Hoffman, we are going to share these pictures with him. But for now, we are thinking of dropping the motor, pulling the heads, sending them out to Hoffman and having them refurbished and then putting everything back together. Jeff bought this car because of the shape it's in and the huge stack of no expenses spared on maintenance including a new LN IMS bearing installed at the Porsche dealership at about 140K miles. So we don't think we won't need to do $$$$$ of "why we are in there" items. We are trying to be budget minded. I did do a leakdown on the three cylinders above, and they were all about 20%, even number 5, and the leaking air could be felt at the exhaust for all cylinders tested. We welcome comments on the valve conditions and how it could related to low compression on cylinder 5 or any other comments! Looking forward to sharing the story on this one.

Cil 6, 2nd photo shows the exhaust valves at 6 and 9 o'clock. Cil 5, 1st photo shows them at 4:30 and 7:30 on the clock. Typically the exhaust valves will have a dull light grey (ash) colour and are the smaller ones, whereas the intake valves tend to show more black deposits. Cil. 5 shows a lot of wetness , either from oil or fuel (or both). See if you can scope through the intake with cil 5 at tdc on exhaust stroke, I.e. during valve overlap and see if you can spot carbon build-up to such an extent it would prevent valve from closing. However, I believe it is more likely that valves in cil 5 are bent. Edit: just read in the linked post, that indeed that is the case. 0 compression whilst leakdown is 20% begs to run the tests again on cil 5 and on the others too, really.

There is no way to get zero compression AND 20% leak down. BTW, nice pictures, what scope is that?

This is an example of carbon build up leading to zero compression. The carbon builds up on the back side of the valve, preventing the valve from touching the head fully. The heat from combustion cannot escape to the head and therefore the valve burns up.

This is a bent valve

Excellent report, I look forward to the next one!

Thanks everybody for the input! I did not perform the compression testing, but I do have videos of each of the cylinders being tested at the previous shop, so I believe it. I did start the car after we got it off the trailer so I could get the cam deviations, but my PC would not play nice with Durametric. So it ran for about 30-45 seconds before it was shut off, so it's probably fuel in cylinder 5, last night I was thinking about that, why is there so much oil (now I think it's fuel) in cylinder 5. The scope I got was this one: Amazon Amazon If you can't see the link (sometimes Amazon links don't work on rennlist for me), it's $129, the TD450S Articulating Borescope with Light

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@yelcab That picture speaks a thousand words! It is really easy to visualize what could be going on. Len Hoffman also mentioned that a valve could be bent. Appreciate it!

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Yea, I tested it with the engine in the wrong position, 360 degrees out. It's been a while since I did a leakdown test and the pistons would move when I did have it in the right position (did not have crank locked by any means), I will do leakdown testing on all cylinders this week. That will tell the story. Thanks.

These probably are the marks left by the piston. Valves turn during operation, that's why they don't line up, in case you didn't know. How could you miss the owner's report that the piston had hit the valves?

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Paul was very generous with his time and knowledge on Saturday. My son and I had great time and we learned more about what is happening with the the car over a few hours in Paul's shop than over three months with the "Pros." Any input the board can offer is welcome to help Paul get my girl back on the road. She is a beautiful car and worth saving.

Whether it's a bent valve or a carbon-caked valve, the solution is the same. Heads off, valve job, heads back on, engine in. At this point, i would not bother with more leak down test.

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So, the valve is bent due to a chunk of carbon becoming wedged between the valve and head? Using Top Tier fuel such as Shell, Mobil, BP, et al plus additing a fuel injection cleaner such as Techron, Red Line or Driven should clean the carbon from the valves, according to the literature and wisdom found on the interweb. Apparently, using cheap fuel results in carbon buildup such as this. Tangent: Many years ago, I owned a new 1983 BMW 733i with a five speed manual. I loved to hear that big six sing and regularly ran it to redline. After only a few thousand miles, it developed a high RPM miss. The dealer and factory rep could not figure out why so they pulled the head. They found carbon build-up on the underside of the intake valves. (Not the exhaust valves) The theory is that unburnt fuel clings to the underside of the intake valve, eventually turns to carbon and the carbon heats up until it glows and ignites the fuel charge before it can enter the combustion chamber, causing preignition. They cleaned the head by blasting it with walnut shells.and told me to use Techron. The miss, by the way, did not go away.

