dbonds

Here's an odd one for the group. I have a 1992 Porsche 968 with a permanently mounted fuel gauge installed on the fuel rail. A few times I've noticed the car showing full pressure (55-60psi) when the car was not running and hadn't been started recently. At other times, the pressure was down to zero.

I started watching the gauge more closely over a weekend and found that in the morning/evenings with cool (70-80F) temps, the pressure was down at zero. As the temperature starts to rise (or fall) the gauge will read partial pressure (15-25psi). At the hottest part of the day, the gauge will read 55-60 psi). Again, all of this occurs without the keys being anywhere near the car or even a door being opened.

So - my question is - is it normal for the pressure in the fuel rail to change with ambient temperature without the car being started?

Zirconocene

I would think that's pretty normal. All liquids will expand (more expansion = higher pressure, until it bleeds off over time) as temperature rises and in Houston, in a black car, the temperatures under the hood are bound to get pretty toasty, even if the car hasn't been running. Heck, you probably don't even have to be in the sun much to get some nice temperature swings in that engine bay as the day starts to warm up.

Cheers

dbonds

yeah, I knew about the basics of fluid thermodynamics, was just curious if there was a design to bleed off excess pressure, but guessing that'd be an issue when it's actually running so not surprising if there isn't.

The car is parked in a garage/metal shop, fairly well shaded, but NOT climate controlled (yet). Still gets AS hot as ambient temp in there, but thankfully not any higher. Just hoping it wasn't putting stress on fuel lines or anything like that. Will have to check it out when I finally get AC in there, OR we get to winter, whichever comes first. ;-)

Gage

The fuel pressure regulator, when functioning properly, will only allow a system pressure that is slightly higher (due to atmospheric pressure balance on the diaphragm) than system design pressure.

Paul Waterloo

I wouldn't worry about it, most cars are designed to maintain fuel pressure in the rail with installed check valves. That way the fuel pump doesn't have to re-pressurize the system every time you start it which can lead to hard starts. The pressure won't damage anything.