'77 M890 Fuel Tank Drop
 

Hi all,
First Post! I recently acquired a 1977 D200/M890 with the 5.9 360 in it. It hasn't ran in 15 years so I'm going to start cleaning out the fuel lines and fuel tank. I bought the service manual for reference, but it's vague on how to remove the bolts holding the straps to the fuel tanks. The question I have is how do I get to the nuts in the frame? I'm having trouble accessing the nuts on the bolts holding the straps. Does anyone have any tips or way to do this?
Thanks!
Colin

I can't remember if the frame side fasteners thread into captured nuts (meaning a cage, clip or welded nut) or not. If so, they may have rusted and broke free and are now spinning free.

So far I've removed two of those tanks.

The first one was on a salvage vehicle so I cut the scraps (obviously don't do that) and the second one was to retrofit a different tank from another vehicle application so I could have an in-tank pump for my efi setup. I didn't cut the straps but, I ended up making new ones.

I vaguely remember the frame bolts also being an issue. I recall getting all 3(?) straps loose on the inboard side and bending the straps down out of the way to drop the (empty) tank. Now granted this was on a truck that was previously converted to a flatbed with hydraulic dump, so it was a lot easier for me to get to stuff.

But, I kept the original straps in case I ever wanted to revert back to the stock tank. Bending them didn't seem to ruin them, I just bent them back, but rust is always a factor I suppose.

Be sure to soak everything in a good penetrating oil. Sorry I can't offer better advice.

However, I do have some other tips after you get the tank removed.



Check for cracks.

Those plastic tanks tend to like to crack. If they’re at the top you can try to fix them. Drill smalls holes on each end of the crack to stop the spread, (maybe) slightly bevel the crack and apply a 2 part epoxy that’s fuel resistant. But, if the cracks are anywhere else I’d look for a replacement or do a retrofit. Fuel leaks aren't nothing to mess around with.


Test or Replace sending unit
It's nice to have a working fuel gauge and this part is usually the most difficult to replace so now is the time. Test ohm range before reinstalling. 73 (E) to 10 (F) Ohms.

It will be difficult to remove the screws for the sending unit without striping or snapping them. Use penetrating oil and take your time. Last time I used a pair of vice grips. Still had a couple break and had to drill and retap.

Clean, inspect and paint the frame

Mud tends to get packed into the C-channel frame where it is boxed off by the fuel tank. Over time this can cause rust damage.



Replace brake line

This is a good time to make and replace the brake line that runs between the tank and frame. If it hasn’t failed yet, it will. I’ve had it fail on two different trucks. Don’t try to unscrew the line directly from the combination valve. It most likely won’t unscrew and you don’t want to risk damaging a part you can’t get an oem-style replacement for. Instead, cut the line leaving at least one foot, re-flare and use an inverted flare union NOT a compression fitting with ferrules. Line size should be 3/16. If using hardline that has a coating, sand it off at the ends so your flaring tool doesn't slip.

You'd probably also be ahead to replace all the rubber brake hoses, wheels cylinders, calipers and master cylinder after sitting for 15 years. Brake fluid is hygroscopic unless it's still running DOT5 (silicon).

To test:

Put a small amount of it in a glass, add a drop of water. If the water is absorbed it is 3. If it stays separate it is 5.
Also, DOT 5 is purple, DOT 3 is amber. However, The purple fades over time .

Hope you find this helpful.