New here, 94 E350 7.3 IDI school bus. Hoping to swap axles for increased MPG's

Charles Moore

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Hi all,
I've been looking through these forums for awhile. Tons of good info around!

I've got a 94 short school bus with a 7.3 IDI non turbo, at about 280k miles. Its an E350 frame and so far every part I've ordered for an 94 E350 as worked out.

Recently my MPG's have dropped down to about 10-11 MPG! It used to be 14-15.

I did noticed the fuel return lines were leaking around Cyl #8. So I've replaced that. Haven't put much mileage on it since then, so I don't know if that's helped or not yet.

But anyways, I've been pondering ways to increase my fuel economy since I'll be putting some miles on this. Planning on doing an electric fan conversion. I'll try to take some pictures of that setup when its done.

The slightly major project I'm looking at is swapping out the rear axle for one with 3.55 gearing. Currently I'm at 4.10. This thing is 90% highway miles, and zero towing so I figured that could help.

I'm wondering if someone could point me in the right direction as to what axles are going to fit? Is something from an F series truck going to have the same mounting points as the E series van?

Thanks for any help!
 

Cubey

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Check your tire pressure. It can kill your mpg on a big vehicle. Costs nothing to be sure they are property inflated. Mine all equally dropped by 10psi within about 500-750 miles a few months ago, I guess altitude and temperature changes.

Winter diesel can make your mpg take a noticeable hit too.

Electric fans are mostly questionable since your alternator has to use more power to run it and if your alt goes, your batteries will be gone fast and your fan will still stop. They don't cool properly too.
 

Charles Moore

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Thanks for the reply. Yeah I keep an eye on tire pressure. Tires don't leak much fortunately, and I keep them at about max pressure.

I'm up in Minnesota right now so temperature has dropped the last few months, but it started getting poor fuel economy in the summer. I'm heading South in a few weeks, so we'll see if warmer climates help or not.

I've got a PV system and battery bank for the camper side of the bus. So at some point I'm going to wire it up so that I can feed the engine batteries from the PV panels when my Camper battery is at 100% charge. Figured that would help reduce the load on the alternator at least some of the time. That's on my "when I get around to it" project list...

So with the Electric fans, most of what I've read around here lead me to believe that the electric fans would help reduce fuel consumption. My understanding was that the mechanical fan has a slight drag all the time, but the electric fans would have no draw added draw on the alternator when they aren't running.
I'm only going off what I have read here and other places so I can't say for sure.

Another thought I've had was to swap out the injection pump and get new injectors. But I don't like replacing parts (especially expensive ones!) without knowing for sure they need to be replaced.
 

Cubey

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So with the Electric fans, most of what I've read around here lead me to believe that the electric fans would help reduce fuel consumption. My understanding was that the mechanical fan has a slight drag all the time, but the electric fans would have no draw added draw on the alternator when they aren't running.
I'm only going off what I have read here and other places so I can't say for sure.

My point is that electric fans (I think you will need multiple) will put a bigger load on the alternator, causing more engine power to turn it, at least partly counteracting it. They might run all the time depending on their set off temp. These engines run 180F minimum, 190-200F easy at highway speed. You'd need fans that don't kick on until a high temp like that, unless you want them running a lot more.

You still have to turn the water pump which is what the fan is attached to, so the only thing you can remove is the fan clutch and fan blade itself. The belts still have to turn the water pump so I doubt it's THAT big of a load, all things considered. I honestly doubt it would make much of a difference, UNLESS you have brand new fan clutch.

My RV's engine is running much cooler (10-20 degrees!) with a new Motorcraft fan clutch. (180ish at 55-60mph). It's hard to say if it actually hurt my MPG because A) fuel switched to winter blend right around the same time & B) I think my fuel system got fouled at the same time, which can hurt MPG. After treating my fuel system with some Hot Shots Secrets Extreme, it helped my MPG some. (keep that stuff off of your skin, it WILL burn you if it soaks in!!) It's still lower mpg than it was in summer, but again it's probably the winter blend fuel causing that, at least partly.

The highest "true" MPG I've ever gotten in my RV (see sig for info & fuelly) is about 9-9.5 interstate driving at an average of 60mph in May 2019, before the new fan clutch. Come April or May, I'm hoping my highway MPG is back to at least 9. I can't expect much more ss it is. This thing is big and heavy, with a GVWR rating of 11k.

