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Old 11-30-2019, 05:59 AM   #1
1979 GMC Sierra
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Water in gas tank or something else?

Hey guys, lately i've been having problems with my 76 3/4 ton. I will be driving on the hwy for half hour to an hour and then she starts surging and losing power and speed and then stalls. I pull over and try to start and she wont start. I wait a minute or two and try to start again and she starts and runs for a few seconds then dies. I repeat this process over and over until she stays on....then i try to drive and as soon as she gets going she surges and stalls again. I repeat this process over and over again until i make it home or have it towed (had it towed 3 times now in the past 2 weeks).

I thought there might be water in my gas tank, so i've been buying water eliminators/deicers that you pour into your gas tank, as well as pouring gas line system cleaners into the gas tank. But after 2 weeks of this, i'm wondering if there's something else wrong?

I'm about to siphon my gas tank to see if there's water in it, but before i do, any ideas of other possible problems i could be having? Thanks in advance.
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Old 11-30-2019, 08:44 AM   #2
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Re: Water in gas tank or something else?

Taking a sample is a good start, you'll have to drain the whole tank or make sure you draw from the lowest point of the tank since water settles. I do not put much stock in pour in chemicals so your mileage may vary there.

It may even be an ignition issue, a weak ignition coil may be erratic and not allow enough spark or not be able load back up in time. You can test it with a decent multimeter or you may even be able to take it to your local parts store for testing.

Good luck!
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Old 11-30-2019, 09:28 AM   #3
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Re: Water in gas tank or something else?

Depending on the contamination level 99% Isopropyl alcohol can clear up water in the fuel tank.
You can't use rubbing alcohol. Rubbing alcohol is a 70% Isopropyl solution... you're trying to remove the water not add more.
Methanol is cheaper so that's what you see a lot of... it can work but it's far less effective.

If you're using E10 pump gas you probably don't have water contamination.

You may have an ignition problem that shows up when things get heated up. Get a can or three of Dust-Off. When she acts up pull the distributor cap then spray the ignition module and the underside of the distributor body. Dust-Off is HFC152 refrigerant. It'll cool things down fairly quickly. I use it to find thermal faults on circuit boards.

If she starts with the cooled module replace it with a new AC Delco Professional D1906 four terminal ignition module. Be sure to thoroughly clean the mounting pad with alcohol and completely cover the bottom of the module with heatsink paste.
If she doesn't start pull the coil dust cover off the top of the distributor cap exposing the coil and cool off the coil with Dust-Off then try starting the truck. If she starts then... Replace with an AC Delco Professional D504A coil and replace the cap and rotor set with a good brass terminal cap. MSD5501 is black with brass terminals.

If the wiring pigtail between the module and the cap has even slightly damaged wires replace it with an AC Delco D221. This is listed as the condenser... It's a molded plug, condenser, and pigtail.

Delco has several quality levels. Original Equipment level should be the same as GM parts Professional is close to OEM and plenty serviceable.

In most cases aftermarket electrical is just plain worthless.
I would be very careful of Amazon parts suppliers. Some of them advertise AC Delco parts and ship no-name Chinese crap.
RockAuto and Summit ship what you order if you want to order parts.
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Last edited by hatzie; 12-03-2019 at 12:23 AM.
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Old 11-30-2019, 10:12 AM   #4
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Re: Water in gas tank or something else?

Your fuel pump might be kaput!
Pull the fuel line off the carb, disable the ignition, stick the line in a litre jar and crank it over and it should fill the jar in 30 seconds. Watch for how strong the fuel pulses are too.
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Old 11-30-2019, 10:59 AM   #5
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Re: Water in gas tank or something else?

You may have an old or cracked fuel line hose letting it suck air.
Or,even a partly clogged fuel filter.
Do the cheap stuff first!
Mike.
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Old 11-30-2019, 04:48 PM   #6
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Re: Water in gas tank or something else?

I know the tech somewhat different but when I was a kid, we had a boat that did pretty much what your truck is doing. Turned out it had a crack in the ignition coil core. When it got hot, the crack would open and mess up the transformer effect needed for good spark.

2nd the idea to check your ignition and distributor wiring.
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Old 11-30-2019, 08:55 PM   #7
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Re: Water in gas tank or something else?

I agree with Geezer and 100%chevy, you're running out of gas.
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Old 11-30-2019, 10:22 PM   #8
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Re: Water in gas tank or something else?

Back in the good old days, the first thing we would check is that stupid little filter where the gas line connects to the carb

I used to carry a spare or two in the glove box, although I have just pulled it out and driven to the next town (it was thirty below in the dark!).
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Old 12-01-2019, 08:39 AM   #9
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Re: Water in gas tank or something else?

I agree electrical problem, coil or ignition module
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Old 12-01-2019, 02:21 PM   #10
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Re: Water in gas tank or something else?

How about a fuel tank not venting and pulling a vacuum in the tank? Easy to check, just pull the cap next time. If you feel\hear a vacuum....

If you are thinking you are losing spark, get a $7 inline spark tester and check it next time.
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Old 12-01-2019, 03:01 PM   #11
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Post Cutting Out

Ignition problems usually are sharp on and off, this slowly loosing power sounds more like fuel starvation, might be crap in the tank or the filter, always check those first .
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Old 12-02-2019, 02:28 AM   #12
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Re: Water in gas tank or something else?

