Butchered fuel system / Chasing Air Leaks

Howdy folks, so brief re-intro I purchased a 1985 CUCV a couple months ago off eBay and I have been inspecting it and taking notes and (re)reading the gee golly willakers out of Steel Soldiers and my reference material and I am stumped.

So a little background the PO (Previous Owner) told me that he was selling the truck because it had an air leak in the fuel system and that it had gotten so hard to start that he had given up on on it. I knew this prior to purchasing and had planned to replace the fuel lines anyways. What he didn't disclose is that he tried to troubleshoot it. I'm not going to get too gnarly with the details because I am making a video of my troubleshooting journey.
What I will say is this any part of the fuel system that was easily accessibly has been hacked horribly. By this I mean that 1/4 line is coupled to 3/8 line (poorly) that then goes through the fuel filter and turns back into 1/4 line but then there is also a separate fuel line stuck in an old bottle of gatorade hanging off the fuel filter with a brass petcock. I'm not sure if he was trying to bypass the lift pump or bleed the system of air. I do know I have about $30 worth of plumbing fittings on the fuel system and its all non-sensical the way its hooked up. I can rotate a couple of the fittings by hand in the fuel line so I know for a fact air is getting into the fuel lines that way.

The PO kind of sort of followed advice similar to what I have seen online for troubleshooting and kind of freelanced in ways that make my head hurt. I tried reaching out to him to try to understand what he was trying to do but I haven't heard back. Doesn't really matter its all coming out anyways.


I put a vacuum jar on the end of the fuel system just prior the fuel filter and pulled a mild vacuum with my mity vac and I was impressed with the amount of air the I was able to draw along with the fuel. He had split the line and put a hose coupler inline. I assumed it was to put a combo fuel gauge (vacuum and low pressure fuel gauge) inline but once I separated it I found it was a 1/4 line coupled to a 3/8 fuel line with screw clamps. That one got a verbal "WTAF" from me. After the fuel filter on the way to the IP there is a section of clear PVC and another chunk of brass adapters before changing back into fuel line. By my count I am just under a dozen places where air could be getting into the system and I haven't even gotten to the mechanical fuel pump yet.


I also sucked a chunk of rubber through the line as well while I was using the jar. This is the second chunk of rubber, that last I found while examining the fuel in the old fuel filter (that has since been changed). Part of the reason I am working my way through changing all the lines out.
So my question is where in the fuel supply chain is the 1/4 line used and where is the 3/8 line used. I assumed I would be able to pull the old lines off and use them as a template but the more I dig the more I am realizing that the entire fuel system has been hacked up and I just can't trust the way the truck came to me. It seems to me the 3/8 should be the supply side and the 1/4 should be the return line but before I go chopping up $50 worth of fuel lines and doing it wrong I wanted to confirm with someone. My hard lines are in tact on the truck but I haven't had a chance to test them yet.

Also I am planning on decommissioning the petcock that is hanging off my fuel filter. I don't have the plug for that port on the filter housing and I had already planned to put a fuel pressure sending unit on my fuel system already so I am looking for recommendations for a brand if someone has already done this kind of modification. I am planning to keep an eye on my lift pumps performance. I am planning to keep the truck mechanical (personal preference) and AFAIK it has the original pump on the engine.

Sorry this is so long.

Generally speaking, you'll use 3/8" ID hose for all of the supply lines up to the fuel filter housing, and 1/4" ID hose for the remainder... The "vent" tube line off the fuel tank and the small injector return hoses are both different sizes, but don't sound relevant to your plumbing issues.

This thread has some info on each piece of rubber fuel hose in the fuel system, but I dunno if the list itself is complete, or if the listed cut lengths will be accurate for every single M1008 out there. The below numbers should get you close, but verify on your specific vehicle to be sure, especially if you have an M1009 or M1010.

Carl1008 said:

Fuel Tank Sender <---> 3/8" ID x 5 3/16" Long <---> Supply Hard Line

Supply Hard Line <---> 3/8" ID x 13.5" Long <---> Mechanical Fuel Lift Pump

Mech. Fuel Pump Hard Line <---> 3/8" ID x 10" Long <---> Fuel Filter Housing

Fuel Filter Housing <---> 1/4" ID x 17" Long <---> Injection Pump


Return Hard Line <---> 1/4" ID x 5 3/16" <---> Fuel Tank Sender

Click to expand...

Guessing the gator aid bottle may be to catch fuel when trying to bleed air out of system, if line is attached to top of filter housing. Pictures might help.

Terracoma said:

Generally speaking, you'll use 3/8" ID hose for all of the supply lines up to the fuel filter housing, and 1/4" ID hose for the remainder... The "vent" tube line off the fuel tank and the small injector return hoses are both different sizes, but don't sound relevant to your plumbing issues.

