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New to the forum and new to 720s


MaxG

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Hi everyone, 

 

I just recently purchased a 1985 Nissan 720 4WD King Cab (manual transmission) 223,xxx miles on it, and since I've never owned a 720 before, I have a few questions about what is normal and what is not for this truck before I attempt any repairs. All help is appreciated!

 

1.) When I start the truck, it will not start unless I apply pressure to the throttle at the same time that I turn the key. I've never had a manual car this old so I'm not sure if this is normal for the era or if there is something wrong. Also, once the truck starts, if I release the throttle and just try to sit in neutral, it will shut off. 

 

2.) The truck idles at around 1000 rpm, but sometimes it will shut off if a stoplight is extra long or if I'm sitting in a drive-thru. Ideally, what should the idle be set at? Also, if I shift from a mid-range gear like third and coast to a stop light, the engine will shut off at times as well. All of these idle questions lead me to believe something is not right with the idle haha. 

 

3.) The speedometer does not work at all, but the odometer and all other gauges are fine, so I imagine it needs a new speedo cable or speed sensor. Before taking it apart, I've been trying to figure out where the speedo cable and speed sensor are located, both online and in my Haynes manual, but I can't find any concrete diagrams or images. Is the location extremely obvious once I take the cluster out? I don't want to mess any of the other gauges or sensors up by removing things thinking I'm removing the speedometer cable. 

 

4.) Just changed my oil and oil filter, but oil light is still on. Also, the oil pressure gauge seems to be reading low. What is the normal PSI? It's on the gauge that has 0, 45, and 90 and it always hovers around the bottom third of the gauge. Never exceeding 45 or approaching 45.

 

5.) The ride is bouncy over speed bumps and other bumps. I've replaced struts on a previous car and that car was making noises going over bumps, which this truck is not doing. It doesn't sag either. The truck is 4x4 and has a lift from the factory so I'm not sure if I need to fix anything with the suspension or if it's bouncy because it always was. How bouncy is too bouncy? 

 

6.) Also, please see the image. https://imgur.com/a/SBk08GD What was in the blank spot with the wires hanging out? In all of the other images I've seen of 720 interiors, it looks like that spot just has another little plastic cover?  Should those wires be attached? If so, what do they go to? 

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If anyone is wondering about number 6, that's for the sensor light, which was discussed here: 

 

 

Hopefully someone else will find it helpful to see where the wires are on the inside of the truck with that image. 

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1/ To start when cold from overnight sitting, step on the gas 2 or 3 times and release, turn the key it should start right away.

 

2/  Not sure what carburetor setup you have. Is there an electrical connector at rear of carb and does it have 2 or 6 wires to it?

 

3/ If odometer is working then the speedometer cable is working and it must be the gauge. 

 

4/ Anything above 45 PSI is ok 55 would be better. The light comes on below 8 PSI so I would replace the sender (which powers the red light and the gauge) located above the oil filter up under the intake on the side of the block. 

 

5/ Replace the 4 shocks. 4x4s are stiffer you may have to live with it.

 

6/ Does the rear window have wire threads across the glass?  Could be for the rear window defogger switch. Can you show the plug? Should have these wires...

Yellow/Blue

Red/Blue

Red/Yellow

Yellow

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I'm far from a guru on these issues but in my experience. Generally my 720 starts by barely turning the key over just for a fraction of a second and runs with no throttle. If the ambient temp is below 45°f or so then it will slowly idle down and die if you dont feed it some gas but that is only when cold started on a cold day. When I started having issues with failure to idle at stops it was due to the choke not opening I believe replacing the relay was what fixed it 

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Agree, there are so many things that it could be. The 1,000 RPM idle is way too high and that could be the idle cut solenoid isn't working and the idle speed is set by running on the primary barrel. This is bound to be iffy. Choke not shutting off is another and will run over rich all the time and foul the plugs.

 

I asked 2/ to find out if this is an ECC set up or regular carburetor.