Losing?
If siphoning, push ~6' hose in the tank, plug the end, then pull out and down. I'll leave the innuendo there. lol.
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Old 12-02-2019, 01:16 PM   #13
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Re: Water in gas tank or something else?

I never ever could get a siphon hose down into any of my square body saddle tanks
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Old 12-02-2019, 02:24 PM   #14
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Re: Water in gas tank or something else?

Quote:
Originally Posted by 1976gmc20 View Post
I never ever could get a siphon hose down into any of my square body saddle tanks
Me either!
Now a days,I use a bulb pump used on a outboard motor.Take the special ends off.
Then,pull the suctiion hose from the fuel pump,stick it and the hose from the bulb pump together (piece of copper tube) ,run to a can and squeeze the bulb!It will drain the whole tank,if the sock is open.
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Old 12-02-2019, 02:59 PM   #15
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Re: Water in gas tank or something else?

Quote:
Originally Posted by 100%Chevy View Post
Me either!
Now a days,I use a bulb pump used on a outboard motor.Take the special ends off.
Then,pull the suctiion hose from the fuel pump,stick it and the hose from the bulb pump together (piece of copper tube) ,run to a can and squeeze the bulb!It will drain the whole tank,if the sock is open.
Mike.
That would be a lot of work to get a gallon of gas for a small engine

On my old two ton with the gas tank behind the seat, there was a piece of rubber line connecting the steel lines just under the back of the cab. I left one of the hose clamps off (it never leaked) and would just pull off the rubber line and let it gravity fill a one gallon jug and then stab the rubber line back onto the steel line. The truck was high enough that it wasn't a big deal for my younger self to crouch down and do this. There was no bed either, just log bunks.

Quote:
Ignition problems usually are sharp on and off, this slowly loosing power sounds more like fuel starvation, might be crap in the tank or the filter, always check those first .
Yes !!!!!

I could always tell by how it ran or quit whether it was fuel or ignition. Of course if you can't find an obvious problem with one then it makes sense to start looking at the other. It's a bit harder to be sure when you're just reading someone's description.

Generally it takes a lot wrong to actually stop one of these old pickups. Usually they begin to run really badly for a long time before they actually quit.
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Old 12-02-2019, 07:55 PM   #16
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Re: Water in gas tank or something else?

Thanks a lot guys for all the info and insights! I appreciate it. So far, indications are that it was what I suspected, water in the tank, making it a fuel problem as a lot of you suggested. I checked the gas filter and it was hard and you could barely blow through it. Had to clean it up (bought a couple new ones now, but wanted to see if the truck would run first), and I could then blow through it better.

Also, was able to siphon most of the gas out with a hose (was hard to get the hose into the tank, worked at it for awhile) and there was a bit of water at the bottom of each pale we used. So I had water in the tank, got bad gas somewhere then. Put the gas back in the tank after separating it from the water. But I don't know if I got all the water out of the tank cause you can't see how the siphon hose is sitting inside the tank. I started the truck and let it idle for a half hour, then I drove the truck for awhile and there was no issues. Dumped some more water eliminater/de-icer into the tank today.

I'm thinking of buying some 99% Isopropyl, like hatzie mentioned, and dumping it it to try and clear up any water I missed. Any idea how much I should or could dump in?
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Old 12-03-2019, 05:25 AM   #17
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Re: Water in gas tank or something else?

That's good news that you got it sorted out! Always nice to hear a happy ending that doesn't break the bank too.
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Old 12-03-2019, 05:41 PM   #18
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Re: Water in gas tank or something else?

Quote:
Originally Posted by 1979 GMC Sierra View Post
Thanks a lot guys for all the info and insights! I appreciate it. So far, indications are that it was what I suspected, water in the tank, making it a fuel problem as a lot of you suggested. I checked the gas filter and it was hard and you could barely blow through it. Had to clean it up (bought a couple new ones now, but wanted to see if the truck would run first), and I could then blow through it better.

Also, was able to siphon most of the gas out with a hose (was hard to get the hose into the tank, worked at it for awhile) and there was a bit of water at the bottom of each pale we used. So I had water in the tank, got bad gas somewhere then. Put the gas back in the tank after separating it from the water. But I don't know if I got all the water out of the tank cause you can't see how the siphon hose is sitting inside the tank. I started the truck and let it idle for a half hour, then I drove the truck for awhile and there was no issues. Dumped some more water eliminater/de-icer into the tank today.

I'm thinking of buying some 99% Isopropyl, like hatzie mentioned, and dumping it it to try and clear up any water I missed. Any idea how much I should or could dump in?
It comes in 12oz bottles. You're supposed to use 12oz in roughly every 10 gallons of fuel. I'd dump three bottles in a heavily contaminated tank... That's 36oz of isopropyl in the tank... then fill it all the way up from a high traffic fuel station that moves lots of fuel.
If you're close to a Pilot or Flying J or TA or Williams or similar truckstop get fuel from them. Their fuel doesn't stick around long enough to get condensation and the gubmint goons ride them pretty hard on inspections and the like.
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1969 Dodge Polara 500 B383, A833 SOLD
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1976 Chevy K20, 6.5L, NV4500/NP208 SOLD
1986 M1008 CUCV SOLD
2000 GMC C2500, TD6.5L, NV4500
2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.
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