I find it very unlikely that a piece of carbon would be causing that. A, two actually, valves stuck in the guide or broken valve springs are other possibilities. The fact that both exhaust valves are bent says something towards the cause but I don't know what. Further analysis needed during and post head removal to ensure it won't happen again.

There could be several explanations as to why it's sticking open. The top of the piston looks fine so there was no connection between valve and piston. There is no way a chunk of carbon would do that. Does the car have spark in #5? The build up is from the gas & oil not firing so I wouldn't really worry about that. A broken valve spring would certainly cause it to remain open but would also drop the valve into the cylinder. I think it's slightly bent

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Nope, I did not say that the valve is bent due to carbon deposit.

Well, sorry to say, while a cause at this point with what we know/see on this thread is impossible, can say without a doubt this has nothing to do with carbon build up. As 'Yelcab" said above, no matter what the cause only thing to do now is "Heads off" to start with... You could do a DME report of over-revs. As a valve float from a 'money shift' could be the cause if nothing else shows up after tear down... While some/many think a head repair on a 160,000 mile m96 engine is acceptable, I strongly disagree as would most engine builders that know the m96. Top end job on high mile m96 is kind of like doing the imsb on a 80k car and not doing the clutch/rms/throw out bearing/ flywheel. Imagine worst case, the not so uncommon possibility of months later throwing a rod or D-chunking a cylinder, etc, and destroying your recent $$$$ spent on Len Hoffman head job repair for $$$$. Or the best case, engine starts burning oil badly/or rod knocking, from bad oil pressure instability, or way way out of round cylinders/worn bearings that always occur on high mile m96 engines . So now its time to start again from scratch, engine back out, engine heads off, engine totally torn down. Yes, I know there are some' guys here running around with good running high mile m96, keep in mind the % of high mlle m96 with bad engine problems is very very high. Every week/month we see a thread or 2 or 3 about some type of m96 engine failure. Nope, to me it doesn't make any sense financially or time wise to just do the heads on a m96 with 160,000 miles,,, unless one just wants a quick fix to dump the car with a running engine that just got a "top end rebuild". A 160,000 mile 996/M96 owes its owner absolutely nothing, its done its job very nicely if it lasted that long, as many many m96 fail way way before 160K. . If you're in this for the duration, its time to do it right or do it twice... jmnsho

So I did more testing today. I did leakdown and compression testing on all cylinders. Initial leakdown testing was a maximum of 8% for all cylinders, cylinder number 5 (misfiring problem child) was 5%. That threw me. Compression testing was good for all but cylinder 5, it was 150 PSI after the first or second crank and built up from there. I then did multiple compression tests of cylinder 5, once it built to 120 PSI, but most of them were less than 30 PSI. When it did build pressure, the pressure would just bleed off within about 30 seconds down to 30 PSI. I then repeated the leakdown test on cylinder 5, it was 80%!!! I rotated the engine two turns, and it returned to 5%. Here are the results: My 71 year old friend was over, he was a mechanic his whole life, he suggested it is a lifter issue. I also called Len Hoffman and he thought it could be a lifter or potentially a broken valve spring. The real issue is how far do you go down the rabbit hole with replacing items, if it was indeed a lifter, you could replace that pretty inexpensively and put the motor back in. Or do you refurbish the heads, or do you go all in? It's not my money, so I can't make the decision. Interesting issue though.

Once you pull the cams from the engine and the lifters, you will see the height of the Valve stems and compare to the others, you will know what your next step is. Your still dealing with a motor with 160K miles on it, have ask yourself are you keeping it or selling it.

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Nope, you did not.

Well, the engine is out. Next up, pull the cam cover and inspect the #5 cylinder lifters. Just call me Hernia Man!

Well, the jury is in! Broken valve spring on one of the two intake valves. I was hoping it was a stuck lifter, but it's not. So the heads will have to come off and be refurbished. Will have to talk to Puddles about the final decision.

You don't need to remove the head to replace the valve spring..... Feed a bunch of 5/16 rope into the cylinder and rotate the engine till the piston/rope holds the valve up......Then get a tool (or fashion one ) that will depress the valve retainer to remove the keepers...then reverse the process with new spring....

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Brilliant! Love learning new techniques from the Porsche brain trust!

