1985 720 base shutting off randomly

[Jp2]

I am at a loss here. I can't get a consistent result but any ideas as to what's going on would be much appreciated. 

 

About a month ago a mechanic had "replaced the seals on the carburetor" and replaced my fuel pump. He also replaced a seal in front of the engine because it was dumping oil (this was a known issue but I just kept topping up because I didn't want to remove the radiator, so I just had the mechanic do it.) 

 

I sent the truck to the mechanic for a fuel pump because she quit on my way to work and I couldn't hear the normal repetitive sound of the fuel pump when the truck was on. After that all said work was done. 

 

Truck hasn't leaked since. Truck has had a strong idle since, for the most part. 

 

Normal errands here and there, no problem. But sometimes it will just want to turn off. It's hard to replicate the results. I am following the starting procedure on the sun visor. It feels like between gears the truck is extremely lean sometimes and then when I put it in neutral it will turn off. I hear the fuel pump is working, so I don't think it could be that. The mechanic said the new fuel pump actually had more PSI than my old fuel pump which is why he had to replace the seals on the carb. 

 

Other than 3 instances in the past week, she runs like a charm. strong idle, no problem between gears. Also whenever it happens, I have been able to just start again and truck runs just fine. 

 

Do any of you have an idea? My leading theory is that there is something that happens in the carburetor when I turn it or when the truck moves a certain way thus choking the engine and leading to the truck shutting off. That being said I don't know enough about carburetors as this is my daily driver and I'm not keen on messing it up by attempting to rebuild it. 

 

Thanks in advance

[Jp2]

Oh also I thought it could be a vacuum issue but I just went through all the vacuum lines from a niko club thread about a guy going through all the vacuum lines on his 720 and everything looks to be in order. 

[datzenmike]

Mechanics will say anything to justify their extra work. The only possible problem with a higher pressure fuel pump is it may force more fuel in and over flow the carburetor. Seals have nothing to do with that. Correct fuel pressure is 2.7 to 3.4 PSI.

 

There is a fuel pump relay located above the passenger side kick panel up out of sight. It's job is to shut the fuel pump off in the event of an accident. One of the several inputs to turn the fuel pump on and keep it running is the starter. Maybe this resets the pump relay and allows an easy restart. If faulty and intermittent you are basically running out of gas each time it quits.

 

 

 

 

Disconnect the plug on the relay. Make a short jumper wire and connect the L (Nissan code for Blue) wire and the White/Black stripe wires. This will turn the pump on whenever the ignition is on. It the problem goes away and stays away then the fuel pump relay is bad.

[DwayneOxford]

Mike can you find the solder fix for the relay you posted years back?

What I did on mine was get it down to where I can rap it when it quits.

[720_Jeff]

2 hours ago, DwayneOxford said:

Mike can you find the solder fix for the relay you posted years back?

What I did on mine was get it down to where I can rap it when it quits.

Are you talking about this article?

https://axleaddict.com/auto-repair/The-Datsun-and-Nissan-Pickup-Fuel-Pump-Relay-What-the-Haynes-Automotive-Repair-Manual-Wont-Tell-You

[datzenmike]

That's the one. Probably easier to buy a new one. The manual describes the conditions of running but it makes no sense to me. Ignition on should start the timed relay and I would assume that if you didn't start the engine the timer would shut the pump off. There's alternator charging, a good indication that the engine is running and the oil pressure switch another indication of the engine running. Should either stop working the pump should shut off. I thought the starter was also involved.

 

Anyway, if you make a jumper this goes around the relay. If the problem goes away then it's assumed the relay has a problem.  Does your charging system work properly? On the 720 a failing alternator can cause the choke heater relay to chatter.

[DwayneOxford]

13 hours ago, datzenmike said:

That's the one. Probably easier to buy a new one. The manual describes the conditions of running but it makes no sense to me. Ignition on should start the timed relay and I would assume that if you didn't start the engine the timer would shut the pump off. There's alternator charging, a good indication that the engine is running and the oil pressure switch another indication of the engine running. Should either stop working the pump should shut off. I thought the starter was also involved.

