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

[jkeese01 0]

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 1,815]

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 2]

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 0]

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 0]

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 1,815]

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 504]

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 0]

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 51]

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 504]

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 2]

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 0]

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?

[Zed Head 1,815]

I originally installed mine to see if I could fix the hot start problem by adding fuel (it helped the engine run better but you still had to wait for the injectors to cool down). Then I installed an AFM with a lean spot at low RPM and ended up using it to get rid of the lean spot. So I tune mine by throttle feel. A lean spot will feel like a lag in response followed by a surge as the AFM vane passes through the area.

Not the best fix, since it's probably rich in other areas, but it's a pretty handy band-aid.

[FastWoman 51]

@darom: Glad it all worked out for you! Nice implementation of the pot!

@Kjeese: I adjusted my pot on the basis of a few things: (1) Exhaust flow and smell, (2) idle vacuum/RPM, (3) off-idle vacuum under constant load (A/C compressor), (4) plug color. I found a happy, all-around middle ground that gave me good results on all of these.

[jkeese01 0]

Drove the Z today for the first time after adding the 5k pot. Car seems to run smoother when set at 1k. This mod is simple to build and definitely worth adding.

[78sid 0]

Very timely post for me. I've been rebuilding a Z for three years and I'm about to get it back on the road. The engine has been rebuilt and all the components tested per the FSM. In testing my engine which is almost completely stock for leaks and other issues, I've noticed that it runs fine until it warms up. Once it warms up, the idle drops and the car shuts off. If I hold the throttle only slightly, it will run fine, but if I rev it up slightly and back off, it shuts down immediately. I can restart easily with slight throttle, but I have to hold it.

I'm thinking I need to try adding this fuel pot, but I'm also wondering if the idle set screw could help. Turning that screw seem almost as hopeless as getting out a spindle pin. Any input would be appreciated.

78 280Z

[FastWoman 51]

Your idle screw won't turn? That might be all your engine needs.

[78sid 0]

I did get it to turn using vise grips, it is all the way down or at least the spring seems to be tight. It did improve if only slightly but the engine still slowly idles down to shut off once the engine warms up. I'm assuming that tightening the screw increases the idle speed. I was also wondering if I could adjust the idle screw on the AFM and get some results. I was going to try that before trying the pot.

[Zed Head 1,815]

The idle speed screw actually controls air flow through a bypass channel. Turning the screw out (counterclockwise) increases idle speed.

The AFM screw just controls the air/fuel ratio, at very low air flow (low RPM). It might affect idle speed but only indirectly and not much.

[78sid 0]

Thanks Zed!!!!! Holding steady and clean at 800 RPM's. Sweeeeeeet. I might actually get to drive this car on the road tomorrow after a three year rebuild.

[Captain Obvious 2,089]

the engine still slowly idles down to shut off once the engine warms up.

Look at the bright side... You inadvertantly also verified that your AAR is working properly.