Zed Head

Jump the appropriate pins in the relay connector. If the various lights are still dim then it's likely that the relay is fine. That way you don't have to contort yourself to remove it. There are a lot of connections on the brake warning lamp circuit.

It might help to restate the state of affairs. One change seems to be that "won't start when hot" has morphed in to "runs funny after it starts".

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You could also check power at the EFI relay. That's the green links circuit.

Somewhere between the terminal on the battery and the other end of the battery cable at the starter, there should be a another wire with a white plastic connector that has two green wires heading off to the EFI system. Open the cowl cover and take a look. Take a picture and post it.

The link holder is connected to the positive post.

You didn't say if you cleaned and tightened the loose fusible link terminal. If it responds to a wiggle it's loose. (All of this wiggling and squeezing is making me feel funny. Says Garth). You can just just squeeze them tighter with a pair of pliers. Don't forget to check the two green EFI links from the battery positive post. And, of course, check your grounds.

Heat. The bolts typically rust in the bores and rust expands when it is formed. Heat the aluminum to get it to grow and it will help break the bond. If a bolt breaks it will probably be just the head and upper shaft. The housing will still be stuck on the rusty part. Once you get the housing off the remainder of the bolt will probably come by hand.

You can't do SteveJ's test with wire wiggling. Multimeters look complex but most of us only use two of the functions. Voltage and resistance. You can ignore all of the rest. https://www.harborfreight.com/7-function-digital-multimeter-63759.html https://www.harborfreight.com/612v-circuit-tester-with-5-ft-lead-63603.html

You'll need a meter or test light to continue. Wiggling wires might get you there but it will be a lot faster with some simple testing equipment.

I'd diagnose the two issues separately. There's a crank/no-start issue and the gauge power issue. You can test the no-start issue by testing for coil power and injector power with the key On and at Start. It's not clear that the ignition relay is the problem but it could be. You can test for power with the key on at several points on the path through the gauges and on to the sensors themselves. I'd start at the fuse box. If there's power on that fuse with the key On then the ignition relay is fine, for that circuit. A loose fusible link connection seems like a possibility. Many ways to break the system down in to parts.

Here's a ZX relay just to fill the thread out. Maybe it's just two years. https://www.autopartone.com/products/79-80-datsun-280zx-oem-relay-25230-w0500

Yes, I looked at the 76 diagram and it shows the same thing. What's funny is that the ZX's did have a terminal and a dedicated ground wire so the drawing does match a ZX. Weird. It was a spade terminal though, not a threaded stud. I picked up a spare from a ZX just in case. In the ZX's they're behind the glove box. Here's a listing and a picture of 78 relay. You can see the four places to bend to get the cover off. I called them tbas but they're really more like crimps. A pair of straight jaw pliers and four squeezes and it will pop right off. Might be a ground wire connected inside though so be careful, if you get that far. https://www.autopartone.com/products/75-76-77-78-datsun-280z-oem-ignition-relay-25230-89958

Looks like the SARD might be like others. https://www.google.com/search?q=sard+fuel+pressure+regulators+leak+down https://www.rx7club.com/new-member-rx-7-technical-256/sard-fuel-regulator-leak-injector-line-816953/ But then this guy below says his does hold pressure. I'm seeing that many people aren't real clear on things like the check valve inthe fuel pump. The guy in the link above could have a check valve problem. https://aus300zx.com/index.php?threads/holding-fuel-pressure.337928/page-2

Weird that that link is dead now. It should have opened to a page with a couple of SARD FPRs. Google shows them on eBay too. I'd search around first and see if you can find any internet comments about leak-down. Just because one didn't doesn't mean that they all don't. https://www.google.com/search?q=sard+fuel+pressure+regulators

The pinout to the relay is shown in the wiring diagram. And you can easily pry the four tabs up and take the cover off. Don't overlook that it grounds through the mounting points. Looks like they might have added a wire in 78, I don't think my 76 had that.

I was browsing pictures of a new BaT Z and found what looks like an FPR that holds pressure. Might be worth some research. Picture 114. https://www.nengun.com/oil-fuel-fuel-pressure-regulators/ https://bringatrailer.com/listing/1972-datsun-240z-290/

The Isuzu valve was nice because it was all one piece and the metal tube is 5/16" / 8 mm. cut the tube and slide it in. I posted a picture in a previous thread. I can't remember the year, I think it was late 80's. Unfortunately, not really common to find them in the yards.

Which option above? Whichever way you run the hoses there needs to be a connection to the intake system, if you want a sealed system. That's where the crankcase pressure goes. There's always blowby so there's always going to be pressure. The simplest method would be to maintain the stock flow paths but to extend their length to run through the catch can(s).

Doesn't show up on the web site. https://www.sstubes.com/search?q=280z&_pos=3&_psq=280z&_ss=e&_v=1.0

It's definitely a real-time reference. Once I got my Z problems all sorted out I barely opened the hood more than once a month. And that was only to check the oil and coolant reservoir. So, it really depends on where you're at in your project. I also had found a Schrader valve T'ed in to an Isuzu fuel line in the wrecking yard. So I had a Schrader valve for measuring pressure easily. Many different ways to do things. You can probably find electronic methods today and have a gauge in the car.

I made one from some inexpensive blemished bar stock. It wasn't too hard. I had a drill press and built a jig to hold the bar. I think it might have been from here. Still out there. Edit - not sure if one foot is enough though. The eBay listing reads like you can only get one foot sections. Can't remember. Probably in site's thread. https://www.ebay.com/itm/204109661735?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Pm6JCi7cTr6&sssrc=2047675&ssuid=bBzFtXzvTQO&widget_ver=artemis&media=COPY

That seems like a lot. There's really no need for a gauge, unless you plan to change fuel pressure often. Once you set it it doesn't change. Looks like they're charging $30 - 40 just to drill and tap a hole. If you're handy you can do that yourself. https://www.godzillaraceworks.com/fuel/pallnet-billet-aluminum-fuel-rail

It's been off the road for a while. I think I see a 94.

Maybe this guy will run a comparison. He does some pretty neat stuff. https://www.youtube.com/@ProjectFarm