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Was thinking the same thing! Another option; they even sell adapters that allow you to put compressed air from a compressior in the cilinder through the spark plug hole... but rope is simpler and more foolproof. Obviously doesn't hurt to remove the heads either, gives you some 'while you're at it options'.

Yea, as soon as I posted the above, I realized I could just replace the spring. Luckily my neighbor knows another guy who works on Porsche's and he called him and we went over to his shop today and picked up a few springs and keepers. Then my neighbor helped me pull the spring and put the new one in. Definitely need four hands when it's still assembled. I had a valve compressor tool that I set up and used my leakdown tester with the engine locked to keep the valve in place during the process. This cylinder has about a 4% leakdown. Removed the old spring: This is the other half of the compressor tool and the broken and "new" (i.e. used) spring: New spring installed: Broken spring, looks like it might of started to break vertically and then just ripped horizontally. The two pieces fit perfectly together and the broken part was captured in the large part that was still installed: Have the lifter carrier, lifters, and cams back in. Now to order all the needed parts to get it back together.

Based on the 4% leakdown I understand that none of the valves are bent? that is very positive. I've used the rope tip successfully on an engine since compressed air in the cilinder at tdc was not enough to hold the valve closed and only compress the spring to release the collets. The valve always opened.

I do not believe the valve is bent. Once I get it back together, I plan on doing a compression test on the hydraulic table before installing it. That will tell the story.

So the car is finished. In the end, it was a broken valve spring that was the culprit. Did a bunch of other items while we were in. Test drove the car about 55 miles over the last two days, have to say it runs like a TOP! 160K miles to boot! There is absolutely no bore scoring in the engine, runs so good, this is the first time I drove a throttle cable car, I like the long accelerator pedal travel. Stock exhaust, which I also enjoy, I have the Fister. Both are awesome. I feel the torque is slightly better on the stock exhaust. Things done:Spark Plugs Oil, Filter, Crush Washer Serpentine Belt Water Pump LN Low Temp Thermostat 996.2 Coolant Expansion Tank Fuel Filter (old filter had 2014 date code on it) Power Steering Fluid New Oil Cooler AOS with Ultimate AOS Transmission Mount Replacement Rear Rotor and Pads I'm sure there is more. In the end, I kept track of my time. I spent a lot of time troubleshooting and understanding the issue, had not done an engine out or a re-time of a 996 motor before, so I went slow and methodical. I spent 85 hours total doing the repair and replacement items. Really enjoyed it. Here is the as-left compression testing after replacing the valve spring on cylinder 5 intake, I did this before I put the engine back in the car. I'm happy with the as-left cam deviations: She's getting picked up today, just cleaning her up now. My new friend....

Wonderful job !! ... I want to give credit where credit is due ...A broken valve spring has fooled many a full time technician that cased them to condemn the engine pre-maturely!!! Congrats on the precise troubleshooting !!! It saved a lot of what I would call unnecessary work ...

Glad you found the problem, my only concern would be where there's one broken valve spring... another may not be too far behind. With the engine completely removed I think replacing them all would have been a good investment? Maybe it's just my OCD taking over!

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I hear you, but it would take so much effort to replace all valve springs with the heads still on. It took about an hour to get the one spring replaced with my friend present who is was a mechanic for 40 years. I'm sure the others would of gone quicker, but I think if you sent the heads off for refurbishment, then you would consider replacing all of them. About $850 for parts alone.

I want to thank Paul for saving the day and getting her back on the road. I took the car to two well known shops before Paul found me here on Rennlist; both of those shops simply condemned the engine. Notably, one told me that Cylinder #2 had horrendous bore scoring, which is completely inaccurate. The car runs great and I'm already planning a few trips with her this summer. Interestingly, when I was going through the car's service records I found a service record at 146K miles when the prior owner brought the car in for service due to a "misfire and lifter tick noise on cylinder #5 due to a collapsed lifter." During that service the prior owner had all the lifter's replaced. I would think that the bad lifter in cylinder #5 negatively affected the spring to the point of its failure at 160k miles? In any event, she runs great again, thanks Paul!

Hooray for Paul!

I can easily see that the bad lifter and broken valve spring are linked. It supports your decision of keeping all other springs, even though that was based on other considerations. Congrats and enjoy her!

Happy to hear a positive conclusion to what could have been a horror story!

Don't know who Paul is in here, but nevertheless a great guy. Very satisfying thread. Good job