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When replacing the oil sensor always use a wrench on the hex part. Putting pressure on the barrel breaks them. They are bpst  British pipe thread a universal one has the wrong threads. There is a different one for trucks with a guage or just an idiot light. Make sure you get the one for the guage and idiot light.

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Thanks everyone for the quick responses, I appreciate the help! 

 

1.) @datzenmike you're right that it starts up just fine when I press the gas 2-3 times before starting it, but it still dies after about 10-15 seconds if I don't immediately shift into gear or hold the throttle. Even when the engine is warmed up and I've been driving for a bit, it'll still randomly die at lights or when coasting to a stop. I kept an eye on it all day today and it's average idle is about 1000 rpm when it decides to dip down and shut off. 

 

@None_zero did you replace anything other than the relay when you fixed that issue? If I'm not mistaken, the relay is essentially just a plug-in piece right? 

 

2.) In regards to the carb, it has 6 wires on the back end. 

 

3.) In regards to the speedometer, is the best bet just to get a used one and replace the whole cluster? Or is there a way to fix the gauge that isn't too tedious? 

 

4.) I'm doing an oil pressure check tomorrow so we'll see what's going on with that and if it's just the gauge or if the pressure is actually low. 

 

5.) Will replace shocks at some point, thanks for the help.

 

6.) That little wire goes to the sensor light,  which according to past posts on here, should be unplugged anyway so that's good news. 

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No it was the only part I replaced but I also was jacking around with the idle speed setting and the air fuel mixture setting (which I still have to set back right actually once I out everything back together) although i ran for a while with a carpenter's pencil stuck in the choke to hold it open. Eventually it deteriorated to a no start condition due to one of the wires from distributor to coil was fried cant remember if it was the intake coil or exhaust coil..  either way what I'm driving at here is its possible that poor contact on the coil was part of the issue I had .....and yes the relay is just. a squarish black cube that should be mounted on the passenger side under the dash just above the fuel pump relay

Edited by None_zero
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7 hours ago, MaxG said:

Thanks everyone for the quick responses, I appreciate the help! 

 

1.) @datzenmike you're right that it starts up just fine when I press the gas 2-3 times before starting it, but it still dies after about 10-15 seconds if I don't immediately shift into gear or hold the throttle. Even when the engine is warmed up and I've been driving for a bit, it'll still randomly die at lights or when coasting to a stop. I kept an eye on it all day today and it's average idle is about 1000 rpm when it decides to dip down and shut off. 

 

@None_zero did you replace anything other than the relay when you fixed that issue? If I'm not mistaken, the relay is essentially just a plug-in piece right? 

 

2.) In regards to the carb, it has 6 wires on the back end. 

 

3.) In regards to the speedometer, is the best bet just to get a used one and replace the whole cluster? Or is there a way to fix the gauge that isn't too tedious? 

 

4.) I'm doing an oil pressure check tomorrow so we'll see what's going on with that and if it's just the gauge or if the pressure is actually low. 

 

5.) Will replace shocks at some point, thanks for the help.

 

6.) That little wire goes to the sensor light,  which according to past posts on here, should be unplugged anyway so that's good news. 

 

 

1/ On cold start the engine should rev up with a fast idle... around 1,800. Does it??? The fast idle lowers as the choke slowly turns off. Does it????

 

2/ Then this is a ECC or feedback carburetor. You will have an O2 sensor in the exhaust manifold and a ECU under the driver's seat.

 

3/ Replace the gauge.

 

4/ The gauge sounds like it's right. The red light is grounded through the sender in the block. Unplug it and if the red light stays on then the wire is pinched and grounding somewhere. If the light stays off then it's in the gauge.

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On cold start it only revs to about 1200 and then dips down. I'm going to check out the vacuum lines and make sure everything is in the right spot on the carb, but if everything looks good in that department, you think the ECU? I'll also order a choke relay to be safe and just replace that too  

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Without extra air the choke will stall the engine. The choke should allow the fast idle cam to engage,.

