Confusion about original fuel setup

[Arne]

Now I am running early roundtops. So I can maybe get by with just the new mechanical pump???

The original mechanical pump on my '71 is fine for my four-screws. That's the only pump that the '70-72s came with, so it should be fine for your ZTherapy carbs. Assuming that everything else in the fuel system is right, of course.

[bjhines]

I am providing pics just to clear up continuing confusion on the 1973 240Z fuel-cut harness.

The harness has a terminal from the voltage regulator, a terminal from the starter solenoid. It also contains a FUSE for pump power(many people neglect this in their mods).

Here is the harness, it is actually 2 different harnesses

Fuel-CUT relay harness Part number

Fuel pump POWER harness part number

Here is the passenger relay bracket with the fuel cut relays removed. It also has the EGR relay and the stone-cold temp switch removed.

I sincerely hope this clears up the misnomers that continuously float around on the web.

[Carl Beck]

I have to chime in here on some misinformation on the operation of the ELECTRIC FUEL PUMP WIRING. The 1973 240Z uses ONLY THE MECHANICAL PUMP during normal operation. The electric fuel pump is for priming the system ONLY!!!!! It will not operate once the engine is running. None of this is detailed in the electrical diagrams. There are hundreds of things not shown in any of the wiring diagrams.

Hi bjhines:

Either intentionally or unintentionally - you make it sound like all US spec. 73 240-Z's came with the relays and harnesses you describe/show. Just to be clear - they did not.

When the 73 Model Year 240-Z's arrived - they carried basically the same wiring for the electric fuel pump as the 70-72 model years did. To mount an electric fuel pump at the tank, using the existing wiring in the harness - you simply added a jumper with an in-line fuse holder, to connect two wires under the right side of the dash. As Arne outlines... that makes the pump feed hot when the ignition key is on.

Later in the Model Year - after thousands of customers were stranded by vapor lock and/or percolation problems - Nissan came up with a series of Dealer Installed modifications intended to address the starting and running problems, brought about by the 73 Emissions Standards here in the US. These modifications included "fix'es" and/or adjustments to many components, only one of which was the addition of an electric fuel pump.

The harnesses and relays you show - were only part of a Dealer Installed Kit, called the V-3 Modifications Kit - only 73 240-Z's that were returned to the Dealers, along with continuing customer complaints, received the Kit.

It is possible that very late production 73 Model Year 240-Z's started to arrive from the Factory with the electric fuel pump installed... maybe the last couple of months of production. I don't know for sure, as by then - I was no longer working for Datsun.

All this is outlined in Nissan's Service Bulletin titled "DATSUN 240-Z & 260Z MODIFICATIONS PLUS - FUEL SYSTEM MODIFICATIONS, as well as in several other Service Bulletins.

There are a bank of relays on the passenger kick panel for the 1973 240Zs. There are 2 relays associated with the electric fuel pump.

Relay 1) cuts the fuel pump OFF once the alternator produces 12v(engine running).

Relay 2) cuts the fuel pump OFF when the starter is engaged(start pos on the key).

The operation of the electric fuel pump is only to PRIME or REPRIME(vapor lock) the fuel system. It will operate when you turn the key to the RUN position. The relays cause it to shut off once the car is running or the starter is engaged.

I'm certainly no expert on this - but the Service Procedure tells you to check the fuel pressure on the electric pump with the engine running at 1000 RPM. I don't understand how that could happen if the relay turns the pump off after starting.

Vapor lock was only one of the issues, percolation and vacuum break were also major contributors to hard starting after running, stopping then restarting.

FWIW,

Carl B.

[bjhines]

Nice info there Carl. Those are the harnesses.

It was my understanding that the add on kit was available to retrofit all the earlier models but was a factory installed piece on the 1973 models. I have never seen those harnesses in an earlier car. I have seen them in both the 1973s I have owned... and a few other 1973s.

Recheck the service procedures. I have checked the series relays operation and they are there to CUT OFF the electric fuel pump. There is no way you could test the operation of the electric fuel pump with the engine running... unless you disconnect the voltage regulator terminal.

[Six_Shooter]

On my '73 the Electric fuel pump ONLY runs with the engine running, it does NOT, ever come on BEFORE that.