Poor timing can hurt MPG too. If possible, have it checked by somebody with the proper equipment. These engines have NOTHING to control the engine (except cold idle timing advance) so it has to be manually adjusted. I'm trying to arrange to get my RV's engine checked/timed, but it's a little difficult due to it's size vs where folks who can do the timing are.

In regards to axles.... truck axles probably won't fit. Stripped chassis (aka cutaway, aka "incomplete vehicle") have a slightly different axle, if I'm not mistaken. I hate to say it, since it's expensive (at least $1500) but you might have an easier time simply having your gears changed and leaving the actual axle left as it is. That's why I'm leaving the 4.10 in mine, it's too dang expensive to have changed, and I doubt that pre-92 3.55 Dana 70 cutaway chassis axles are easy to find. There is no guarantee that it would really help MPG enough to justify the cost. You might have better luck finding a 92-?? to fit, however.
 
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saburai

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Welcome to Oilburners Charles Moore.

What he said;burnout:cool

This is the best place to learn about our idi powered vehicles and a great community filled with helpful, knowledgeable and colorful people:Thumbs Up

I think that Cubey advice is on the money. In regard to the electric fan conversion, it appeals to me as well. However, it does add mechanical and electrical complexity to a very simple system, which is why many of us love these old mechanical diesels. While it is true that it may improve your mpg somewhat, if you have a failure of one of the many electrical components in a e-fan system that works properly and is temperature controlled, you lose your ability to cool. The same is not true with the bare-bones stock mechanical system.

Speaking for the community, we love pictures!:peelout

When you get time please post some either here or in the show us your ride thread. Personally, I'd like to know the details of your PV system and offer my help if it is needed. I've got a bit of experience, as we live off the grid pulling all of our electricity from the sun.
 

Clb

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check this guy's mpg tech...
Brb...
Welllllll
Looks like I forgot his handle.
There's a guy on here, that's done a ton of mpg testing (on a van I think) and I want to say he has a site or page somewhere...
I will keep looking.
 
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Charles Moore

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Hey thanks for the welcome folks!
I've definitely noticed all the IDI info around here which is why I decided to join up!
Thanks for the info on the axles, I've come across some things around here on FB marketplace & craigslist but I don't want to buy something just to figure out it isn't going to work. And yeah, I'm not going to get the thing rebuilt with new gearing. I'd rather build a turbo setup if I was going to spend that kinda money!

I saw somewhere about a guy that used the factory fans from a Ford Windstar for his e-fan conversion. I found one for $20 at a junkyard locally. There are some adjustable e-fan switches & relay kits around for $60-70, that allow you to set the temperature that the fans kick in at. So should be less $100 for the whole e-fan conversion? Plus a bit of time mounting the setup.
I'd invest in a more accurate coolant temp gauge also so I can keep an eye on the actual degrees and see if the fans are doing their job.
Maybe an EGT gauge while I'm at it.

And I'll make sure to snap & upload some pictures when I get a chance! And I'll snap some of the PV system too.
 

Shadetreemechanic

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Is your current fan clutch unlocking? My clutch started siezing on my 94 and it immediately lost 3 mpg. I diagnosed it by sound. The normal loud roar when starting out cold was there, but it also came and went under low load situations on the interstate. Also, when you shut down after running the unlocked fan should continue to spin for several seconds after the engine is off if its free. The cooling system on these trucks is pretty well thought out IMO, I would try to make sure what you have is working before changing it up with electronics.
 

Shadetreemechanic

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I do like 3.55 gears with the E4OD. I think your spring perch width is different from a truck however so you may have a hard time finding an axle.
 

Clb

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Man we need to clean up the fud in the tech sections....
 

Charles Moore

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when you shut down after running the unlocked fan should continue to spin for several seconds after the engine is off if its free.

Thanks, I'll test that out.

But I tested the glow plugs (checked for 12v b/w + terminal on the relay and each glow plug), there was a range of voltage but the glow plug for cylinder #8 was not reading anything. That explains the hard starting! So I'm going to go ahead and replace all of them. Any particular brand you all recommend?

Another issue I found while I was in there, was a puddle of diesel (or coolant maybe?) under the intake. There was 30 years of gunk blocking a drain hole, that I cleared up. But that fluid got there somehow.
The fuel return line by Cyl #8 had been leaking previously. I've replaced the return lines, O rings & caps so I'm hoping that is just left over that I hadn't managed to clean up properly before.

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