3.4 charging voltage & warm start-up problems

I have a 3.4, which was rebuilt about 8000km ago with a new short block from Porsche, head work (valve seats, seals, etc), new chains, the works. The power cable to the starter and the engine ground strap were also changed and the car worked fine until a few weeks ago I started experiencing really hard hot starts. The engine will barely crank over at first, but once it gets going it's OK and starts. This is if I stop for maybe 15 minutes and come to re-start the car. The dash will show 14 - 14,2V when cold, but as the engine warms up it drops to ~13,5V. The engine will also crank, although maybe a bit sluggish, and start fine when cold. Charging current at idle is only 13,4V at the battery and 13,6V at the engine auxiliary power connector. Battery voltage right after driving is 12,1V, which to me means the battery is only about 30% charged What do you guys think? To me this sounds like a bad alternator and/or battery. I also see there are 120A and 150A alternators for these cars. Can I just order the more powerful one? I don't see any other differences. My car is a manual so I know will need the clutch pulley version.

I've had poor starting before. I first replaced the battery and that only partially cured it. I then rebuilt the starter motor and that cured it. About a year later it went bad again and this time it was just the battery. I didn't arrive at a definitive diagnostic test beyond trying a different battery during my troubleshooting. In terms of effort and cost, the battery replacement was low effort but high cost at around £80, whereas the starter motor was a b1tch to remove and needed a new solenoid and brushes, which cost about £35 in total, but took several days of effort.

Those voltages with the engine running seem normal to me, and are what people often report when they've just fitted a new alternator!

I'm afraid i don't really trust the cars voltmeter , you really need to test with a seperate voltmeter which you can easily buy for £10. Voltmeter on battery and car at operating temp and running , turn on all lights , high beam heated seats fan on full speed etc then read off the voltage. Rule of thumb then above 13 volts and the alternator is fine , below that and it's on its way out , 13.1 - 13.4 is what i would like to see . Bearing in mind this is a load test of a hot alternator so if its on the way out this will show it . My guess from what you have said .. its the starter motor .. it's getting heat soak after 15 mins and struggling to spin over .

The power cable to the starter runs from the battery via the 12v terminal in the engine bay so it's actually in two parts. It'd be worth checking the integrity of the connections and if possible the condition of the crimped end connectors - they can break down with heat and corrosion. Check the connections on the earth strap too.

The power cable and engine ground strap were both changed when the engine was rebuilt 2 years ago. I've changed the oil today and no particles in the filter, so I don't think it's a mechanical issue. Electrical: - Battery tested at the shop, 12.06V even after ~30mins of driving, starting current 700A (Battery is 95Ah 850A) - Charging current measured with a voltmeter: Cold 14.0V and 13.9V with lights and radio on // Hot 13.6V I'm leaning toward either a bad alternator or starter. If the alternator is pushing 13.6V it can't fully charge a battery and the lowish battery voltage is an indication of that. I would expect to see ~12.6V at the battery with the car not running?

If you start the car from cold , i assume it spins over fine , then drive to operating temp then switch it off and leave it overnight then i assume it starts fine in the morning ? If the above is correct then it's something getting hot and not the state of charge in the battery .. batterys don't get hot unless there is an over charge which is a seperate fault to what you have .. already proved by your voltage readings as not over charging . What gets hot .. starter and wiring leads .. leads have been changed so im left with the starter motor .. it's also a typical symptom for this type of fault . Alternator gets hot but if under charging then the battery would be low on charge for a cold start ... 13.6 is not a low charge . Battery may not be 100% . a fully charged open circuit battery should be 12.6 volts or 2.1 per cell .. a dead cell and its about 10.5 . Some Sulfation perhaps . Im still of the opinion its a starter fault .. nothing to say i'm totally wrong , i certainly don't know everything and cant see your car .. but that's what it sounds like to me

If you charged the battery off the car then disconnected the charger and let the battery sit for an hour I bet you would have 12.6v. When connected to the car the small drain always present pull the voltage down a bit. You say the power cable has been changed, do you mean the cable from front to back or the much shorter one that runs from the engine bay to starter. If only one of them has been changed maybe it's the other? You have charging voltage that is better than most people who are not having any issues so as Demort and others are saying the alternator seems the least likely to be at fault to me too. I too think starter or one of the connections on the power cables is most likely I had a fault on my car where the charging voltage started at nearly 14v but then dropped to just over 12v after driving the car for half an hour or so. It started ok if I only did short journeys but when I did longer ones it struggled to start the next time, basically the battery was discharging on longer journeys because 12v wasn't enough to keep it fully charged, a new regulator fitted to the alternator sorted it!This does not seem to be your problem

Thanks guys! I will measure the battery voltage once more when the car's been left for a few hours. It turns over fine cold, but not when hot. Feels and sounds like a dead battery, so it will turn over once very sluggish, pause for a bit and then crank a bit better, just enough to start. And only when hot, say I stop at the shop for 15mins and come back. Any experiences with aftermarket staters, or should I just get Bosch? They seem to be about ~250EUR for a Bosch and ~150EUR for a re-built or aftermarket no-brand one.

Definitely go for the Bosch - its a different animal from other aftermarket starter motors.... By comparison, others seem to be "high rpm" whereas the Bosch seems to be "high torque" in nature (please note: all of this is 100% anecdotal and in no way factual or accurate!) - the sound of a Bosch starter is completely different from others i.e. not a high spinning whine, but more of a guttural "chugging"- I still have the occasional hot start issue where it doesn't catch on the first try, but the Bosch seems far more capable and suitable for our engines than other offerings

Bosch .

You are posting exactly the same symptoms I had before changing the alternator Y cable - I know you said it was changed, but I would look there and make sure it's a-ok before doing anything else, as you may be throwing money away!

Check what gauge cable was used to replace it also, if under spec cable was used and not terminated well that could well be falling apart now and causing your problem

It's the OEM Porsche cable and I'm hard pressed to believe it's at fault. I will check it when I get the chance to remove the alternator for inspection and to change the regulator. It's a cheap part so might as well.