It also would not make sense to have two cut offs, both effected while starting, as in one being a direct connection to the start signal, and one being RPM dependant (voltage signal from the N terminal on the Alt/reg). Only one OR the other would be needed to work the way you describe. Also the fuel pump would sit there and run, for as long as the key is turned on, but not started.

I'll have to disect the add on harness, but I'm sure you'll find that the start signal energizes the circuit to "prime" while starting and then the signal from the alt keeps the circuit powered up, and will shut the circuit down, in the event that the engine stalls/shuts off.

About the fuel shutting off in a hard corner, due to low oil pressure on an oil pressure based fuel pump safety system, sounds more likely a problem with oil pan design, or low oil level.

The ONLY time I have ever had my oil pressure drop, is when it was low, usually by a couple litres (I had a car that had a semi-quick oil leak for a while).

I found this thread (and a few others), because I'm upgrading my alternator and noticed the jumper harness between the regulator and the harness, and followed it back, to discover what looked very much like an add on harness, and wanted to find out more about it, to see how to make it work with the new alternator. Looks like I'll be switching over to an oil pressure based fuel pump control system.

[beandip]

Six_shooter . It is a simple addition. Buy a tee and connect it to the port used by the oil sending unit . Attach he sending unit back on and add a simple pressure switch there. My switch activates at 5 to 7 psi. and has two spade connectors on the top of it. Just wire the switch in the power source to the pump by the counsel/dash where the in line fuse is located. Even after 3 months of non driving my car , she still fired and ran with the fuel in the float bowls. As hard as I have pushed my Z I have never had the oil pressure drop wile cornering. I am not saying that it is not possible . This is one of the reasons race engines run with a dry sump. Gary

[Six_Shooter]

Six_shooter . It is a simple addition. Buy a tee and connect it to the port used by the oil sending unit . Attach he sending unit back on and add a simple pressure switch there. My switch activates at 5 to 7 psi. and has two spade connectors on the top of it. Just wire the switch in the power source to the pump by the counsel/dash where the in line fuse is located. Even after 3 months of non driving my car , she still fired and ran with the fuel in the float bowls. As hard as I have pushed my Z I have never had the oil pressure drop wile cornering. I am not saying that it is not possible . This is one of the reasons race engines run with a dry sump. Gary

Yeah I have installed more than a few oil pressure fuel cut off systems in my days.

I'm just trying to decide whether I want to tap the block or find (make) an adaptor to retain the original thread, since I have read that it is different, I'll be verifying that today, most likely.

I'm not opposed to modifying my car, I just have this real strong opposition to metal shavings in my engine. (yes, I know the grease trick, still worries me.) I'll likely just end up tapping it differently since Iusually just do things that way anyway.

The alternator works great, needs a smaller pulley than I had, but solid 14.7V at just above idle and on up.

I also had my electric fuel pump disconnected, and drove it all yesterday and last night, about 150 miles or so, no appreciable differnce in the way it ran with the pump to without.

[biddljj]

I wish there was a "Premium Thread" option to this web site. Great discussion

zdisease

[beandip]

Yeah I have installed more than a few oil pressure fuel cut off systems in my days.

I'm just trying to decide whether I want to tap the block or find (make) an adaptor to retain the original thread, since I have read that it is different, I'll be verifying that today, most likely.

I'm not opposed to modifying my car, I just have this real strong opposition to metal shavings in my engine. (yes, I know the grease trick, still worries me.) I'll likely just end up tapping it differently since Iusually just do things that way anyway.

The alternator works great, needs a smaller pulley than I had, but solid 14.7V at just above idle and on up.

I also had my electric fuel pump disconnected, and drove it all yesterday and last night, about 150 miles or so, no appreciable difference in the way it ran with the pump to without.

I bought a tee at my local parts store that is compatible with he block and the sending unit. I took the sender with me just for that reason. I wouldn't run a tap into my block either. Another way of safeguarding this is the use of a inertia switch. In case of a crash or roll over it will cut the power. This is the reason I have done what I have here. Not worried about a oil pump failure. I just don't wish to be French fried with the gas running if I crash and cannot turn off the key. I have seen and dealt with this type of crash when I was a Fireman in Los Angeles. Gary