77 280Z Dropping Fuel Pressure once Warm

[Zed Head]

True, at some time in the 1978 model year they changed the type and location of the relays. But the FSM's shows the same picture and location for 76 and 77. 1977 still used the AFM switch, so would have had the dual relay. And, oddly, for some reason, a 1978 parts car that I bought and dismembered a few years ago (with title and proper build dates for a 78) had the 76/77 style dual relay in the same location. It's actually sitting in front of me right now (I keep various odd parts laying around, for inspiration). If I knew then what I know now I would have messed around with the AFM switch and the oil temperature sender on the 78 car to see which controlled the fuel pump.

[siteunseen]

I used about 12 feet of speaker wire, the clamps off an old battery charger and 2 alligator clips to straight wire my fuel pump to the battery. Taped the wire down the passenger's side with painter's tape. I use that all of the time testing different things with straight 12 volts. You can also put your Lowes gauge into a 5 foot piece of hose, you don't need the high dollar fuel injection hose for this temporary testing, coming off that "T" then zip-tie it to your windshield wiper so you'll be able to drive and watch the fuel pressure. Leave the hood unlatched.

[Z-Heap77]

This past weekend I pulled up the EFI Bible and the service manual and started doing some testing. In the troubleshooting section of the bible I found something that said lack of power when accellerating or something like that. It gave some parts to test and then a list of circuits to test at the Ecu connection. All went well until I got to the temp sensor circuit. We actually have a new one of them so we tried to pull out the old. Damn that thing was rust welded in place. I finally got it out but not before cracking the housing and then even after getting it out we couldn't get the nut off the sensor. So I had to order a new nut and used housing. I'll give another update after I get those and get a chance to install them.

I did text the dual relay per the service manual and it checked out ok. Although I know that is while it's cold. My thought process behind moving on to the other testing is that even when it is cold and is running "better" it's still not right. I'm only getting 26 psi at most and as it warms up it drops down to 20. So there's still some issue present even when not completely running like arse.

I'll update as soon as I can. With the holiday and waiting on parts I'm not sure when I'll be able to work on it.

Thanks all.

[FastWoman]

I still think you need to work through posts #5! I can't stress enough that my car was behaving exactly like yours, and the problem was a failing pump. Go to Home Depot and buy some clear vinyl tubing and some speaker wire. That's all you'll need to test your fuel pump right in your driveway. You'll have your answer within a half hour.

I'll mention one other trouble spot, which is the wiring harness main connectors, right inside the passenger firewall. You should check those to see whether you have any black, melty, arcing connectors. Then again, if your pump relay is inside the cabin, the main current may not pass through those connectors as mine does. However, it's a very quick (5 min) thing to check.

BTW, I did mention the coolant temp sensor... in post #5!

This probably won't be important, but I'll mention it anyway, just in case: When you're replacing the thermo housing, be aware that one of the threaded holes runs through to the backside of the timing chain tensioner. You don't want to use too long a bolt there or chase the threads too far with a tap.

[Z-Heap77]

I wanted to give a little update and get some details on how to go about the direct 12v power to the pump test. We replaced the coolant temp sensor, but still ended up with the same results so that rules it out has a possible problem.

I wanted to try the direct power to the pump test to see if I can get the car running and remove the relays and other electrics that have a hand in the running of the pump. But I was wondering if you might be able to provide some details on how to do that. I'm assuming that I remove the stock power wires from the pump and then connect power directly from the battery? Once I connect the power to the pump, won't the pump be running and circulating fuel through the system? Do I then start the car as normal? Driving it in this manner is ok?

Thanks.

[siteunseen]

I straight wired mine with speaker wire and some alligator clips to the battery and the pump's terminals. Try that and pull the bottom hose off the fuel filter, put it in a jug and see how it pumps. Should be a strong steady stream so be careful. Our cars have a return line going back to the tank so it circulates all the time.

[FastWoman]

Before you unhook anything, identify which connector on the pump is positive and which is negative. The easiest way is to do a resistance measurement with the ignition off. The negative/ground connector will show zero'ish resistance to the body. The other will show a few ohms more. When you disconnect the one that shows a few ohms more, the resistance between that wire and the body should be infinite. If that's true, then you've identified the positive side of the pump. Now run a long wire from the battery to the positive side of the pump, and the pump should run. This all assumes your ground connection is good. It might be worth tracing that connection and cleaning/tightening it up. I believe the ground connection is inside the tool box area somewhere, but I might be wrong.

Once you're able to run your pump this way, then do all of your flow and pressure tests in the driveway. Don't drive your car this way! You also don't need to run your engine during the testing, although I suppose you can if you want to.

I would do two tests:

Hook your test hose to the outlet of the fuel pump, so that you can test for restrictions. With the pump running (and obviously not the engine running), you should get a pretty strong and steady stream that won't diminish over maybe 5-10 min, while rocking your car around to slosh the fuel in the tank (e.g. by pushing on the bumpers). If your stream diminishes it might be because junk is clogging up your filter or your pickup screen.

