Well I have needed to do a return line kit for a long time. I finally ordered one since I was leaking fuel like a plane in a wwII movie. I didn't have time to install it because I am so far behind with work so I bought a fire extinguisher just in case, cus I am responsible like that. Anyway I am thankful for all the information on here that gave me the tools and confidence to do it. I was only getting about 10 mpg's over the last two weeks. Now I am getting up to18 mpg! Mainly I am just happy to have one more thing fixed right. 5 of the 8 caps broke when I popped them off and the hoses had grey paint on them. Is it possible that they where original with 290,000 on the clock?

 

That's a massive amount of fuel to be leaking for an 8 MPG gain. I lost one fuel line when it cracked, dumping all fuel to that cylinder on the ground and running on 7, and only dropped 2 MPG for that trip.It was dumping huge amounts of fuel and leaving puddles every time I stopped, a fix immediately type of problem. I can't imagine leaking enough fuel for an 8 MPG gain. I am suspicious of your numbers.

My 4x4 crew cab with 3.55: gears, ZF5, and a fresh 7.3 with total seal rings and turbo has a best of 18.56 MPG over 65k miles and averages 16-17 MPG. Granted you gave no info aside from 290k miles, but I have a hard time believing an old engine approaching time for freshening up in a truck that likely doesn't have overdrive is delivering 18 MPG.

The biggest gain I ever saw in MPG was settings timing from 6.5* to 9* (might have been 9.5*), which got 3-4 more MPG and significantly more low end torque. Going from C6 to ZF5 was only worth 2-3 MPG for my mixed city/highway driving. The engine rebuild was worth about 3 MPG.

 

Fuel return could be higher volume than a high pressure leak as the pump uses fuel flow for cooling... just saying.


I would be curios about how you derived your Timing specs too ... as 8.5 would be optimal at 2K RPM with a Pulse adapter and different with Optical and Cetane ratings of fuel would determine where to set.

are your Timings BTDC or ATDC ... different methods different settings .. I use Lumy Mag settings myself.

 

Fair point on return leakage, I had not considered that but that doesn't explain 18 MPG. Low to mid teens, sure.

I time mine using the pulse method with an adapter and dial back light.Recommended timing at 2000 RPM I've read on forums was 8.5*-10* for a safe range. More timing is better for power and economy but kills glow plugs by eroding them. 10* is about the upper limit before rapid glow plug wear and alternative cold starting methods would be required. Another consideration for more advanced timing would be cylinder pressure and rod bearing wear. The mechanical noise gets noticeably louder at more advanced timing,. At 6.5* it is sound soft and smooth like a gas engine, at 9*-9.5* there is a noticeable diesel knock. I have experienced a stuck injector and that sounded like the engine was about to grenade, amazed a rod wasn't hanging out the block when I pulled over after that.

A different theory is that more retarded timing gives higher EGT which makes up for the wasted fuel and high EGT gives more boost at lower RPM resulting in better power and/or economy. I don't subscribe to that school of thought. I want maximum energy from burning fuel pushing down on the pistons.

 

If it makes any difference, my 86 dually averaged 17.42 over the summer with a best measurement/interval @ 18.98. (the best interval was short and basically driving around on long stretches at 50mph). The only thing on my truck that could really be considered non-standard is a facet pump, I think.

Edit to add: My tuck was a work truck, and everything is worn out. The ODO right now is 02,xxx. I have no idea of the real mileage or, because it has a PTO and worked as a spray tuck in another life, the hours on the engine. But like most of us, Glow plugs, return lines, and a good lift pump made the truck start and run reliably. I also try not to have it in the ketchup, as I have no tach and have no idea how hard one can run these things.

 

Your best at about 50 MPH sounds about right. My best of 18.56 was at 50-55 MPH on two lane blacktop while unloaded. RPM kills MPG for the IDI. Fuel delivery is very linear with accelerator position position, unlike a gas engine that will only pull what it needs depending on load. I think the key to MPG on the IDI is getting it to make appreciable low end torque and keeping the engine at low RPM with appropriate gearing.