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77 280z won't stay running, no idle, out of ideas


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#1 akanning

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Posted 23 February 2017 - 07:27 PM

Okay, I have done a lot of reading through various forums and I cannot find anything alike enough to my problem to correctly diagnose it. I have both the fsm and the efi bible and have done many of the tests as I will explain.

To start off I have a 77 280z, f54 block, p90a head, totally stock. New ngk plugs and wires, gapped correctly, getting good strong spark. The fuseable links have been upgraded to maxifuses, the fuel pump is wired up to a kill switch. I have a fuel pressure tester T'ed between the fuel filter and the fuel rail to check pressure.

It ran strong and perfect for a few weeks after I bought it early last year and then I ran into some charging issues that were solved by cleaning grounds and connectors. But it sometimes would bog down while I was driving and chug until it died. I got it running for a few days by doing more cleaning, cleaning the afm and tips, making sure they were properly adjusted.

I wasn't able to get it started for a few months. I would turn the key and it would do nothing, I tracked it down to a bad battery and corroded starter solinoid, so I got new ones.

Now the problem was a strong crank but no start. I sprayed starting fluid into the throttle body and it would fire up for a few seconds and die, so obviously it was a fuel problem. So I started tracking more down. The dash was out at that point because I was doing repairs on it, and so I threw it back in, and as I was reconnecting connectors I found one that was on the passenger side that was disconnected yet didn't attach to the dash, I connected it, and without looking at my wiring diagram I'm pretty sure that it is one of the efi connectors, one that should probably have been plugged in.

Now that we have a backstory, about a week ago after installing the dash and reconnecting that connector me and my buddy Matt pushed her out of my garage and turned the ignition to on and turned on the fuel pump, the pressure gauge shows a strong 35ish psi as it should be, turned to start, and it cranked and fired right up!!

For about 2 seconds and then the rpm dropped down to nothing and it was dead. We tried again, this time with the throttle slightly depressed, it fired right up again, but this time, again the throttle about halfway depressed, it seemed to have an idle. However as soon as you let off the gas it dropped down in rpm all the way until it died. For the first couple startups if you revved it up it would respond and rev, however all the sudden, it would start with throttle depressed, but when you pumped the gas, nothing would happen, the rpm wouldn't move, as if you weren't doing anything, and as soon as you let off the gas it would die. Also, it was pouring out lots and lots of blackish smoke, which at first I assumed was just from accidentally over gassing the engine by turning on the fuel pump and cranking with no start. But it continued to put out lots of blackish smoke every time it ran. It also would backfire every once in a while, and a neighbor said it almost sounded as if it were only running off a few cylinders, however I've had no experience with that so I cannot know whether or not is is until I find a compression tester and do that.

For those that have read through and have any idea, it is extremely helpful. If you need I can give you more numbers and maybe even a video if I can get to it soon. I have the money to fix whatever I need and yet I cannot figure out what is going on. I know there is a wealth of knowledge flowing around in here, and I am ready to gain some of it.

#2 NewZed

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Posted 23 February 2017 - 07:55 PM

Check the coolant temperature sensor.  Sounds like it's running super-rich, with the blackish-smoke.  

 

That engine is a 280ZX engine, maybe a turbo engine, so whoever put it in might have used the CHTS instead of the coolant temperature sensor.  Either way, they need to be connected and indicating engine temperature, otherwise the ECU sees an arctic-cold engine and dumps lots of extra fuel.



#3 akanning

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Posted 23 February 2017 - 08:23 PM

I will check that. And it's the stock motor, I believe end of 77 is when they started using that head and block combo on the na cars.

#4 JSM

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Posted 23 February 2017 - 08:53 PM

My 79 has the CTS. The CHTS came out in 80. Your running a P90 head and the CHTS would be back by cyl 6. They probably mounted the stock 77 thermostat housing on the p90 head.

Also check the bullet connectors and clean them going to the CTS. I would also unplug the cold start injector and see how it runs.

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#5 Leskinen

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Posted 23 February 2017 - 08:54 PM

You can check the CTS by removing the coolant temp sensor plug and use a paper clip (or something equivalent) to connect the two pins in connector (= no resistance). Check also the resitance of the sensor.

I had the same problem with my -75 with similar symptoms. It was a bad CTS.

Edited by Leskinen, 23 February 2017 - 08:54 PM.

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#6 NewZed

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Posted 23 February 2017 - 09:07 PM

I will check that. And it's the stock motor, I believe end of 77 is when they started using that head and block combo on the na cars.

Good luck.  And you believe wrongly.  78 got the N47 on N42, Mid-80 or 81 got the P79 on F54.  P90 came in 81, on the turbo.

 

If you really want to go directly to what the ECU sees, check the coolant temperature circuit at the ECU plug.  That way you won't have to determine if it's a CHTS or a thermostat housing based sensor.  All that matters anyway, is what the ECU sees.



#7 akanning

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Posted 23 February 2017 - 10:32 PM

You're totally correct, thank you for that, I misread something somewhere, it's good to know where my engine came from.

Any how, I will be checking the CTS or CHTS, whichever is there, and making sure everything is good. I'll test the pins to eachas stated in FSM, as well as from the Ecu.

#8 akanning

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Posted 26 February 2017 - 08:35 PM

The CTS checked out good, 3.6 kiloohms at around 50°F. The insulation resistance was also good, no continuity. There is a little bit of greenish corrosion on the plugs, I'm going to clean those, as well as the bullet connectors, any other ideas?

#9 twofouroh

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Posted 10 March 2017 - 12:44 PM

Check the injectors for flow, one or more may be stuck open.  Check the cold start injector too.  That black smoke means something is flooding it with gas.


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