Low Idle and Dies. Alternator? Carb? Uncertain

[1Blue720]

Hi all! 

 

I've made a few posts on here but (one of) my current problem is that my 720 has an extremely low and rough idle: usually somewhere from 300-100. About a month ago I installed new speakers and a new head unit and my idle went down to around 600 and has been dropping ever since. I suspect it may have something to do with my alternator but I wanted some other opinions. Currently, when I turn tightly at idle my truck tends to die. My thought was since its already idling at about 200 the load increase (from the power steering) overloads the alternator and the truck dies. Lights also dim at idle. I've been pressing the gas and brake with my right foot so it idles higher 😂.

 

The alternator is 50A and stock so I wouldn't be surprised if it is time for a new one. If it is time for a new one, what's the best bet? I've seen some people swap to a D21 alternator instead of buying a new 720 alternator or rebuild their alternators. 

 

The carb is aftermarket (not sure where from). Prior to installing the new sound system I had the carb professionally tuned to make sure everything was good and everything ran great after that, until I put in the sound system. I was thinking about just tightening the throttle cable more so that it idles higher as a temporary fix. 

 

Thank you!! And happy holidays! 

[datzenmike]

The engine stalling when turning tight turns at low speeds is the added load of the PS on an already low idle speed.

 

The addition of a stereo is just a coincidence.

 

Idle of 100-300 is, well impossible. You're reading it wrong or tach is wrong. I'd be more inclined to believe that you don't have a tach and are guessing at an extremely low idle which is impossible to do accurately.  

 

Idle speed is set with the idle speed screw NOT by tightening the throttle cable.

 

To lay to rest the alternator as the problem first keep in mind that a low idle will cause a very low charging output. Now get a meter and measure across the battery. If you have a good charged battery, it will read about 12.6v with engine off. If charging properly running about 14.3v with the idle at or around 700 RPM.

 

 

Now here's what I believe the problem is for a sudden unexplained low stumbly idle:

 

Idle does not lower from a stereo because when not in use it would return to normal and you don't mention this. I think the engine is in the mid stages of blowing the head gasket between two cylinders. It's already blown but only beginning and disrupting the idle. Get a compression tester and record the compression in all cylinders. Look for two adjacent low cylinders as in 150, 125, 130, 150 or 150, 150, 140, 135 something like that.

[1Blue720]

41 minutes ago, datzenmike said:

The engine stalling when turning tight turns at low speeds is the added load of the PS on an already low idle speed.

 

The addition of a stereo is just a coincidence.

 

Idle of 100-300 is, well impossible. You're reading it wrong or tach is wrong. I'd be more inclined to believe that you don't have a tach and are guessing at an extremely low idle which is impossible to do accurately.  

 

Idle speed is set with the idle speed screw NOT by tightening the throttle cable.

 

To lay to rest the alternator as the problem first keep in mind that a low idle will cause a very low charging output. Now get a meter and measure across the battery. If you have a good charged battery, it will read about 12.6v with engine off. If charging properly running about 14.3v with the idle at or around 700 RPM.

 

 

Now here's what I believe the problem is for a sudden unexplained low stumbly idle:

 

Idle does not lower from a stereo because when not in use it would return to normal and you don't mention this. I think the engine is in the mid stages of blowing the head gasket between two cylinders. It's already blown but only beginning and disrupting the idle. Get a compression tester and record the compression in all cylinders. Look for two adjacent low cylinders as in 150, 125, 130, 150 or 150, 150, 140, 135 something like that.

I do have a tach but it is possible it is off/wrong though it has worked fine in the past. When at idle the truck isn't just stumbling it is barely running.

 

I have never had the stereo not in use (I can't figure out how to fully turn it off so I was planning to disconnect to fully from the battery and try running it without the stereo system at all) but when I have my lights off it idles slightly higher. I did the head gasket in July as well as some other gaskets and thermostat and have only driven it about 1.5k miles since then (with on and off idle issues, that's why I got the carb tuned and post tuning everything was running great). I will look into compression testing after ruling out the alternator. 

