Troubleshooting Intermitent loss of power on 84 720

[northoak]

Hey y'all, luckily it has been a minute since I needed to seek help on here.  My 1984 720 is randomly loosing power.  It's sputtering like its not getting enough fuel and will continue to do so in all gears.  The strange thing is that it happens at random.  I'll go entire days without problems and then it will happen at random when driving a short or long distance.  I recently drove about an hour  and half out of town and back on a hot day without trouble.  The other night I drove about 15 blocks and the problem arose.  It will act like it is not getting enough fuel so Ill slam the gas but still get nothing and then suddenly it will lunge forward for a second and then act like its starved again.  It did this to a friend who borrowed it on the highway today so they pulled off and then shortly after the problem went away.  When its not happening the truck runs great.  I can hit 75 easy on the highway without a problem for a good distance without any trouble.  As far as I can see there is no factor such as heat that causes the problem because sometimes it happens when the car has barely warmed up, sometimes when its hot and most of the time not at all.  I also assume Its not vapor locking because there doesn't appear to be pressure built up when I remove the gas cap and it happens when the truck is cold sometimes.

 

I'm going to go ahead and buy a new fuel filter for starters because its cheap and easy.  I also ordered the filter for inside the pump.  I know throwing parts at a car is a bad idea but I am considering just buying a new fuel pump since the current one is probably 34 years old.

 

I'm suspecting sentiment in the tank or filter or a faulty fuel pump.  I have already swapped the fuel pump relay with one from pic and pull.  I already tried shaking the break prone wires to the carb and relay to see if I could recreate or remove the problem with no luck.  I'd love any other suggestions on where to look.  Thanks in advance,

 

Nick in Oakland

[datzenmike]

The filter in the pump will tell you if it's sediment in the tank, which I doubt.

 

Do you have a tach in the dash? If so does the needle drop when the engine quits? This would indicate a loose power wire to the distributor module.

 

Find the fusible links attached to the positive battery post. Try wiggling them while running and see if it quits.

 

[northoak]

Thanks for getting back, unfortunately there is no tach on this one.  I’ll give shaking the fusible links a try after work and check that fuel pump filter when the new one comes.  I’m not super sure how to check the wiring for the distributor module beyond visual inspection but I’ll take a peek there and report back.  I really appreciate the help!

[northoak]

Was experiencing the problem slightly just now on my way home from work.  I removed my fuel cap and there was pressure built up that released.  Could this be telling of a problem with the charcoal canister?

 

i also gave those fusible links a shake with no changes 

Edited September 4, 2019 by northoak

[datzenmike]

I think you'll find that is a vacuum and air is sucked in. This is from the tank not being properly vented and as the gas is removed there is nothing going in to replace it. Try loosening the cap just enough so it's not sealed and see if the problem goes away.

[northoak]

Thanks will do, any idea if drilling a small hole in the cap would be a terrible idea? Is there a vented one that would fit? So grateful for the help

[datzenmike]

Try this first a hole may not be necessary. The tank should vent through the charcoal canister. Line may be pinched.

[Charlie69]

When you remove your gas cap to get gas is there pressure built up in the tank?  If not then the cap is either wrong or the seal is bad.  Replacing my bad cap helped with mileage per gallon.  Not sure why this affect the MPG but it did.  Mike might be able to clarify this.

[northoak]

Thanks Charlie, there is pressure when I remove the cap, I am a little confused, should there be pressure built up or not?

[datzenmike]

I thought it was pressure on my 620 on a trip. Every time I took the cap off there was a big rush of air. Eventually the tank imploded. If not vented air can't get in to replace the gas taken out.

 

Think about it. How or by what mechanism would air get in under pressure??? Where's it coming from???

 

Is there maybe an exhaust leak blowing on the gas tank???

[Charlie69]

I believe it is a vacuum not postive pressure.  It is part of the emiisions system.  My gas cap failed this year going through emission testing.  I replaced it and and it passed but now every time I pull the cap off I here air either sucking in to the tank or escaping from the tank.  My old cap was locking and my new cap is not.

[Awkyeow]

Northoak,

I had a similar problem on my 84 many years ago. My problem was the fuel pump relay located under the passenger side dash wall right above where a speaker would go.

It was intermittent and would leave me stuck at times or sputter. I replaced it. Problem was solved. I'm Hoping it could be that?? 

If you are stuck and pulled over, try listening to see if you can hear the ticking of the fuel pump with the key turned to the right.(no need to turn over the engine) or look ( if you have a carb front window) to see the fuel level. It should always be at the center dot if your float is calibrated correctly. If it is low I suspect it is the relay perhaps not sending fuel the the carb.