 

 Pe6vtj2.jpg

 

Find this thing on the back of the carb, it may also be metal. See the underside? has steps?

 

PdRuWRS.jpg

 

When choke comes on the plastic part should lift up into the way of the closing throttle and hold it open more. As the choke turns off the fast idle will move down those steps lowering the idle. Spray the linkages liberally with WD-40, hold the throttle part open and push the choke flap closed and the fast idle cam should move. Loosen any linkages till it moves freely.

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Thanks again for the help! One last question, in regards to the suspension and steering system - I have power steering on the truck so does it use a rack and pinion? I can't find anything in the Haynes manual about that. The steering wheel is way off and has excessive play so I'm thinking that's what it is if the truck uses a rack and pinion 

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Not R&P.

 

Play in the steering wheel can be the wheel bearings worn or not properly pre loaded. The idler arm bushing worn out or any or all of the 6 steering ball joints. Be a shame to replace the power steering box and it does nothing. The steering box can be adjusted but eliminate all the above first.

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To check this slide under the truck and have a volunteer move the steering wheel back and forth and look for movement. There is the centerlink, inner and outer tie rod ends and idler arm. Stock idlers suck. Don't bother with the bushing kit get a moog. I run big tires and have rebuilt my front end many times in 32 years of ownership. Just did it again last summer and a fresh front end feels awsome! Also balljoints need to be checked but don't have much to do with the steering. Tire wear is an indicator of issues also.

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On 10/5/2020 at 7:32 PM, datzenmike said:

 

 

4/ Anything above 45 PSI is ok 55 would be better. The light comes on below 8 PSI so I would replace the sender (which powers the red light and the gauge) located above the oil filter up under the intake on the side of the block. 

 

 

I've been meaning to figure out what is up with my oil gauge.  It shows high most of the time, either pegged or close to pegged. It does go to 0 when the engine is off.  Usually high oil pressure, if real, means worn rings with high blowby, yes?  I have not done a leakdown test but all 4 cyl have compression within specs and the truck does not smoke or consume oil.  Is this worth looking into or just leave it alone? I'd want to be sure the gauge is working properly so I don't have a situation where pressure goes low but the system does not pick it up and the idiot light does not go on.

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34 minutes ago, MaddieCycle said:

I've been meaning to figure out what is up with my oil gauge.  It shows high most of the time, either pegged or close to pegged. It does go to 0 when the engine is off.  Usually high oil pressure, if real, means worn rings with high blowby, yes?  I have not done a leakdown test but all 4 cyl have compression within specs and the truck does not smoke or consume oil.  Is this worth looking into or just leave it alone? I'd want to be sure the gauge is working properly so I don't have a situation where pressure goes low but the system does not pick it up and the idiot light does not go on.

 

I have never heard of this above, oil pressure is not related to blow-by, blow-by is a ring issue, oil pressure is oil pressure, if it is low all the time that may be due to the engine getting worn out.

I do not trust the Nissan 720 gauges, they work most the time but not always all the time, my Nissan diesel is an example, the gauge rarely read over 45psi, sometimes it would be lower, it just didn't act right in my opinion, so I put a mechanical oil pressure gauge in and it read 90psi compared to the 45psi the Nissan electrical gauge was reading at the same time, over time the electrical gauge fixed itself, but I kept the mechanical gauge, I used one of them sandwich pancake things that you put between the oil filter mount and the oil filter, I was skeptical about the product and watched it constantly but it has yet to leak, but I don't even drive that truck anymore, I like my 521 kingcab turbodiesel way better.

DSCN5559.JPG

DSCN5560.JPG

Edited by wayno
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 I'll tell you this... back in the mid '80s I split my oil filter right after changing the oil.  A tiny bit of casting flash got into the pump and jammed the pressure relief valve closed so the pump put out almost maximum pressure. It was ok around town but when revved up on the highways boom. So it's possible the the high pressure is real.

