Think I have vapor lock...
 

Hello,

I have a new situation here, since this summer, my 400E has a rough idle problem after start when weather is hot, rpm drops below 400 immediately after start, sometimes it almost stalls, however, rpm returns to normal after about 10 seconds, it also helps if I give some gas. If I stop the engine and start again, it fires right up and idles normally.

I hooked up a fuel pressure gauge to the fuel rail, I measured a 55 PSI with engine running, I then shut down the engine, within 1.5 hrs, fuel pressure dropped to only 5 PSI. I turned key to "on" only let fuel pump build up some pressure, then drained fuel from the fuel pressure gauge into a long thin plastic tube, I could see a lot of bubbles in the tube, they actually broke the fuel delivery in the tube, I repeated the procedure several times, after 5 drains, I no longer saw bubbles.

So is it a sure sign that I have "vapor lock"? if so, which part should I start to look at?

Once you open the line, pressure has dropped, and the bubbles will form. The purpose of pressurization is to move the fuel state into the region of the temperature-pressure curve where vaporization will not occur, and you have upset that.

Steve

You may be right, Steve.

After several pumps&drains, I no longer had bubbles, I guess the reason may be the fuel has cooled down after several drains, it's not easy to evaporate then.

Do you happen to know how long it's supposed to hold the pressure, is 1.5 hrs from 55 PSI to 5 PSI too fast?

Some updates on the solution:

Today, I checked the fuel delivery, found out there was a slow fuel leak at the gasket on one of the fuel pump check valves, it was just sipping fuel and wasn't significant, but it was enough to leak down fuel pressure from 55 to 5 PSI within 1.5 hrs.

I replaced the gasket and measured the fuel pressure again, it has been 8 hours, fuel pressure only dropped from 55 PSI to 25 PSI, looks like it's going to hold it forever, the vapour lock is gone!

I have suspected fuel injectors and check valves but it turned out to the cheapest part in the whole car, less than 1$.