I have a ‘91 e150, 351w, 5.8l, e4od. Purchased 3.5 years ago, it has been a saga to say the least. I think I finally have it running fairly well, BUT my mog is still very low at 5.3 mpg. I don’t expect any kind of real fuel economy, if I could get to 8 around town and 10 hwy I would be thrilled. Here is what I have already done to remediate the previous owner’s poor maintenance..what do you all suggest next or do you think I am at my best mpg with the 5.3 mpg?
Repairs I have done..(I know not all apply to mpg but this is the list)
Steering gear box, Rebuild steering column, tie rod ends, control arms, alignment, front brakes all new could not be resurfaced, shocks and tires, air cleaner housing and filter, fan clutch, throttle body, throttle position sensor, iacv, mlps, fuel sensor, fuel filter, engine temp sensor, fuel regulator and pressure checked, accelerator cable, accelerator return spring, battery+cables+connectors, O2 sensor, Upstream catalytic converter (downstream removed and found ok so reinstalled), ignition coil, ignition switch, distributor cap and rotor, spark plugs and wires, Alternator, voltage regulator, starter motor, compression check all cylinders in acceptable range, transmission service, 3X oil change engine, 1 change oil differential.
I might be forgetting a repair or 30, lol.
.At this point it starts on first crank, idles smooth, accelerates nicely, no engine missing, and shifts smoothly at the proper time. This is vast improvement!
It will go over 45 mph now, yay!
But that mpg is still low, even for this vehicle.
What would you all try next?

 

There are many variables that affect MPG. Are you hauling anything? What does the van weigh?

How do you drive in-town? Is it a lot of stop-and-go driving in a big city and you sit at traffic lights frequently?

Do you drive easy or do you regularly do 'jackrabbit starts' or other aggressive driving?

Do you live in a hilly or mountainous area?

Do you know what differential ratio you have?

Is it driven every day or does it sit unused a lot? Do you warm it up for very long after it's started?

When on the highway, are you sure it's going into OD?

Have you checked for dragging brakes? Probably a few more I can't think of right now.

Finally, how are you figuring MPG? Have you checked that the odometer is accurate? (compare to highway mile markers over 10 miles or so).

With a 35 gall. tank you should drive at least 200 miles (combined hiway/city) before dividing by gallons.

By contrast, I get about 10-11 city and 15-17 highway with my 93 E150 w/same engine.

PS: What does the mechanic that did all this work say about the poor MPG?

I would suggest hooking up a vacuum gauge and reading the results.A vac gauge can tell you a lot about what's going on.

https://www.motor.com/magazine-summa...-vacuum-gauge/

 

Exactly how are you calculating fuel consumption?
What codes do you get when your code reader is connected?

 

My 85 E150 with a AOD transmission and 5.0 engine and carburetor, got around the same mileage as econo93's van.
My 88 E150 with a AOD transmission and 5.0 engine and fuel injected gets about the same. My 88 though has always had a smell from the exhaust of gas like it is running rich but at 15 to 17 MPG, I would think it is not.
I don't know if any shop these days would still have the older emissions equipment where they used to put a sensor in the tail pipe and could analyze what emissions is coming out. That might help guide you in a direction.

You said you changed the engine temp sensor. That sensor can cause the engine to run rich if it thinks the engine too cool. It sill keep the system in open loop.
Another sensor that comes to mind is the IAT (intake air temperature sensor) also called ACT (air charge temperature sensor). it feeds information to the ECM about the air temperature in the intake and the ECM adjusts the fuel/air ratio as needed.

Conanski and Econo93 might know better then me but I don't think either sensor can be tested on the engine with a code scanner since we have OBD1 systems. I believe a OBDll may tell you if they are working or not.. I used a ohm meter and tested mine at different temperatures to make sure they were working properly. There are charts online that will tell you what the sensors would be reading at different temperatures. Our ECT and ACT use the same chart.

The ACT looks a lot like the ECT
ACT

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And is located just behind the distributor. Easy to get to with the engine cover off.

Those are 2 sensors to check.A bad connection, broken wire, or bad ECM perhaps but the sensors would be what I would check first.

Conanski and Econo93 both have very valid questions! Perhaps checking for codes should be first! I use a vacuum gauge on my engine now and then just to check its health.

 

Much of the work was done trying to solve an intermittent acceleration issue. So many mechanics that did not diagnose properly, probably 10-12..in the end it was a failing TB. The most recent mechanic replaced the ignition switch, coil, distributor cap and roto, that took care of hard starting and missing. . His opinion on the mpg was that it was normal for this van, I think he like most mechanics here, do not like difficult diagnosis issues..they tend to be “ read code, bolt off, bolt on” type mechanics, most refuse to work on OBD1 at all. I drive daily, but only in town, no highway. I am not aggressive on the pedal. The brakes were inspected, front replaced. The OD has been checked. My tank is only 17 gallons, the forward tank was already removed by previous owner. I do not know my differential ratio. I am figuring mpg by odometer and gallons used, but have not checked odometer for accuracy.
‘9

 

Filling tank, noting odometer, at refill dividing mikes driven with gallons filled.

 

Use the same pump at the same gas station facing the same way for a few fill ups and recheck the mileage
Tune it up (replace the spark plugs and check the ignition timing)
Looks like you've done the rest
I rod the hell out of mine trying to get the van out of its own way
Those 5.8's are not known for big power
Are you rodding yours around too?

 

Are you starting it and driving it short distances several times a day? If so, it probably never gets up to operating temp and the engine won't be very efficient---bad for MPG. With a small tank and checked with fewer miles, there is more room for error in your calculations. That's why you should follow what ManicMechanic said about using same station/pump,etc.

Again, do you sit idling very much? Either warming it up or running the A/C or heater during the day without going anywhere?

In town only will give poor results. My Class B Dodge van only got about 6-MPG around town but it was pretty heavy.

 

If you have a standard E150 then you should have room for a 22 gallon tank on the rear.

My van will vary 1 to 2 gallons depending on the angle the van is sitting when filling up the gas tank.

It is 4 miles to the grocery store and 4 back. In just 2 trips to town I notice the gas gauge has dropped. The engine is just about up to open loop temperature by the time I get to town. Run in the store and the engine cools back down a little. Probably back to open loop. But even then In my van it gets much better then 5 or 6 MPG i am going to guess and say around 10 or so, maybe 12 at best on these short trips.

IS your E150 a standard cargo van or a camper loaded down? I have slowly been converting mine but so far have kept it very light weight since i don't live in it full time.