Dear all, I have an odd problem on my 400i. When the outdoor temperature is high and after a few hours driving (3 hours today at roughly 60-70mph, ambient temp about 27C/ 81F) the left bank starts to stutter and then stops. After some cooling down its all fine again for 20-40 minutes and then same happens again. Adding cold fuel has the same effect as cooling down. Up to the stuttering she spinns like a kitten and runs very even on all cylinders. Engine temp and oil temp is solid at 80-85C. Oil pressure is good. All fluids changed end of last season. CO2 set to 1,7% and vacuum set on both banks with gauge. Idling at 800rpm. A little valve cluttering around 1000rpm but not bad. Engine has done just over 30k miles. Fans are working and she runs an MSD ignition box. Can it be that the left fuel pump is intermittent faulty? Both relays changed last year. Ignition is probably unlikely as there is only one distributor? Why would only one back suffer if faulty. Many things are recently exchanged or calibrated so it is probably not; Injectors, new Fuel lines in engine bay, new Fuel distributor, newly serviced and calibrated Fuel pressure regulator, newly serviced and calibrated I will change the fuel lines at the tanks but cannot see that they could cause this albeit that they are old and should be exchanged. I have asked for advise on length and dimensions in a separate thread. I also need to figure out what the different hoses do in the rear. There is a crossover hose between the tanks that is joined in a square metal box, what is that. I sits on the front wall in the trunk. I do have the work shop manuals but the page with the different tank parts are all in Italian. One observation is also that the tanks themselves becomes rather warm after a few hours driving when hot outside. When I got back today and when the left bank cut I measured with an IR gauge and they were about 47C/117F on the surface with the cover taken off. They do probably get warm from the fuel return from the engine. Is that normal? Had about 50 liters in the tanks at the end, so a decent amount. At what temperature does vapor locks become a problem? Can it bee that a vapor lock only affects one bank? And I guess the final question is, can a fuel pump stop working at high temperaturs or do you think it is an electrical problem. As all is perfectly normal when cooled down it is a bit odd. Not very easy to trouble shoot as this only happens after long driving stints in warm weather when everything is very warm, including the fuel in the tanks. All input much appreciated //Rob Can this be due to back fuel accumulators?