 

Thank you! 

[datzenmike]

If the stereo is turned off it has no effect on the charging system.

 

Check battery voltage with engine on and off. This will tell you if charging or not.

 

Why was the head gasket replaced??????

[Draker]

My guess is you have vacuum leak.

[bottomwatcher]

4 hours ago, Draker said:

My guess is you have vacuum leak.

Bingo. Check all of your hoses and carburetor base gasket. Go big while you are at it if you want reliable transportation give it a tune up. Replace one component at a time so if you screw it up you are only dealing with one component and if the budget is tight only do one item for pay period. All vehicles need maintenance. Sparkplugs, ignition wires, cap and rotor, air filter, fuel filter, belts and hoses. 

For you current problem bend and flex your vacuum hoses looking for cracks or extremely stiff hoses. If the look crappy go buy the hose by the foot at your local autoparts store is the cheapest. If one is bad replace them all because they will be failing also.

Edited December 27, 2024 by bottomwatcher
Spelling

[datzenmike]

While a small vacuum leak can cause this symptom they generally don't happen without having something disturbed. The Z24 is problematic for blowing head gaskets.

[NC85ST]

Try adjusting the idle set screw. Probably a vacuum leak, but at least with the truck idling it should be easier to find. I know you said that you had the carburetor professionally tuned, but that doesn’t mean that they know a lot about carburetor’s. Post a picture of your carburetor.

[banzai510(hainz)]

adjust the idle speed screw.and the mixture at the carb base. Maybe even turn the dist. To get idle up. If you get above 13.8 volts w everthing off. 14 better.at1200rpm. If loads down at 1200rpm then alt real week

 

[1Blue720]

9 hours ago, datzenmike said:

If the stereo is turned off it has no effect on the charging system.

 

Check battery voltage with engine on and off. This will tell you if charging or not.

 

Why was the head gasket replaced??????

 I replaced the head gasket because it blew. I suspect it was in the beginning stages of blowing when I bought the truck (April 2024) as the previous owner just kept dumping gasket repair into it. Prior to the head gasket going I did the oil pan gasket since it was leaking.

[1Blue720]

4 hours ago, bottomwatcher said:

Bingo. Check all of your hoses and carburetor base gasket. Go big while you are at it if you want reliable transportation give it a tune up. Replace one component at a time so if you screw it up you are only dealing with one component and if the budget is tight only do one item for pay period. All vehicles need maintenance. Sparkplugs, ignition wires, cap and rotor, air filter, fuel filter, belts and hoses. 

For you current problem bend and flex your vacuum hoses looking for cracks or extremely stiff hoses. If the look crappy go buy the hose by the foot at your local autoparts store is the cheapest. If one is bad replace them all because they will be failing also.

Thank you! I will see if I can get it idling higher with the idling screw and then begin the vacuum leak investigation.

 

And also I will check the battery to rule out alternator (I currently don't have a multimeter so I'm seeing about getting one/borrowing). 

[datzenmike]

17 minutes ago, 1Blue720 said:

 I replaced the head gasket because it blew. I suspect it was in the beginning stages of blowing when I bought the truck (April 2024) as the previous owner just kept dumping gasket repair into it. Prior to the head gasket going I did the oil pan gasket since it was leaking.

 

Did you check that the head was not warped? Previous owner may have over heated it. Also not fixing and continuing to drive it can cause the metal between cylinders to erode away making a poor seal and blowing again.

[1Blue720]

9 minutes ago, datzenmike said:

 

Did you check that the head was not warped? Previous owner may have over heated it. Also not fixing and continuing to drive it can cause the metal between cylinders to erode away making a poor seal and blowing again.

