996 3.4 C4 Tip – zero compression / suspected burnt valve. Repair or sell?

I’d be grateful for some advice from those with experience of both the engines and the economics of repairing early 996s. I have a 2000 996 Carrera 4 Tiptronic, now on approximately 150,000 miles. It has been serviced every year and has had oil changes every 6,000 miles. The car has an interesting engine history. From memory, Porsche replaced the original engine at around 70,000 miles, and that replacement engine subsequently received a full Hartech rebuild at around 80,000 miles. My daughter has been using the car for the last five years and, until recently, it had been running well. The latest problem appeared quite suddenly. She had been driving for about an hour without fault and both the oil and coolant temperatures were normal. The CEL then came on and the car started misfiring. She drove approximately another five miles to reach a service station on the dual carriageway and called me. I knew there was a cheap code reader in the passenger door pocket, so she plugged it in. It showed: P0301 – cylinder 1 misfire P0302 – cylinder 2 misfire P0304 – cylinder 4 misfire P0305 – cylinder 5 misfire P0306 – cylinder 6 misfire So, oddly, every cylinder except No. 3 recorded a misfire, across both banks. There were no other fault codes. The garage has subsequently carried out a compression test and found zero compression on cylinder 4. Their current suspicion is a burnt exhaust valve. They say it can be repaired, but obviously until it is investigated further/opened up it’s difficult to know precisely what will be found or how large the bill might become. There are really two questions I’m trying to answer. Firstly, the technical one. Would a burnt exhaust valve/zero compression on No. 4 satisfactorily explain the sudden misfires being recorded on cylinders 1, 2, 4, 5 and 6? Or, before authorising significant engine work, should I be asking them to establish whether there was another underlying problem which caused the multiple misfires and perhaps contributed to the valve failure? I’d be particularly interested to hear from anyone who has had a zero-compression/burnt-valve problem on a 3.4. Once investigated, did it remain a relatively contained cylinder-head repair or did it turn into a much larger rebuild? Secondly, the economics. I don’t actually need the car and realistically I’m unlikely to use it very much even if I have it repaired. My daughter has been using it, but she’s getting married next year, so I have another fairly substantial drain on the motoring budget approaching! At 150,000 miles, and being a C4 Tip rather than one of the particularly valuable 996 variants, I’m conscious that I could spend many thousands on the engine without adding anything like the same amount to the value of the car. On the other hand, it seems a shame to give up on an engine which has previously had a proper Hartech rebuild, particularly if the present problem really is confined to a burnt valve/head work rather than something more fundamental. My rough thinking is that the car might be worth around £11k repaired. If this turns out to be a £3–5k repair, I can see an argument for doing it. If opening the engine turns it into a £10k+ job, I’m much less convinced. Alternatively, if someone would pay around £5k as it stands, knowing exactly what is wrong with it, I’m beginning to wonder whether that is the sensible point at which to hand it over to somebody who actually wants a 996 project. I’m not particularly interested in squeezing every last pound out of the car. I’m really trying to work out whether repairing it makes economic sense or whether I’d simply be throwing good money after bad. Any thoughts on the multiple misfire codes, likely repair scope, and what a complete but engine-fault 996 C4 Tip might realistically be worth would be very welcome. The car is currently at a garage in Vauxhall, although I’m also considering whether it would make sense to get a second opinion from a Porsche engine specialist before making the decision.

Sorry to read about your situation. How much do you know about the Hartech work, do you have invoices detailing what exactly was done ? Obviously zero compression isn’t good, you won’t really know what you’re dealing with until you get it apart, it’s the mother of all catch 22’s I’m afraid. If it’s done a valve my first question is why? I assume it’s a coupe ? I think only you can judge if it’s worth repairing, What does it mean to you? Pragmatic opinion - if you’re paying garage bills it’s hard to see they make sound financial sense to repair, unless it’s absolutely pristine or has real sentimental value.

M

the borescope has shown the No. 4 inlet valve physically bent/stuck open .

My first question is, as you have not been driving it and it’s been with your daughter, is she aware of the engine warm up procedure these cars need. Ie don’t rev beyond 3000 revs before fully up to temp? I would also get a second opinion on the engine issues. If it is a catastrophic failure, I personally don’t think it’s worth spending £10k plus on a tiptronic unless the car has personal meaning to you.

A friend assisted his mate fix a boxster with a dropped valve. Wasnt bent but turned out to be a broken spring. Is it one valve in that cylinder or both? I’d want to understand what has caused the bend - has it ingested something or spring / collet failed? Sometimes the repair can be simply fitting a new valve but there could be damage to the seat / guide. Getting as much information as possible will help if you decide to sell as spares / repairs.

If the valve is stuck open, did the borescope show any damage or witness marks on the piston?

