Non-Mercedes 6.5 GM diesel problem

[kerry]

Got a thread on this over at the 6.5 forum at Diesel Place but I thought I'd pick people's brains here too. About 3 weeks ago I bought a 93 GMC Sierra pickup with a 6.5 turbodiesel and 104k miles. Started it cold and hot before purchase. Fired right up. Finished up the third of tank of fuel that was in at purchase, filled it up and ran 400 miles on that tank. Filled up again. Immediately it began showing symptoms of harder starting and 130 miles later it is extremely hard to start when hot. It does start if I pour water on the IP to cool it down, which is the standard test for a worn out pump on these IP's. But I'm suspicious of that diagnosis since the hard starting occurred immediately after filling up. I've replaced the fuel filter. Glow plugs are controlled by a temp sensor in the engine and don't operate once the engine is up to temperature. I installed a fuel pressure gauge and lift pump is providing about 7-8psi all the time. Bleeder valve shows plenty of fuel when cranking or running. I was wondering if I got a tank of thinner winter fuel and it showed up the problem of a worn pump right away over summer fuel. Other than the hard starting it runs fine.