1977 280z Idle Fuel Pressure 28 psi - Factory Service Manual says 36 psi

[jkeese01]

New on fuel system:

- Fuel Hoses

- Vapor Hoses

- Pressure Regulator

- Fuel Pump

- Clean tank

- Rebuilt injectors

Vacuum is 19.

When driving the car, the fuel pressure will increase to 36 psi at full throttle. It is 28 psi when idling and sounds like it's running lean.

Pinching the return fuel hose to the tank increases the fuel pressure to 52 psi. Removing the vacuum hose to the Pressure Regulator ups the pressure to 36 psi.

Also ran the car out of a gallon gas tank and still 28 psi idle.

Factory Service Manual says it should be 36 psi.

Any ideas on getting it to 36 psi?

Thanks.

[Zed Head]

If you read the test procedure in the FSM you'll see that there is no intake vacuum applied to the FPR when the test is performed. Your numbers all look good.

[Wade Nelson]

Could you please describe what a lean idle sounds like?

If you enrichen it with faux fuel (starting fluid, spray brake cleaner, anything aerosol and flammable...) does the idle sound better?

[jkeese01]

If you read the test procedure in the FSM you'll see that there is no intake vacuum applied to the FPR when the test is performed. Your numbers all look good.

The Electronic Fuel Injection Book says "The pressure regulator thus maintains a constant balance between fuel and manifold pressure keeping the difference between them at 36 psi." So if you remove the vacuum, it should show 36 psi, difference with no vacuum is + 36 psi. With my vacuum of 19, I'd think the pressure would be 55 - 19 = effective pressure of 36 psi.

[jkeese01]

Could you please describe what a lean idle sounds like?

If you enrichen it with faux fuel (starting fluid, spray brake cleaner, anything aerosol and flammable...) does the idle sound better?

At idle, the exhaust makes a puffing sound. I'm probably being too critical on this. Car cranks right up and seems to have good power. Never driven any other 280z, so don't have anything to compare what "normal" is.

[Zed Head]

The Electronic Fuel Injection Book says "The pressure regulator thus maintains a constant balance between fuel and manifold pressure keeping the difference between them at 36 psi." So if you remove the vacuum, it should show 36 psi, difference with no vacuum is + 36 psi. With my vacuum of 19, I'd think the pressure would be 55 - 19 = effective pressure of 36 psi.

It's great that you're doing the calculations to figure out what's right but you're off-track a little bit. It took me a while to get things straight when I got back in to cars, besides the fact that I never really knew what I was doing when I used to work on them.

The intake vacuum that you're measuring is in inches of mercury. You can convert that to psi and subtract it from 36 to determine what your fuel pressure is at idle. That's why you're getting 28 psi at idle, because the fuel pressure regulator is reducing the fuel pressure to give 36 psi against atmospheric pressure. When you lower the pressure in the manifold (create a vacuum), the FPR automatically compensates so that the same amount of fuel will flow for a certain injector open period.

The key is remembering that we all live in about 14.7 psi pressure, outside the manifold. In a turbo application, fuel pressure will go down with vacuum and up with boost, varying with manifold pressure.

Edit - and my point about the FSM procedure was that there's no intake vacuum when you run the pump alone. So 36 psi with no vacuum (like WOT) is the spec.

Edited February 22, 2013 by Zed Head

[EuroDat]

Zed's post is spot-on. I couldnt explain it any better.

I think there is nothing wrong with your FPR. Its doing exactly what it should do and that is maintaining 36psi differential pressure over the injector at all times when the fuel pump is running.

If you think your car is running lean, you can have that tested and adjust the mixture with a variable resistor in temperature sensor for the EFI system. See link for details on how its done.

http://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

Chas

Edited February 22, 2013 by EuroDat

[jkeese01]

Thanks for the info. and link. Have you or anyone here installed the 1k pot to fine tune the Temp Sensor?

Someone seems to have already added a resistor inline on the Temp Sensor. Adding a 1k pot to make this adjustable looks pretty easy and would be interesting to see how well it works.

[FastWoman]

I've done that mod, although my ideal resistance is around 2.5k, so I use a 5k pot. Instead of the 1-turn panel-mount pot recommended in the Atlantic-Z writeup, I'd use a 20-turn pot, which adjusts with a screw. My results were excellent, and my engine now runs strong and clean.

A couple of notes, though:

1. It makes no sense to do this without first ruling out all other EFI issues, including especially vacuum leaks.

2. You're not really fine-tuning the temp sensor, but rather tricking an analog ECU that's drifted in its accuracy/response over the decades. The '78 ECU frequently drifts leaner as it ages, and the '77 likely does too, being more or less the sister year to the '78.

Edited February 22, 2013 by FastWoman

[EuroDat]

You could read up on this thread. http://www.classiczcars.com/forums/help-me/47597-77-280z-water-temp-sensor-issue-runs-very-rough.html

Chas

[darom]

I use the 5k resistor mounted in the old Clifford alarm enclosure. I added 500 Ohm to the circuit. The car passed CA smog with flying colors.

The idle fuel pressure is the same as the PO's (29-30 psi) at idle. I tested the fuel pressure with 2 new fuel pressure regulators - there is an electric fuel pressure sender in the line, the manifold vacuum is 16 (no leaks, thanks to FastWoman's yogurt cup :-) testing tool).

[jkeese01]

Great job with the Clifford alarm enclosure.

I modified a 5k pot using the instructions from Atlantic-Z write up and calibrated it with a multimeter. The idle sounds better with it set to 1k. Raining here today, so didn't take it out for a drive.

Did you guys adjust the pot based on the sound of the exhaust?