I did check though only with a straightedge. I also checked the whole block for any cracks or fractures. The truck sat for a while after the head gasket blew while I waited to fix it but I didn't drive it at all since I was worried about it damaging things. Everything looked pretty good when I opened it up, but I know just because it looks good doesn't mean it is. 

 

I also replaced what I believe was the original gasket. Also the engine currently has about 250k miles on it. 

 

 

[datzenmike]

Lets leave the blown gasket theory for now and eliminate other possible causes.

[Thomas Perkins]

Go with a 60 amp alternator.What I use.The 50 amp is too weak.I also have a.c.My gas gauge needle would not go pass full when I filled up and my stereo didn't work right with a 50 amp alternator.All my grounds are clean.Make sure your power steering pulley bearing is not sticking.Take belt off it and turn by hand.I have a new one on mine.A.C. pulley is good.Make sure belt is tight.I also have a new stereo.Check your fat black fusible link wire going to white wire and follow it and see if it is okay.My battery reads 12.85 volts,my alternator reeds 14.5 volts.Make sure you have no corrusion on your battery terminals and post.Take off and clean.Make sure connector from positive battery cable to starter is good and ignition wire going to starter.And your negative ground going to battery tray and power steering bracket.Clean with sand paper.You have 4x4 similar to mine,I have a Weber and have my idle set at 900 rpms.It runs great at this.Stock carb says in the book at 800 rpm's for the 4x4's.But Webers need to be a hair higher.That is my take.Been this way on mine for 8 years.When I got my Redline.When these trucks were made they had the cheapy stereo's.Not enough ass for the now day Stereo's.Go 60 amp.My Kenwood has a built in amp,Bluetooth and other features.

Edited December 28, 2024 by Thomas Perkins

[1Blue720]

On 12/27/2024 at 3:58 PM, Thomas Perkins said:

Go with a 60 amp alternator.What I use.The 50 amp is too weak.I also have a.c.My gas gauge needle would not go pass full when I filled up and my stereo didn't work right with a 50 amp alternator.All my grounds are clean.Make sure your power steering pulley bearing is not sticking.Take belt off it and turn by hand.I have a new one on mine.A.C. pulley is good.Make sure belt is tight.I also have a new stereo.Check your fat black fusible link wire going to white wire and follow it and see if it is okay.My battery reads 12.85 volts,my alternator reeds 14.5 volts.Make sure you have no corrusion on your battery terminals and post.Take off and clean.Make sure connector from positive battery cable to starter is good and ignition wire going to starter.And your negative ground going to battery tray and power steering bracket.Clean with sand paper.You have 4x4 similar to mine,I have a Weber and have my idle set at 900 rpms.It runs great at this.Stock carb says in the book at 800 rpm's for the 4x4's.But Webers need to be a hair higher.That is my take.Been this way on mine for 8 years.When I got my Redline.When these trucks were made they had the cheapy stereo's.Not enough ass for the now day Stereo's.Go 60 amp.My Kenwood has a built in amp,Bluetooth and other features.

Where did you get your 60 Amp alternator? I am having trouble finding one that doesn't look cheap. I have also been thinking about upgrading to a weber, I think it's about time to bite the bullet and get one, it would probably save me a lot of trouble. Thank you! 

[Thomas Perkins]

Got it from Auto Zone.I bought one about 25 years ago from them and when it goes bad I get a free one with a lifetime warranty.Some people say Auto Zones are not good.But I never have to buy another one.50 amps are too week,especially with stereo's with amps.You can get a alternator else where but get no warranty.I believe this one is about 3 years old.Battery is also 3 years old,got them at the same time.Battery is from Walmart.Battery is 12.8 volts and alternator is 14.5 volts.Get a Weber.Nissans carbs are junk.Always tinkering with them.Plus old as dirt.When you order one make sure you can see the screen they order from and it is a 60 amp one.Part number 14886.A dollar more than the 50 amp one here.Also 50 amp ones made me have other electrical issues,gas gauge would not go to full when filled up.Stereo had issues.In the morning when I would start it up took a minute for the dash lights to come on,battery light on dash would come on dim.When i put the 60 amp on,this all stopped.Head lights didn't work right.

