Trucks been sitting for 30 years, best course of action for maintenance.

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Jan 16, 2025
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Hey, I’ve got a 1981 GMC K1500 that's been sitting for quite a while. When I finally got the motor running, the first thing I did was change the oil, but the old oil came out really sludgy. I went ahead and put in a cheap oil filter and added some 10W-30 with zinc. Now, I'm trying to figure out the best course of action moving forward. Should I change the oil again after 1,000 miles and keep doing that for a bit to help clean things out, or would it be better to try flushing the engine to get rid of all the sludge more quickly? Also, do you think I should consider switching to a full synthetic oil after a few changes to help keep the engine cleaner and running more smoothly? What’s your advice?
 
No pictures of this truck?

As long as your oil pressure is verifiably good, I would get a few cheap good filters...... Supertech, and some cheap API certified high mileage 10w30 and do some short changes like 500 miles on the filter, several times. Both Supertech.

One good thing might be to drop the pan and physically clean the pan, if this is feasible.

I would not use any flushing agent until after a few filter changes, if at all.

It is likely that a leak or two will surface.

I am not an expert, but that is what I would do.

Good luck.....
 
Hey, I’ve got a 1981 GMC K1500 that's been sitting for quite a while. When I finally got the motor running, the first thing I did was change the oil, but the old oil came out really sludgy. I went ahead and put in a cheap oil filter and added some 10W-30 with zinc. Now, I'm trying to figure out the best course of action moving forward. Should I change the oil again after 1,000 miles and keep doing that for a bit to help clean things out, or would it be better to try flushing the engine to get rid of all the sludge more quickly? Also, do you think I should consider switching to a full synthetic oil after a few changes to help keep the engine cleaner and running more smoothly? What’s your advice?
Definitely a transmission fluid and filter change, along with new brake fluid, power steering fluid and differential fluid.
 
Personally, I would just plan on changing the oil every 3,000 miles or so for a while, and focus my attention elsewhere in the meantime. I'd spend my money instead on good tires and restoring the brakes first.

Probably some repair panels for the cab corners and floor boards, lol
 
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Definitely a transmission fluid and filter change, along with new brake fluid, power steering fluid and differential fluid.
Yeah that’s my plans, but is it save to change the transmission fluid, everyone online tells you not to or this certain situation where you can?
 
Yes please post pictures!

Why would you not change the oil?
Hey, I’ve got a 1981 GMC K1500 that's been sitting for quite a while. When I finally got the motor running, the first thing I did was change the oil, but the old oil came out really sludgy. I went ahead and put in a cheap oil filter and added some 10W-30 with zinc. Now, I'm trying to figure out the best course of action moving forward. Should I change the oil again after 1,000 miles and keep doing that for a bit to help clean things out, or would it be better to try flushing the engine to get rid of all the sludge more quickly? Also, do you think I should consider switching to a full synthetic oil after a few changes to help keep the engine cleaner and running more smoothly? What’s your advice?
Personally, I would just plan on changing the oil every 3,000 miles or so for a while, and focus my attention elsewhere in the meantime. I'd spend my money instead on good tires and restoring the brakes first.
Do you think the old rotor and break pads would be fine after breaking a couple times or old and brittle? And need replaced.
 
Do you think the old rotor and break pads would be fine after breaking a couple times or old and brittle? And need replaced.
It's not the rotor and pads that are likely to be a problem, but the possibly rusted brake lines and the components with rubber. Brake hoses and wheel cylinders especially, calipers, and master cylinder, too.
 
I would go through the brake system completely, replacing all seals, hoses, etc. If rebuilding the master cylinder, calipers, etc is beyond your skill level, I would find a mechanic to do that for you.

I would replace all hoses in the cooling system too.
 
What's the backstory on this old thing?
How was it stored? In a nice dry garage in Arizona maybe?
That you got it running and apparently driving would seem to indicate favorable storage conditions.
 
Replace all fluids. Flush the brakes. Change the transmission fluid 3 times, but a few thousand miles apart if you are feeling cautious. Rubber hoses are all suspect - they may look good but not be strong enough to hold up. Be thorough with the coolant flush. Give the brake system a good thorough evaluation.

Put a very mild lowering on it, some modern wheels, clean the upholstery, and hit the highway.
 
Absolutely change the brake linings, they can delaminate from sitting. My RV did this on both drum shoes. Incredibly they still sort of worked as the linings just sort of wedged in the right spots. I've chucked a disk brake pad lining as well-- that is NOT fun as it takes a few pumps of the pedal to get the space back and be able to stop.

Replace rubber brake lines (cheap on Rockauto), rotors if they're rusty, repack wheel bearings, do all the fluids, see if chunks are coming out of the gas tank. Put a cheap inline filter between the fuel pump and carb like everyone does anyway and cut that open to see what's passing through.

Dropping the oil pan on a 4wd is probably no fun. Since the first change was so chunky I'd give it another change in 2 hours or 20 miles with whatever cheapo oil left overs you can find around the farm... or 10w30 if you have to buy new. I wouldn't go with synthetic until you have it running right and a few hundred miles of drive time to prove it.

As for the transmission absolutely change it! If the filter clogs you get less pressure which means it'll shred itself.
 
Put the fuel filter before the pump, not after it. I went through 4 fuel pumps on an old F100 before I realized tank crap was ruining the pumps. After installing a filter before the pump, I never had an issue.

AS for the rest, if it's rubber, replace it. Check coolant, air filter (they do deteriorate) ALL fluids replaced. You can do a drain and fill on the tranny. Then look at tire's age and condition. If it's been sitting for 30 years, probably just replace and be done with it. yes, you'll dump about a grand into it, but it will be dependable and safe.
 
Cheap oil, cheap filters. Who knows what is going to come out. Maybe change after running a tank of gas through it, see what is in the filter, see what drains out. As confidence grows, then go to "better" filter and oil.

In the meantime, you might find out why it was parked, and might be sad if you toss too much money at this. All the rubber bits need attention, and anything that can be greased. Change all fluids. Check the radiator, make sure it's not corroded.

Is this one of the motors with a rope seal in the rear?

You have it running, but is it pulling gas from the tank? Or did you flush that out already? If it's sat for 30 years I would flush the tank; no sense running what is in that, it's probably long turned to varnish.

Fun project I bet. Good luck.
 
A 30 year idle vehicle which you started with no mention of fuel or how it idled?
Then you go into a fat paragraph about proper oil?
I think you're pulling our collective leg.

What are the long term plans for this vehicle?
Are you going to just poop it around a farm/private land for possibly very short distances or register it and make it roadworthy?
Your inspection load will be less than you might think as many parts will be automatically trash, but it'll still be huge.

Thinking about a lowering kit sounds a tad premature at this point....or perhaps that was gentle sarcasm?
 
Like others have mentioned put a new fuel filter before the fuel pump. Here is a old hot rodder trick. Get some clear fuel filters so you can see what is going through them. Install a couple in a series so when the first one plugs up you can just remove it and reattach the hose to the next filter in line.

Did you drain the gas tank? I'd be worried it is completely full of junk.

And I'd go with SuperTech High Mileage. It will help swell the seals and might prevent some leaks.
 
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