Then hook the fuel rail back up with a pressure gauge between the fuel filter and the fuel rail. Connect your test hose to the return barb on the fuel rail (the one not connecting to the fuel filter). Run your pump again, and observe both the pressure and the return flow rate. They should remain steady over time. If your pump is tired, you might find that the return flow rate starts to diminish until it stops altogether. If this happens, then you probably need a new pump. (This assumes your system passed the first test without clogging up.)

Note that to do these longer run-times, you can put the end of your test hose right into the filler neck of your fuel tank. If your hose isn't long enough, I suppose you could fill one gas jug while you dump the other into the tank.

Edited July 23, 2014 by FastWoman

[siteunseen]

Reference pictures, http://atlanticz.ca/zclub/techtips/fuelpump/index.html

[Z-Heap77]

FastWoman: Thank you for the response and detailed instructions. I wanted to reiterate a few things from previous updates that you might have missed based on some of the comments in your last message.

The fuel pump has been replaced with an aftermarket style, smaller/thinner not the fat OE style. That didn't change the flow rate at the fuel rail mounted gauge. I did remove the fuel filter and attached a gauge to the hardline coming from the pump and was just over 40psi. It wasn't a fully sealed connection at the gauge so the pressure may have been higher, but I thought that range was adequate. I also tested the AFM flap by turning the ignition on and holding the flap down. I could hear the pump immediately kick on and the gauge on the fuel rail read 36psi. That's the only time that gauge has ever read that high. doesn't that prove that the pump is working, the FPR is working, and the system can reach the correct psi?

The only thing I haven't done yet is put the gauge between the fuel filter and the rail. I can do that and direct wire the pump to see what I get. I also have a question about the return line. Can you explain why I would run the test hose from the end of the FPR to the filler neck of the tank when the return line of the fuel system accomplishes the same thing? Is it just so that you can see the rate of flow coming out of the hose itself? Just trying to understand as much as I can about these tests as I do them.

Thanks, everyone, for your responses.

[FastWoman]

ZH, I did miss the part about having a new fuel pump. I think 40 PSI static is shaving the pressure pretty close. When you stomp on the accelerator, vacuum goes to almost zero, and therefore pressure goes to 36 (ideally). At the same time your fuel demand is quite high. Your pressure might drop when you're dumping lots of fuel, causing you to lean out at WOT. I know that's not your "cutting out" problem, but it would still be a worrisome point to me. It would be interesting to see if you can maintain 36 PSI at WOT (accelerating on the road, with your fuel pump hooked up normally and a fuel pressure gauge zip-tied to your windshield wiper), but that's a test for another day.

Yes, the reason to hook to an external test hose is so that you can watch the fuel come out the end, measure flow rate, etc. Of course you're not looking for a "tired" pump, but maybe one that's overheating and/or malfunctioning. Oh, and here's something to consider: If you don't have a good return flow, and if you're not using a lot of fuel (e.g. idling), then your pump might get hot. (It uses the fuel flow to cool itself -- or at least the OEM style does.) Anyway, by doing this, you can verify that the pump is working correctly and sufficiently. If that's the case, then the problem is in your wiring or relays. You're systematically eliminating possible trouble points this way.

Here's another possible cause for your fuel pressure cut-out: Let's say you have a bad electrical connection somewhere in the line. When it passes current, it heats up, and when that happens the contacts flex slightly, breaking the circuit. You should check all the electrical connections from A to Z. You might find a black, melty one somewhere. You might have already done this, but I don't want to re-read the thread. I can't really remember what's in it -- sorry.

[rossiz]

another way to get the fuel pump to run with the engine off: turn the key to on and disconnect the oil pressure sender (located on passenger side of the block, down low, below #2 cyl, should have a rubber boot on it, just pull the plug off) and the pump should start right up and run till you plug the sensor back in. i've used this method a lot to try and prime my fuel rail during hot conditions - simple, no tools required.

[Z-Heap77]

UPDATE:

Sooooo..... you guys (and gals) are going to love this! :-)

We finally gave up on trying to figure out the issue and we had a mechanic that we've had some history with, pick up the car and try to help us figure out the problem. Long story short, the mechanic noticed that the coolant level was really low (we lost a bunch doing the intake swap and other misc tasks). He filled that up, checked the calibrations on the AFM and TPS and a few other things..... car has run perfectly since. He's had the car for a week, letting it sit running to get hot, took it very several rides and never had one problem. Apparently the temp sensor was throwing some ugly info back the ecu since the coolant level was so low and once hot it had big time negative affects on fuel supply.

He does believe the gauge we have on the rail is off as it's still only showing 20 psi, but I think he did put the gauge right after the fuel filter and was at 30 psi or more.

4 weeks of BS for low coolant level. Pretty amazing!