 

It's also more likely the sender is bad and next the gauge is off.

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33 minutes ago, wayno said:

 

I have never heard of this above, oil pressure is not related to blow-by, blow-by is a ring issue, oil pressure is oil pressure, if it is low all the time that may be due to the engine getting worn out.

I do not trust the Nissan 720 gauges, they work most the time but not always all the time, my Nissan diesel is an example, the gauge rarely read over 45psi, sometimes it would be lower, it just didn't act right in my opinion, so I put a mechanical oil pressure gauge in and it read 90psi compared to the 45psi the Nissan electrical gauge was reading at the same time, over time the electrical gauge fixed itself, but I kept the mechanical gauge, I used one of them sandwich pancake things that you put between the oil filter mount and the oil filter, I was skeptical about the product and watched it constantly but it has yet to leak, but I don't even drive that truck anymore, I like my 521 kingcab turbodiesel way better.

 

From what I've gathered, the blowby causes increased crankcase pressure which then manifests as increased oil pressure via back-pressure into the oil lines.  Maybe not a thing tho.  I had an '82 turbodiesel Mercedes with the same gauge problem, had 300k miles and did 100mph, did not burn an ounce of oil, I ignored it then and will probably ignore now. Anyone know voltages I could follow from the sensor and see if they are in range?

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If you had enough blow-by to raise your oil pressure I am pretty sure you would also blow out every seal in your engine.

I had a lot of blow-by in my work truck L20b(a LOT of blow-by), it didn't raise my oil pressure, but I did have issues with my engine seals/gaskets, when the oil fill cap was off while the engine was running it would make a wet round oil spot on the bottom of the hood and that is when the hood was up.

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10 hours ago, wayno said:

oil pressure is not related to blow-by, blow-by is a ring issue, oil pressure is oil pressure

High oil pressure would be created by blockage. Engines wear as do oil pumps and the clearances get larger creating LOW oil pressure. Blow-by cannot show as high oil pressure as you'd have many other engine problems before that could happen - excessive blow-by creates crankcase pressure which would find it's way out through the easiest means, your PCV valve first off would be dumping all that crap into your carb and then it would try to escape through the next weakest point. To back pressure through the oil system would be way down the list, and your oil filter would be bulged.

 

The most logical issue is sender first then gauge gone bad. Have you dropped the oil pan? What's the sludge like in the bottom of the pan?

Weirdest thing I've ever seen was guy with high mileage Bronco brought it into the dealer complaining of engine sounding bad. No oil showed on dipstick,  added 1 quart and suddenly it was overfilled. WTF??? Turned out the guy had NEVER changed the oil so the pan was full of hardened sludge - there were grooves that the crankshaft spun in. Amazing that the engine even still ran.

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Oil pressure is regulated by the pressure release valve in the oil pump. At or around 55 PSI the spring inside is pushed back exposing a path for oil to escape and it is recirculated to the inlet of the pump. No amount of blockage in the lubricating system will raise the pressure.

 

When blow by exceeds what the PCV can handle, (which isn't very much) it backs up and pushes out the valve cover vent hose which directs it to the mouth of the carburetor. The valve cover vent tube is 1/2" at least. It would take a lot of blow by to build any pressure in a system that has a 1/2" hole in it. On top of this the oil pump is a positive displacement pump and you can't force oil through the gears.

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  • 2 weeks later...

Rebuilt and adjusted the carburetor and now it's idling perfectly at stoplights, etc. The vacuum lines all look good too. The only idle issue I'm having now is that it is still sputtering to a start even after depressing the gas peddle two to three times. If I apply the throttle simultaneously to turning the key, it starts just fine. Should I just leave this alone or any ideas about what I might fix next? Just got the choke relay in the mail today so I'll throw that in just in case, but other than that I'm not sure. 

 

In regards to the carburetor - opened it up and parts were melted so that part was self explanatory as to what was going on with it. 

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