I hope this helps. 

 

Here is one on eBay.

I suggest possibly trying this route.

Its a cheap check it off the list option...and they are good to have an extra one when it goes out.

I have just replaced another one a few months back.

The relay is the big black box.

The connection is hard to remove. So be careful if yours is hard to remove too.

Let's us know....

 

https://www.ebay.com/i/283455548563?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&itemid=283455548563&targetid=596465943108&device=m&mktype=pla&googleloc=9031615&poi=&campaignid=1689407492&mkgroupid=74365778348&rlsatarget=aud-622524040958:pla-596465943108&abcId=1140476&merchantid=114712396&gclid=Cj0KCQjwwb3rBRDrARIsALR3Xeb81R_qCArHqGsTutw3NH1RCy-YyDEudSQDHWtP_Sx08eLCfX7XMO0aAkJUEALw_wcB

 

[northoak]

Thanks Awkyeow, I'm pretty sure my relay is ok because I have two and swap them with no difference.  I also opened them up and checked to see that the solder joints were still in tact.

 

I just want to double check, does this routing on my charcoal canister seem correct?  None of the lines appear to be pinched and the filter element is a little gross but not excessively

 

Also here is the cap I'm working with

 

[Charlie69]

I filled up with gas today and it is positive pressure coming out of the tank when I take of my gas cap.  When removing the gas cap air rushes out.

 

That looks like the proper gas cap.

 

My 85 and 86 king cabs are California emissions and the charcoal canister has 5 hoses.  The federal trucks have the 3 hoses.

[datzenmike]

 

The hose on the left should allow fumes from the tank out into the canister for storage. It should also allow air back into the tank as the fuel level drops. There is a fuel check valve on the tank end. If you blow from the tank side toward the canister there should be flow but noticeable resistance. If you blow towards the tank there is much easier flow.

 

The small hose also is the vacuum advance for the distributor so it's important that it not leak and hold a steady vacuum. This is the purge signal and tells the canister to open and allow the fumes stored inside to be sucked out by the hose behind it and into the intake to be burned.

 

The bottom of the canister is open and air is drawn up into it displacing the stored fumes. The canister acts like a sponge.

 

 

 

 

The gas cap has a one way valve. If vacuum rises above a set limit a check valve is pulled open allowing air in preventing the tank imploding. Suck on the tank side of the cap and air should flow through it from the outside.

[northoak]

Thanks for the description, it makes troubleshooting this thing make a lot more sense.  Checking the cap and the line that allows fumes out of the tank and air into in seem pretty straight forward.  

 

Im still a bit confused about checking the vacuum advance/purge signal hose.  Should I put a vacuum pump on facing the evap canister and see if it holds vacuum?  Would putting a vacuum gauge on the line in place of my evap canister be a good way to check to see that there is vacuum coming from my distributor?

Edited September 5, 2019 by northoak

[datzenmike]

I don't see how this system NOT working would cause your described problem. Leave the gas cap loose and the tank will be vented. If the problem goes away then look into this further. If it remains then not the tank venting.

 

It's very possible to have a small pressure in the gas tank from evaporated gas. It may seem a lot because you hear a small whoosh but in reality it's only a mall fraction of a PSI and a large tank to equalize when opened. Alternatively there may be a small vacuum as the tank empties before the valve in the gas gap is pulled open.

[northoak]

Hello again, I’ve given it some time driving around with no cap and as datzen mike suspected venting is not the cause of the issue!  The problem persists.  

 

This is Tuesday I have time to troubleshoot.  So far my plan is to replace the filter in the pump and check it for sentiment as well as the main fuel filter.  

 

I imagine the feeling of not getting fuel could actually be a spark issue.  

 

Any suggestions on other things to try when I have a day to troubleshoot would be much appreciated.  Thanks! 

[datzenmike]

Well if fuel related we should eliminate the fuel pump relay. It's job is to only allow the fuel pump to work under certain conditions. In a accident the pump would keep pumping which could be a bad thing if a rollover or fire. They can be problematic so give this article a good read... https://axleaddict.com/auto-repair/The-Datsun-and-Nissan-Pickup-Fuel-Pump-Relay-What-the-Haynes-Automotive-Repair-Manual-Wont-Tell-You  ... There is a jumper wire you can make to eliminate the relay and keep the pump working. If the problem goes away it can be assumed the relay is working intermittently.

