Bore score and very cold weather - Page 1 - 911/Carrera GT - PistonHeads UK

I’m trying to diagnose an issue with my Subaru flat six and thought this would be the best place to ask this question. When temperatures get below zero my otherwise faultless car turns over beautifully with fuel flowing perfectly towards new spark plugs where new sensors and filters do their stuff but the engine doesn’t start. It’s a common problem in very high mileage Subaru flat sixes in cold parts of America. I can only troubleshoot this when temperatures are below zero which makes the job difficult and unpleasant. My latest idea is I have slightly worn cylinders/piston rings which contract just enough when it is very cold to change the compression ratio and prevent ignition. Once the engine has started it can be immediately turned off and restarted with no problem. My question is, do the Porsche flat sixes ever have this problem, particularly in the lead up to a full bore score diagnosis? Cheers

My Subaru flat six has run for only 134k with no issues, but in recent years it's not been that cold - I've owned it for just 12 years, and took it to the alps just the once, but the mileage was probably only around 80k when in the extreme cold, with no starting issues. If it's a common problem for the US of A, what is their cause/solution?

If your theory is correct, you could do a compression test on a warm morning in the summer months and get the readings, and then again when the ambient temperature/engine is below zero, and compare them. If the difference is noticeably lower, you could then spray a small amount of oil into the combustion chamber, and try to start the engine, if it then starts easily/easier, it would tend to confirm your theory about cylinder/ring wear.

Not sure on diagnostics, but surely a block heater might be the simple solution if you have an accessible outside socket?

I’m on 204,000 which is not at all uncommon in the States. Have owned the car since new so I know it extremely well. The Subaru forums have loads of chat about this issue but no one has ever found a solution. The last thing I’ve changed is the MAF sensor and there is nothing else left. I could go down the route of boroscopes and compression tests but there is little point as a rebuild makes no sense. I have thought about a portable heater but it’s not a long term solution, particularly as I often have to leave for work at 4.30 and I don’t want to faff about at that time of the morning - I only still have the car because it’s otherwise faultless and I’m sentimental about it. I was just curious if the Porsche flat sixes ever have the same problem. Some of the bore scoring looks quite severe so compression would presumably be affected. Maybe 911 owners don’t drive much in sub zero temperatures?

As a serial owner of Subaru and Porsche flat sixes, I'd be looking at the battery, especially if the vehicle is outside and may not get started for a few days Cold in the USA can mean much lower than -20C. We don't have that sort of cold here (well maybe if you live up a mountain in the Cairngorms), just mild and wet

New battery tried twice. Top of the range batteries.

SonicHedgeHog said: I’m trying to diagnose an issue with my Subaru flat six and thought this would be the best place to ask this question. When temperatures get below zero my otherwise faultless car turns over beautifully with fuel flowing perfectly towards new spark plugs where new sensors and filters do their stuff but the engine doesn’t start. It’s a common problem in very high mileage Subaru flat sixes in cold parts of America. I can only troubleshoot this when temperatures are below zero which makes the job difficult and unpleasant. My latest idea is I have slightly worn cylinders/piston rings which contract just enough when it is very cold to change the compression ratio and prevent ignition. Once the engine has started it can be immediately turned off and restarted with no problem. My question is, do the Porsche flat sixes ever have this problem, particularly in the lead up to a full bore score diagnosis? No.

SonicHedgeHog said: I was just curious if the Porsche flat sixes ever have the same problem. Some of the bore scoring looks quite severe so compression would presumably be affected. Maybe 911 owners don’t drive much in sub zero temperatures? Perhaps try asking on a more technical/old-skool forum such as impact bumpers - there will be higher mileage owners, and european owners who have proper winters. But (if fuel is flowing to the engine at correct pressure) it sounds more likely to be an electrical issue, rather than any engine wear cause, off the top of my head, sitting in the comfort of my Chesterfield wingback. Good luck with fixing/living with your mostly faithful Scooby.

Could this be a head gasket issue? The reason I ask is that I had a Lancia Beta HPE during a ski season many years ago and the head gasket used to leak between one of the cylinders and a water way. Once up to temperature the leak and accompanying exhaust steam stopped. I wonder whether it could be a leak between cylinders in this instance

SonicHedgeHog said: I have thought about a portable heater but it’s not a long term solution, particularly as I often have to leave for work at 4.30 and I don’t want to faff about at that time of the morning - I only still have the car because it’s otherwise faultless and I’m sentimental about it. Scandinavian block heater is just a little fly lead on a timer to warm the block at the right time. Subaru do OEM kit.

Thanks for the ideas. You get all these kind of responses on other forums as everyone tries to be helpful. At some point or another everyone has tried changing every sensor, filter, battery, spark plug, coil pack…you name it, and the problem remains. It’s only when it’s very cold and the rest of the time it’s perfect. You can even plug in a pro fault reader and no fault codes will appear, even when sub-zero temperatures. Good to know Porsches aren’t affected though. Cheers.

Evanivitch said: Scandinavian block heater is just a little fly lead on a timer to warm the block at the right time. Subaru do OEM kit. Thanks for that. I’ll take a look.

GreatStuff said: Could this be a head gasket issue? The reason I ask is that I had a Lancia Beta HPE during a ski season many years ago and the head gasket used to leak between one of the cylinders and a water way. Once up to temperature the leak and accompanying exhaust steam stopped. I wonder whether it could be a leak between cylinders in this instance Just saw this reply. It is possible. I’ve done about 30,000 miles since it first occurred so I would think the head gasket would have properly failed in that time. Having said that I drive it so gently, maybe not. I’ll have a look and see if there are any signs of head gasket failure.