Edited January 1 by Thomas Perkins

[banzai510(hainz)]

proplem is 25years later they can get cheaper parts or dont stock it anymore Lifetime or not. If you think your going to keep the rig get 2 parts. spares

[datzenmike]

First of all the alternator output has nothing to do with the idle. You could unplug the alternator and start it and it would run and idle on the 12v battery for hours and hours.

[Thomas Perkins]

The only warranty part that I could not get anymore was a good thing.That was the heater core,the after market ones do not fit the box and causes problems.They gave me my money back.I did put three in there.I now have a complete custom made heating and air.That was a blessing that Auto part stores no longer sell them.I fill for ya'll with Nissans garbage.I fought it for a long time.Those days are a distant night mare.I don't miss having to take off a million parts for that heater core.Dont need a spare alternator when I get them for free.That is a part they will always have.Same as starters,never have to buy one of them either.Prices  go up.Had my rig for over 30 years,also get free brake pads and shoes.Shop smart.Same as a water pump,radiator,coils,brake and clutch master cylinders.All are free.Both temp.sender and sensor.Slave cylinder.If any warranty part is longer available,they will give you your money back.So you don't ever lose.Rock Auto Parts has no lifetime warranty.They have good prices on stuff like spark plugs.I buy some things from them.But shipping is getting sky high.Some places want 15 bucks now.Autozone has free shipping.So does your local Nissan Dealer.On line Nissan is high.Napa is sky high on there parts.

 

[1Blue720]

Update! I played with the idle and got it running much better, drove it for a couple of miles to a friend's house for New Years where it promptly died. I started it around 11pm New Years Eve and waited for the idle to drop which it did, then I tried to accelerate and it died. I knew the battery was dead since the starter wasn't going and I was just getting the click.

Day after I tried to jump it and nothing so I took the battery in to have it tested: It was good but completely dead so they charged it for me. Put it back in today and the starter will go and all lights will come on but for some reason the fuel just won't ignite (it's got that machine gun noise). Finally towed it back to my house this afternoon and started pulling the alternator which I will take to get tested tomorrow. If its not the alternator I will probably begin checking grounds. Spark plugs all looked good when checked them while doing the head gasket in the summer (is it possible it could be them?). 

[banzai510(hainz)]

17 hours ago, 1Blue720 said:

alternator which I will take to get tested tomorrow.

YOU Could have  just put a Volt meter across the batter to see if it was charging? Before taking the alternator out.

check with lights on also inn case the alternator loads down.

 

maybe you have a battery drain? does it not start days later if not driven?

[banzai510(hainz)]

get one on these and you can monitor whats going on

 

https://video.search.yahoo.com/search/video?fr=uh3_news_web&_tsrc=uh3_news_web&p=alternator+cig+lighter+adapter#id=3&vid=9efc78191ffe26ef1c3da26d2897d784&action=click

[1Blue720]

On 1/4/2025 at 3:35 PM, banzai510(hainz) said:

YOU Could have  just put a Volt meter across the batter to see if it was charging? Before taking the alternator out.

check with lights on also inn case the alternator loads down.

 

maybe you have a battery drain? does it not start days later if not driven?

I don't have a volt meter and the truck is no longer starting so I decided to just take the alternator out and get it bench tested, which it passed. In the past, it has always started right up. In early December I went on a trip for 2 weeks and it sat the whole time, started right up when I got back. 

[1Blue720]

I took the alternator to get bench tested today and it passed. I am going to put it back in and try a different battery sometime this week as an experiment before I start getting into things. I have tried to jump it with two different vehicles and I'm still just getting that machine gun noise. 

 

I will also check my spark plugs again. 

 

I have a few ideas of where to start looking but I'm kind of lost lol. Maybe the distributor? Fuel delivery? Grounds?