My view would be “the zero compression on No. 4 is the important finding, but I wouldn’t yet accept ‘burnt exhaust valve’ as the complete diagnosis.” The distinction matters because a compression test tells you that cylinder 4 cannot seal, not why it cannot seal. The five misfire codes are interesting. The fact that you have P0301, P0302, P0304, P0305 and P0306 doesn’t necessarily mean that five cylinders have independently failed. On the early 996, the DME detects misfires indirectly from crankshaft speed/engine-roughness fluctuations rather than actually measuring combustion in each cylinder. A cylinder with zero compression and a very severe misfire can potentially affect the misfire detection on other cylinders as well. So I wouldn’t conclude from those five codes that cylinders 1, 2, 5 and 6 necessarily have separate faults. The fact that No. 3 is the only cylinder without a code is actually quite interesting: Bank 1: 1, 2, 3 Bank 2: 4, 5, 6 It doesn’t form a neat bank-wide pattern. If there were a straightforward fuelling or timing problem affecting one bank, I would normally expect a cleaner 1-2-3 or 4-5-6 pattern. A completely dead cylinder can also upset the oxygen-sensor/fuelling picture, potentially causing the DME to alter fuelling for other cylinders. So No. 4 causing other misfire codes isn’t implausible, although I wouldn’t assume that without looking at the actual DME data. Before authorising substantial engine work, I would definitely ask the garage to do another diagnostic stage. First, repeat the compression test on all six cylinders and get the actual figures, rather than simply “No. 4 = zero”. Second, do a leak-down test on No. 4. This is probably the most useful next test. If air is clearly heard coming from the exhaust, that would strongly support an exhaust valve/seat problem. If air is coming through the intake, that points towards the inlet valve. If air is escaping into the crankcase, that suggests piston/rings/cylinder damage. If there are bubbles or pressure appearing in the cooling system, that points towards a head/gasket/crack issue. Third, I’d borescope No. 4 through the spark-plug hole and inspect the piston crown and cylinder wall. Fourth, if the leak-down points towards a valve problem, I’d want the valve train properly inspected before simply declaring it a burnt valve — valve, seat, spring, retainer, lifter, cam/lobe etc. Finally, I’d get a proper Porsche-capable diagnostic interrogation rather than relying on the cheap OBD reader. I’d want the stored fault information, freeze-frame data and, if possible, the individual misfire/rough-running counters. The really important question is therefore not simply “has No. 4 got zero compression?” — we already know it has. The question is “why has No. 4 got zero compression?” If the leak-down test shows air escaping clearly through the exhaust and the borescope shows no obvious piston or cylinder damage, then I would be quite comfortable that the garage has probably found the primary fault. In that situation, the other misfire codes could quite plausibly be consequences of the severe No. 4 failure rather than five separate mechanical failures. However, if the leak-down shows significant leakage into the crankcase, or the borescope shows piston/cylinder damage, then the situation changes considerably and I’d want to stop and consider the economics before authorising anything. One other thing I’d investigate is the history of the Hartech rebuild. If you can find the original invoice, I’d want to know exactly what was done at 80,000 miles — particularly which cylinders, pistons, liners, valves, seats, guides and other components were replaced or refurbished. That could be very useful in understanding why No. 4 has now lost compression after another approximately 70,000 miles. So, in summary, I don’t think the garage’s diagnosis is unreasonable at all. A burnt exhaust valve could certainly produce zero compression and a very severe misfire. What I wouldn’t do is authorise major engine work based solely on the compression test and the assumption that it’s a burnt valve. I’d ask them to establish exactly where the compression is being lost before dismantling anything. Personally, I’d be saying to the garage: “We know No. 4 has zero compression. Before committing to dismantling the engine, can you please carry out a leak-down test and borescope No. 4, give me the compression readings for all six cylinders, and retrieve the full DME misfire data? I want to establish whether No. 4 is definitely a valve problem and whether there is any evidence of piston/liner damage or another underlying fault.” If that comes back as zero compression, leakage clearly through the exhaust valve, and no evidence of piston or cylinder damage, I’d regard the burnt/damaged exhaust valve or seat diagnosis as pretty convincing. I also wouldn’t drive the car any further in this condition. A severe misfire can send unburnt fuel into the catalytic converters and potentially damage them.

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If a valve has been identified as bent / stuck open, how does the leakdown test work? Surely all the air will escape via the easiest route ie the stuck open valve?

I would say unless you can do the work yourself it might not be financially feasible. A bent/stuck valve means the head has to come off and that will the beginning of a costly repair , I'd say £2K min but more likely to be much more. You need to get the head off to see what the damage is or sell as is as a project for someone else.

I would go with what the Baron 3 said,sage words.

My bet is that it has dropped a valve seat out of the head and bent the valve, just take the head off and have a proper look, it’s worth fixing if it has hartect liners. Is the body in good condition?