Engine hard to start when warm

[Mufu]

Guys, I have a problem with my C6 2009. When the engine is warm (operating temperature) and I let it cool down for a while it often won't start on the first try, it cranks a bit and then dies. But usually starts on the second try. And sometimes it cranks maybe 6 seconds before it starts. This never happens when the engine is completly cold. Any ideas?

[SteveG]

Any error codes ?
If not then could be anything. O2 sensor,plugs ?

Prolly wait to see what breaks as cheapest fix

[Jim Haseltine]

I don't yet know much about the C6 fuel system so this might be rubbish, I'm just going on my experiences with other cars, but my first thought would be that you've a loss of residual fuel pressure. The fuel rails are pressurised by the pumps and when the pumps aren't running the pressure is retained by a sort of non-return valve. If the valves don't work or there's a line leak the engine has to turn over until the pressure has built to the point where the injectors can pass enough fuel for the engine to start. The difference between cold and hot starts is that the cold start enrichment compensates for the loss of pressure.
I'd check the post filter fuel lines for leaks and the injectors in case one or more are weeping fuel when they are closed. If there's no leak I'd then suspect the pressure regulator which I believe is part of the filter on these cars.

[Mufu]

Any error codes ?
If not then could be anything. O2 sensor,plugs ?

Prolly wait to see what breaks as cheapest fix

No error codes unfortunately. Might do that but gonna try some other things first.

I don't yet know much about the C6 fuel system so this might be rubbish, I'm just going on my experiences with other cars, but my first thought would be that you've a loss of residual fuel pressure. The fuel rails are pressurised by the pumps and when the pumps aren't running the pressure is retained by a sort of non-return valve. If the valves don't work or there's a line leak the engine has to turn over until the pressure has built to the point where the injectors can pass enough fuel for the engine to start. The difference between cold and hot starts is that the cold start enrichment compensates for the loss of pressure.
I'd check the post filter fuel lines for leaks and the injectors in case one or more are weeping fuel when they are closed. If there's no leak I'd then suspect the pressure regulator which I believe is part of the filter on these cars.

Well, to be honest I don't know much when it comes to engine problems, but what you're saying makes sense. Have gotten some other suggestions from people that it could be a bad crankshaft position sensor or the EVAP system. Also the fuel pump check valve, but maybe that is related to what you're saying? I'm thankful for your reply, gonna have these things checked to begin with.

[Jim Haseltine]

Yep, the pump check valve is there to stop back flow to the tank when the pump isn't running, so that could be the problem if there's a loss of pressure.

[Jim Haseltine]

Having browsed a parts list or two, if there is a check valve (I'd be surprised if there isn't) it's part of the pump, not a separate part. There also appears to be at least 3 pressure sensors, so I'd expect a code stored somewhere if there's low pressure.