[canon3383]

I wonder if I have the same issue intermittently? 2 nights ago I had to idle my truck for an hour on a slight incline (looking for a deer my friend shot with an arrow, never did find it) when we got back to the truck it was still idling fine (I had filled the tank right before going out to meet him and the truck has run run great before ) when we were leaving crossing a pasture the truck wanted to die when I put put my foot on the brake and took it off the gas, I could keep it running if I kept my foot slightly on the gas and left foot brake. On the way to town it died at a stop sign and ran terrible yesterday. Now it acts like nothing was wrong and acting totally normal. The only think I did this morning was pumped the gas once and started it like I always do when the engine is cold. I do have an 1985 king cab with a z24 and an automatic. I have read the article

[northoak]

Thanks for the article, I had some strange results.  I unplugged my relay and tried to run a jumper.  If I run a jumper from the blue wire to the white/black wire like  article suggests the truck will not run at all. Turns over but no fuel. If I plug the relay back in the truck will start and run.  Also the fuse for the pump is not blown 

 

Im totally confused.  Why would it start with the relay but not with a jumper in its place?

 

in other news filters looked fine and the problem continues with or without a fuel cap

Edited September 27, 2019 by northoak

[datzenmike]

On 9/23/2019 at 10:40 AM, canon3383 said:

I wonder if I have the same issue intermittently? 2 nights ago I had to idle my truck for an hour on a slight incline (looking for a deer my friend shot with an arrow, never did find it) when we got back to the truck it was still idling fine (I had filled the tank right before going out to meet him and the truck has run run great before ) when we were leaving crossing a pasture the truck wanted to die when I put put my foot on the brake and took it off the gas, I could keep it running if I kept my foot slightly on the gas and left foot brake. On the way to town it died at a stop sign and ran terrible yesterday. Now it acts like nothing was wrong and acting totally normal. The only think I did this morning was pumped the gas once and started it like I always do when the engine is cold. I do have an 1985 king cab with a z24 and an automatic. I have read the article

 

This could just be a very slightly rich idle setting that ran for a long time and maybe sooted up the plugs so the didn't fire very well. Given the chance spark plugs will heat up and like a 'self cleaning oven' will clean themselves. Driving home probably cleaned them

[datzenmike]

7 hours ago, northoak said:

Thanks for the article, I had some strange results.  I unplugged my relay and tried to run a jumper.  If I run a jumper from the blue wire to the white/black wire like  article suggests the truck will not run at all. Turns over but no fuel. If I plug the relay back in the truck will start and run.  Also the fuse for the pump is not blown 

 

Im totally confused.  Why would it start with the relay but not with a jumper in its place?

 

in other news filters looked fine and the problem continues with or without a fuel cap

 

 

 

Try to find a 12 volt source that is switched on with the ignition like the W (fuse #5) What you want is to remove or eliminate the fuel pump relay entirely and run the pump from the ignition switch. Make sure the pump turns off with the ignition and try this for a while. If problem has disappeared then the relay is at fault.

 

 

On the in line fuel filter I wouldn't go by looks. If over 2-3 years since the last change or you don't know.... change it.

[northoak]

On 9/27/2019 at 9:15 PM, datzenmike said:

 

 

 

Try to find a 12 volt source that is switched on with the ignition like the W (fuse #5) What you want is to remove or eliminate the fuel pump relay entirely and run the pump from the ignition switch. Make sure the pump turns off with the ignition and try this for a while. If problem has disappeared then the relay is at fault.

 

 

On the in line fuel filter I wouldn't go by looks. If over 2-3 years since the last change or you don't know.... change it.

Thanks for the help.  I was able to figure out jumping the relay thanks to that.  After about 3 weeks driving every few days with no trouble with the relay jumped the problem returned twice today 😞
the idle seemed to drop and then surge right before it happened the second time.  After a few minutes of struggling to keep up with traffic it went back to running well.   It’s so hard to trouble shoot this thing since the problem comes and goes. Unfortunately it’s not really safe to get on the highway because of this.  Anymore ideas of where to look are much appreciated 

Edited October 15, 2019 by northoak

[datzenmike]

On 9/22/2019 at 2:19 PM, datzenmike said:

Well if fuel related we should eliminate the fuel pump relay. It's job is to only allow the fuel pump to work under certain conditions. In a accident the pump would keep pumping which could be a bad thing if a rollover or fire. They can be problematic so give this article a good read... https://axleaddict.com/auto-repair/The-Datsun-and-Nissan-Pickup-Fuel-Pump-Relay-What-the-Haynes-Automotive-Repair-Manual-Wont-Tell-You  ... There is a jumper wire you can make to eliminate the relay and keep the pump working. If the problem goes away it can be assumed the relay is working intermittently.

 

Well I guess we have eliminated the relay so that leaves the pump.