Over 155k mile on the original engine and transmission; third owner since 2006 at 110k miles; never needed major repairs other than brakes, engine oil changes, front and rear shock door check/latch replacements and replacement of a rear-right low control arm.

I noticed that the gas tank has pressure build up even when it is in the parking mode: after the car parked overnight and open the gas filler cap in the morning and I can hear the sound of pressure release. I read on the Internet that pressure build-up in the gas tank is bad and dangerous. The car appears running fine and I use it for short local trips (< 10 miles) a couple of times a week. I have no clue as when the pressure build-up problem started because I fill the gas tank only 2 or 3 times a year.

Please advise on how to go about diagnosing the pressure build-up problem.

 

If you have the KE-Jetronic system, the fuel tank holds about 30# of pressure when you shut the engine down. This is to keep the engine from vapor locking.

 

Are you saying pressure build-up in the gas tank may be by design?

I have no clue as to what fuel system is in my 420SEL. How do I check it out?

 

I feel sure you have the KE-Jetronic system. Yes, the pressure is there on purpose. Here is a ink to a manual explaining how it all comes together.

http://www.berlinasportivo.com/Techn...tronic-OCR.pdf

 

Thank you for the pointer and your advice is very much appreciated.

 

Both my 1986 560SEL and 1989 420SEL have t his pressure build up. From what I've read, it is designed that way.
Oat

 

Forgot to mention, both have the KE Jetronic fuel injection system.

Oat

 

Thank you for the inputs - very assuring to put my mind in peace.