greg jorgensen

Does anyone know what psi #s you get from a fuel pump directly? My shop manuel says on top of the fuel filter you are supposed to get between 64 and 84, I only get 40. I took my Fuel pump out and checked it directly for psi and got 35 psi. O by the way my car starts and only stays on for about five seconds...

Scott 89t2

40 sounds normal as the it's 38 in the rails... I'm not sure where the 64-84 comes from.

'Vert in Vegas

The pump has a high and low pressure mode. Controlled by relays.

Scott 89t2

ahhh

the N/A pressure stays the same. the turbo pressure will raise under boost. there are no relays controlling pressure.

dvls-7

Also if you are getting varying readings, you might want to look into rewireing your fuel pump.

HAILERS

The fuel pump puts out 71.1~92.4 psi. The fuel pressure regulator , regulates the rail pressure to 28-40 psi, depending on the intake manifold pressure at a given time. Actually the 28 is on the whole only at idle or when you let off the accelerator and have a large vacuum in the intake. That's for a n/a.

The turbo is a bit different. When in boost, the fpr will gradually raise the fuel pressure in the fuel rail up to fifty psi.

So the fuel pump puts out 70-90 psi, the regulator keeps the rail pressure at approx 40psi, and the excess pressure gets dumped back into the fuel tank. That's why you don't see the fuel pumps 70-90 psi at the fuel rail.

'Vert in Vegas

I don't know for sure that I was right about the fuel pressure relays.
But - I'm looking at page F1-57 of the factory manual
(1990)
AND it shows a fuel pump resistor relay controlled by the ECU.
You can bash me, but I'm just reading from the "bible".
I'm always willing to learn.

PS: My head is starting to hurt. Can you edit that out now?

HAILERS

The fuel pump resistor relay determines the amount of voltage at the fuel pump. I only have 86/87 n/a and a 87turboii. On these models, only the turboii has the fuel pump resistor relay. When there is no boost, the pump gets approx 9volts. When under boost, the ECU puts a ground on the fuel pump resistor relay and pulls a relay in. That relay changes the path of the battery voltage to the fuel pump by bypassing a resistor. Now the pump gets 12 plus volts as long as your under boost.

Just the same, if you look into the Fuel section of that manual, you'll find a checkout of the fuel pump and its pressure. It'll be somewhere close to 80 plus or minus.

HAILERS

Oh! P.S. Whether its 12v or 9v, the pressure on the rail will be the same. That pressure is controlled by the fuel pressure regulator. The 12v adds volume, not more pressure if the fpr is up to snuff.

HAILERS

87 TURBOII fuel pump pressure.

HAILERS

The resistor:

'Vert in Vegas

So, you are saying the 9 volts at the pump puts out the same pressure as the 12 volts?
Or, that the regulator keeps it down, so there is no effective difference in pressure?
My book shows a maximum pressure, but does not show a minimum or low pressure number.
Oh, and your diagram is different than mine. Is it for a different year than mine? (1990)

HAILERS

I'm saying the fpr, the fuel pressure regulator is what keeps the fuel rail pressure at approx 40psi, even though the pump itself is putting out 70 plus psi.

The jump from 9 to 12 volts adds more volume so the rail pressure will stay at 40psi during heavy boost.

HAILERS

MODERATOR, I HAVE NOT POSTED THIS IMAGE BEFORE.

Well, I was going to post the page out of the manual that shows how to check the fuel rail pressure with the engine at idle, but he said I had already attached it to this thread. WRONG. I DID NOT. So you'll have to read the manual to find it.

NZConvertible

Actually, the turbo is no different. All EFI cars (turbo or not) use the FPR to increase fuel rail pressure as manifold pressure increases, usually keeping fuel pressure a set amount higher than manifold pressure. Turbos just increase it further, because manifold pressure goes higher.

BTW, you may not have posted that particular picture before, but you've probably posted one with the same name. Give it a different name and try again.

NZConvertible

S4 NA's don't have the resistor/relay, but S5 NA's (and all turbos) do.

Jimmy325i

I have a good (working, runs the engine just fine) 86 n/a fuel pump I'll send you for the cost of shipping. PM me if you're interested.

HAILERS

OK. Here's a new jpg called freddygreenteeth:

HAILERS

Really don't need a tester as shown in the picture. Just tee into the fuel filter output hose. Same thing.

Just to be clear: If you put a fuel pressure gauge on the output of the fuel filter output(no tee) and then cause the fuel pump resistor relay to default to the higher voltage, I'm sure the fuel pump will put out more pressure. It's just that with the whole system connected together, the fuel pressure regulator will not let this extra pressure from the fuel pump to change the fuel rail pressure (unless it's a waldo that puts out so much more that it overwhelms the fpr).

Yes, the n/a fpr works the same as the turboii. If you put pressure on that vac line to it's fpr, and raise that air pressure to 10psi, then the fuel pressure will rise to approx 50psi.