Hi All:
I have an 1987 928S4, automatic, with a brand new adjustable fuel pressure regulator purchased from 928 Specialists. It's a beautiful unit, I'm sorry it resides under the air filter housing. Anyway, My problem is that I don't know what the stock fuel pressure should be. I know I should have the manuals. Could someone let me know what it should be at idle, with and without the vacuum tube attached. Also, what are other owners adjusting their pressure to?

I have a fuel pressure gauge mounted on the fuel rail and a recently installed Air Fuel Meter which I will ultimately use to fine tune the pressure but I need a starting point and any advice will be appreciated.

Thanks,
Charlie

 

For a stock '87:
Engine not running, fuel pump relay jumpered - fuel pressure should be 55 +/-3 psig; Minimum delivery 1250 cc in thirty seconds.

with the engine idling - approx 48 psig.

Minimum pressure twenty minutes after engine shutdown - 43.5 psig.

 

Hi Charlie,
The fuel pressure at idle should be 48psi with vacuum on the regulator.

With the fuel pump relay jumpered, engine off, the pressure should be 55psi +/-3psi.

After 30 minutes engine off, the pressure shouldn't drop by more than 3psi

Hope this helps

 

.........simmultaneous posting with Wally !

 

Thanks Guys. Tommorrow I'll brush off the 6 inches of snow from the two tarps and one car cover and adjust my pressure. Spring can't come soon enough!

Charlie

 

BTW Charlie,
Welcome to the list........

 

What is the leak-down parameter for? Are upstream components are also implicated in this parameter? Would it include check-valves and fuel dampeners?

 

Do we know a standard "good number" for the fuel pressure when you are trying to get more out of other changes to the engine?

 

Hi Brian,
The leak down test is important in terms of hot starting. Problems with hot stsrt can mean that fuel pressure is not holding. Fule can leak back to the feed (Pump) side if the check valve in the end of the pump body os faulty (replacable).

Fuel can also leak forwards (back to the tank) if the pressure regualtor is leaking.

 

Hi Brendan,

It "depends", as they say. Can't give a straight answer.

If you are upping the power by some means, then you should always check that the mixture strength is OK at WOT. Weak mixture could destroy the engine. If the mixture is weak, then a rising rate fuel pressure regulator might do the trick.

Or if a reprogrammed computer gets to the point where the injectors are open for more than about 80% duty cycle, you might have to fit larger flow injectors, and recalibrate again.

The standard 928 fuel pressure is OK really. The best way is to change other things to get the effect you want.

 

It is certainly interesting how different cars with similar problems attract opposite diagnostic considerations and remedies. My son is the proud owner of a series 2 Mazda RX7 (which I think is an unapologetic rip-off of the 928 body design). They have a very common hot-start problem which their Forum members have diagnosed as "flooding". That seems an unlikely possibility with EFI, but their hypothesis is that the injectors "leak" and deposit fuel into the combustion chamber when the engine is at rest. If not given enough time to evaporate this fuel, hot-start flooding is the diagnosis. Their remedy is to put a switch into the fuel-pump circuit to switch off the supply when hot-starting, and to remove the F/P's check valve.

Curious-er and curious-er! <img border="0" alt="[ouch]" title="" src="graemlins/c.gif" />

I'd clean the injectors! <img border="0" alt="[icon501]" title="" src="graemlins/icon501.gif" />

 

John -

To specify, I have removed the Cats and added headers (MSDS). The exhaust will be dual 25s to the back.

Eventually, I will add a euro intake and throttle body, but at that point I may also have an aftermarket EFI, and will simply be able to control dutie cycle. Until then, however, I was trying to find out if anyone had played with the fuel pressure to get that last bit of oomph out of a mildly changed 928.

I will be installing it soon, and was looking for some numbers apart from the manuals for the L-Jet cars. I am sure a simple cleaning of the injectors would do almost as much good.

Thanks,

 

Brendan: How about getting a A/F ratio guage and watching it to determine the need for more fuel.... They are not very expensive.

 

Yes, this is on the list.

Thanks,

 

Which of your cars has the mods, Brendan ?