Motronic: Fuel Pressure @ Idle?

[WydRyd]

Curious, what should the fuel pressure be at idle for the Carrera Motronic EFI?

I'm registering approx 43psi at idle, which I think is a bit high. Shouldn't it typically be around 36psi or so?

What could be causing too high fuel pressure?

I have fitted a new fuel pressure damper, because my old one had a torn diaphragm and bled fuel into the intake via the vacuum port.

Could it be the fuel pressure regulator on it's way out?

What else could cause too high fuel pressure? A leaky injector that's possibly stuck open?

Ideas/suggestions?

P.S: Car performs flawlessly, apart from consuming a bit more fuel than usual.

[89911]

At idle with vacuum line connected, 2 bar or 30 psi.
At idle with vacum line disconnected, 2.3 to 2.7 bar or 34 to 40 psi.


Yes, It's a bit high. Generally not a bad thing since your system is going to bleed off the excess back into the fuel system. Much better then being too low.

[WydRyd]

WOW, so I'm ~13psi OVER spec at idle with vacuum connected

Something is definately not right there?!?

[WydRyd]

If I disconnect the vacuum line to the FPR, it makes no difference in fuel pressure reading! Still stays at 43psi

It should be approx 2.5bar at idle (vacuum connected) and then jump to 2.8bar if the vacuum line is disconnected to the FPR.

Looks like I need a new FPR, or perhaps there is some restriction in the return line back to the fuel tank

[dougn1]

is your vacuum line sucking? if so then it seems you need a new regulator.

i'm particularly interested in this subject ...specifically how the fuel pressure regulator effects throttle response.

the loss of vacuum when the throttle is abruptly opened should cause the fuel pressure to instantaneously rise and richen the mixture like an accelerator pump. i think in a modern fuel injected engine, the off idle throttle response is designed to provide low emissions at the expense of quick throttle response.

i'm curious to hear if you notice a change in your off idle throttle response with a new regulator

i'm currently looking for the fitting so that i can measure my fuel pressure vs. vacuum.

they make a fuel pressure regulator that is designed to effect a greater change (increase) in fuel pressure when the vacuum is reduced. it's called a power boost valve or something goofy like that.

[WydRyd]

Originally posted by dougn1
is your vacuum line sucking? if so then it seems you need a new regulator.

Sorry, what do you mean? The vacuum line from the regulator to the "T" at the rear of the throttle body is sucking. That's how it's suppose to work, isn't it? i.e: the vacuum line to the throttle body is pulling vacuum.

My main concern is that the fuel pressure does not jump when I disconnect the vacuum line to the regulator, indicating the regulator is faulty in some way

At least it's not the other way, and causing a too lean condition

What adverse effects would be present with too high fuel pressure, apart from increased fuel consumption? Hasn't really affected drivability. The car runs great (smells a bit rich at idle). I'm just concerned about the extra fuel consumption at this stage.

[dougn1]

yes that's what i mean ....if your vacuum line to your regulator is sucking then it should lower the pressure.....i believe in a 1 to 1 ratio.

i suppose if the pressure was too high and you don't use an oxygen sensor, you would be running rich.

keep us posted as to what happens when you install a new regulator

[dougn1]

did that whole throttle response theory make sense to you?

[WydRyd]

Yup, makes sense.

I haven't run an O2 sensor for years and I have never had the fuel consumption issues I'm facing at the moment.

So, let me get this right in my head. If the fuel rail pressure is too high, the amount of fuel injected into the combustion chamber when the injector opens is increased too, right?

Having a faulty FPR may explain the buzzing noise I can hear under acceleration

[dougn1]

yes because the injectors are opened over a prescribed (mapped) period of time. at a higher pressure...more fuel.

the oxygen sensor provides a feed back loop so the length of time that the injectors are open can be adjusted to provide the correct mixture

[WydRyd]

Running an O2 sensor would have masked the fault with the FPR though, because the DME would automatically adjust the injector duty based on feedback from the O2 sensor.

High rail pressures will over-drive the fuel pump(s) and eventually cause them to overheat and fail So, I guess I'm lucky I found the faulty FPR now.

[greglepore]

Merv-I'd just email Todd and find out what the specs are for the RR fpr that you're using.

[WydRyd]

Yup, Todd said to disconnect the vacuum line AT the FPR and see if it affects the fuel pressure at all. If not, then the FPR is definately screwed cause the regulator isn't really regulating

[red964]

Is your fuel pump stock, or could it be a higher pressure aftermarket thing?