 

Anyway, if you make a jumper this goes around the relay. If the problem goes away then it's assumed the relay has a problem.  Does your charging system work properly? On the 720 a failing alternator can cause the choke heater relay to chatter.

The one I remember had only one "trouble" joint pointed out.

Went to WIN 10 from 7 and the browser I had then didn't make the trip. Emailed them, hope to get back in.

[datzenmike]

No idea if this is a common problem or random. About all you can do is open an look for something burnt or on the other side a loose solder connection.

[Jp2]

I'm trading cars back with my dad today so I will be looking for the relay after work. I lent it to him to see if he could replicate the issue and troubleshoot (the truck was a hand-me-down from him so he has a bit more experience with it) he said that it's a little bit jerky from when he last drove it but he that he just needed to be on the gas earlier than he used to when shifting gears so he thinks the mechanic adjusted the carb somewhere while they were fiddling but he said it didn't quit on him all weekend. 

 

To address some things: I am way to scared to mess something up if I start splicing wires around. @datzenmike, I appreciate the advice and I see you said it's probably easier to get a different relay to test it out. I know it's impossible to tell without evaluating the truck but do you think I replaced the fuel pump without a proper troubleshooting? Maybe the issue could've been a weak connection with that relay all along? 

 

I see a ton of 1985 Nissan 720 relays on ebay. Even a brand new replacement on amazon https://a.co/d/bfyAhKK. Is it worth just buying one and trying to replace it? And do I need to disconnect the battery if I do? 

[datzenmike]

There's no wire splicing involved. The small jumper simply sens continuous power to the fuel pump when the ignition is ON. This test just eliminates the relay from the system. If it fixes the problem it can safely be assumed that the relay is at fault.

 

Yes you could just replace the relay but it's what? $50 and if it doesn't change anything you now have two good relays.

 

No you don't need to disconnect the battery. There is no power to the relay with the ignition off.

[DwayneOxford]

16 hours ago, Jp2 said:

I'm trading cars back with my dad today so I will be looking for the relay after work. I lent it to him to see if he could replicate the issue and troubleshoot (the truck was a hand-me-down from him so he has a bit more experience with it) he said that it's a little bit jerky from when he last drove it but he that he just needed to be on the gas earlier than he used to when shifting gears so he thinks the mechanic adjusted the carb somewhere while they were fiddling but he said it didn't quit on him all weekend. 

 

To address some things: I am way to scared to mess something up if I start splicing wires around. @datzenmike, I appreciate the advice and I see you said it's probably easier to get a different relay to test it out. I know it's impossible to tell without evaluating the truck but do you think I replaced the fuel pump without a proper troubleshooting? Maybe the issue could've been a weak connection with that relay all along? 

 

I see a ton of 1985 Nissan 720 relays on ebay. Even a brand new replacement on amazon https://a.co/d/bfyAhKK. Is it worth just buying one and trying to replace it? And do I need to disconnect the battery if I do? 

To jump relays and harness connectors I cut a 3" piece of 12 or 14 gauge Romex, strip ends, bend into U to push into rear of connectors.

[datzenmike]

That would work well.

[Jp2]

Just wanted to priced provide an update regarding the 720. 

 

Seems like truck was able to figure it out on its own. Been driving it around a lot and haven't had the same problem at all. 

 

That being said I've been giving it more love. I actually let it idle for 30 secs before revving once half throttle for about 0.5-1 sec now since that's what it says to do on the sun visor. I didn't do this at all for the first twelve years one owned this truck. (never read the sun visor) (also it says to protect the catalytic converter, which I didn't think would have an impact on how it ran) 

 

I also did unplug and replug all the vac lines but nothing was loose or anything.

 

 

Tl;Dr: I did nothing meaningful and now I haven't had any issues with the truck. All your suggestions and recommendations were very much appreciated and I had this thread handy in case anything were to happen and still do just in case.