D jet fuel pressure question

[ABDow]

Thanks - it is strange that it starts when there is little or dropping fuel pressure when the f/p fuse is removed. I will test the fuel pressure and the fuel pump volume delivery when the car is hot and refusing to start and report back.
Graham, by 'leak down' do you mean loss of fuel pressure when the fuel pump has brought the system to full pressure and then has been turned off?
Alastair

[pch2021]

Feel free to call me if you have any issues...

[Graham]

Originally Posted by ABDow

Thanks - it is strange that it starts when there is little or dropping fuel pressure when the f/p fuse is removed. I will test the fuel pressure and the fuel pump volume delivery when the car is hot and refusing to start and report back.
Graham, by 'leak down' do you mean loss of fuel pressure when the fuel pump has brought the system to full pressure and then has been turned off?
Alastair

Yes - Leak down occurs after you have been running. It is due to small leaks through the injectors, cold start valve, pressure control valve and fuel pump check valve. Pressure should be 2bar +/-0.1 (about 29 psi) when you turn the key off. MB have a spec for the allowable leak down. It's a bit cryptic, but they say it may drop to 1.7 bar immediately. After another 5 min a drop to 1.5 bar is permitted (about 21.5 psig).

I have a fuel pressure gauge on the dash - If I stop for say 20 min, leakdown causes the pressure to drop to about 20 psig. At this pressure, summer fuel with RVP of 7psig will begin to vaporize at 165F. With heat soak, you would be way above that in the injectors and probably also in the fuel rails.

So, what it comes down to, is that you will have vapor in your fuel lines and injectors after a short stop. The question then is how to get the car going if you don't have time to let it cool down. The pdf I linked to provides some ideas.

[ABDow]

Dashboard f/p gauge - kewl! I'm in the process of working out an elegant way of connecting and disconnecting an under bonnet (hood) f/p gauge without having to undo the cold start injector line or other crude and messy arrangements. Will put a barbed T in the cold start injector line and a click fitting with a tap. Mate the pdf is a goldmine - thanks for sharing it.

[Graham]

Originally Posted by ABDow

Dashboard f/p gauge - kewl! I'm in the process of working out an elegant way of connecting and disconnecting an under bonnet (hood) f/p gauge without having to undo the cold start injector line or other crude and messy arrangements. Will put a barbed T in the cold start injector line and a click fitting with a tap. Mate the pdf is a goldmine - thanks for sharing it.

Sounds like a good plan.

I have seen fuel pressure regulators with an integral gauge that look good.

I mounted the sender for my gauge on the hose that runs across the front of the engine and added a bracket to hold it in position. See pic.

[ABDow]

Good idea. That position also reduces the likelihood of fuel splashing onto hot parts of the engine.