My 98 E300 has recently developed a problem where it sometimes fails to start after a hot shutdown. It will crank normally, but it just won't catch. Sometimes it will start after repeated attempts. Last night I had to abandon it. I had left it idling and gone into a store. I came out of the store and got within ten feet of the car and it stumbled and then stalled. That is the first time the problem has caused it to cut off. It did not even sound like it wanted to catch after that. I went back this morning and it fired right up. The strange part is it will always start normally when cold.

The car has no fuel leaks. Over the course of the last four years, I have replaced all the clear fuel lines. The o-rings on those lines, the delivery valve seals, and the shut off valve o-ring are all Viton. There is no odor of diesel fuel under the hood, so no fuel is leaking, but some air is getting in. Bubbles are visible moving in the line from the shut off valve to the lift pump when I rev the engine. No air is visible in the lines near the prefilter (other than the usual small bubble when the engine is off). I replaced the filters and associated o-rings when the problem began a week ago, with no change. I loosened the main filter this morning to see if air had accumulated in it. It was full to the top with fuel. By the way, I had a full tank and was parked nose down when the the engine stalled last night.

I plan to replace the o-rings on the clear line between the shut off valve and the lift pump tomorrow, but I am not confident that is going to fix it. I don't understand why the problem never occurs on a cold engine. Could the shut off valve let air in while not leaking a drop of fuel? Could the problem be the lift pump or is there something else going on and the air is not the main problem? Thanks in advance.