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?

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99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.