This thread has some info on each piece of rubber fuel hose in the fuel system, but I dunno if the list itself is complete, or if the listed cut lengths will be accurate for every single M1008 out there. The below numbers should get you close, but verify on your specific vehicle to be sure, especially if you have an M1009 or M1010.

Click to expand...

That is fantastic. Apparently I missed this post. Thanks I will add this to my Evernote notes. This is definitely a step in the right direction. I found the post that had total estimated lengths needed which is how I was able to purchase my gross length of fuel lines.

dependable said:

Guessing the gator aid bottle may be to catch fuel when trying to bleed air out of system, if line is attached to top of filter housing. Pictures might help.

Click to expand...

You are correct photos would be better. I thought I attached them to my original post, turns out my iPad is not totally compatible with this format of forum for uploading images. Let me fix that from my computer.

So let me ask something because this is all still theory to me. The GM Tech training manual indicates that the best way to purge air out of the lines is to put a mild charge of compressed air over the fuel in the tank. This is supposed to push/prime fuel through the lines and onstensibly air with with it. Cracking a line at the fuel injectors being indicated as a way to check that fuel had reached the injectors but that it wasn't totally necessary. As you could crank the starter at 15-20 second intervals (with rest periods inbetween) to clear any additional trapped air. This is assuming of course you aren't sucking in fresh air into the lines which I am currently.

Ok uploading photos now. Thanks for the help.

I have used compressed air in tank to push fuel out to find leaks before the fuel pump. Usually I try to bleed to the filter base and then bleed to injectors.

You can get fuel to the injectors by cranking engine. Does not take long to wear down batteries and overheat starter. You could remove glow plugs to reduce resistance cranking.

But that does still not find the air leak. You can crank the motor until fuel is at injectors, and it will drain back and fill with air again if you do not locate the leak.

Leaks Ive seen include the hard line where it goes behind cab, bad flex lines and clamps of course, even the one on top of tank,, which is hard to get to, and the primary fuel pump itself.

Looking at your picture, there is a lot of plumbing joints in replacement system. I'd make sure all those were tight, add Teflon tape to any that do not have it.

If that does not help, since you say fuel likes were already botched, replacing it all is not a bad idea. To get them all, you probably will have to move or remove the truck bed. If you do that, you should check the tank sender tube and sock screen while you are at it.

I had a similar problem. I find one good way to start is getting the lines back to the right size and lengths. Once that is done, pull the supply line off the fuel pump and stick a gas can full of diesel up front,run you some line from the gas can into the supply side of the fuel pump. Bleed the injectors and crank it up. If it runs fine then you have a leak between the tank and the pump. It helps break the fuel system into segments to check each piece as you go. I learned the hard way and after replacing everything from tank to sending unit all rubber lines, fuel pump, new spin on filter, it ended up being the IP sucking in air. Once that was replaced its run solid ever since.

Okay so quick follow up to this post.

Previously I posted a photo of a mason jar that I fashioned into a vacuum jar so I could check for air in the line. Since I posted that I really have not had time to get out and play on the truck. But as the photos indicate the jar was mostly full.

I had been doing a lot of reading on this fuel system and read that other folks had put anti-siphon valves inline and or either added electric fuel pumps as an aid to starting or as a direct replacement for the mechanical fuel pump.

One of the things that attracted me to this truck was the "mechanicalness" of the truck and not having to rely on electrical / electronics to keep it going. I am a nerd for machines and this is my indulgence.

One of my concerns with this truck is that the anti-siphon valve in the mechanical pump was no longer working and I read the official way of testing it and I was less than enthused. Especially considering that I know I have an air leak in my system from drawing a vacuum on the fuel system just prior to the fuel filter. I cam up with all sorts of fancy and convulted methods for measuring if the anti-siphon valve was working in a way that would give me faster results. Didn't have time to play with any of them all just theory.

Fast forward to today when I open the hood of my truck and find that my previously 8/10 full vacuum jar of fuel is emptied back to the just 1/10 full. Well I have proof that the anti-siphon valve in the mechanical fuel pump is toast.

So in addition to the air encroachment in the fuel lines the mechanical fuel pump is ready for a refresh. I got up under my truck and identified where the mechanical fuel pump lives.

*gasp*

Under the turbo, between the turbo plumbing and the riveted subframe lives my fuel pump and the soft line for the inlet. I really thought Honda had the patent on pain in the donkey designs for maintenance and repair. But no I am pretty sure Chevy is going to take the prize for this one. I had planned to pull the radiator to have it flushed, and the harmonic balancer because mine is the original grenade with pin pulled and the accessory belts are roached. I guess I am doing all of that stuff at the same time. *sigh* I'm gonna go find a cheap low pressure fuel pump to get my truck started so I can pull it in the garage. Winter is coming and I'd rather do all this indoors. I was hoping to have it in shape for fall camping season but this is too much I'm not going to be happy to go out of cellphone range until I get the major issues fixed first.