I threw a rod my old Vortex motor in my 1996 blazer at 180,000 miles. I went a long time without changing my oil and we went to our annual reunion. I was sick all week I was not planning on going but I got better that Thursday so the next day I went but I didn't change the oil because I didn't have time.

Anyway, I have my mechanic (I stopped working on cars since I got back issues) put a Vortex motor that had 91,000 mile on it, that came from a pickup truck.

this motor is obviously more powerful than my last one, but I have a problem. my old motor gave 300 miles per gas tank fill up (I consider my tank empty at the quarter mark). Well my new motor only give me 200 miles per tank at the same mark.

I've been driving it for 2 years now but that 100 mile loss is starting to be to much to take.

I'm thinking maybe the carburetor is adjusted differently since when I hit the gas now, the truck takes off, whereas it would take it a while to go fast with the old motor, but my brother said that fuel injected engines don't work like that. I put in premium spark plug, I seafoam it and I use high octane gas

where am I losing a 100 miles at?

 

These are modern fuel injected vehicles so there is no carb to adjust. Depending on what year the replacement engine came from, it should have been nearly identical to your original. From '96 thru '01, they were very few changes.

As far as what could be causing your lower fuel mileage, the injectors in the replacement engine could not be spraying as nicely as your original motor which has decreased the efficiency. You would want to get this verified first.

There is an updated fuel injection spider (MFI) available that was used in the mid-'02 through end of production 4.3L engine. It is a direct swap with the possibility of needing a new bracket to mount it into under the plenum. This is covered in a few other posts on the forum so just run a search for 'MFI'.

 

putting in a new one will increase my miles?

 

Only if your current spider is contributing to the lower fuel mileage. Proper diagnosis of the problem is key.

 

i have a 2004 blazer that if i'm just running around town (very urban driving) i get about 220-240 miles to the tank. if i drive strictly highway i get between 280-320 miles to the tank. It all depends on your driving, the newer engines love to have the throttle put to the floor i found haha... its so tempting

 

I thought 96 Blazers were Vortec Engines. 95 used the the different system.
For me I have no idea how you can check mileage accurately by filling at 1/4 tanks etc. I check my mileage by going & filling my tank after the pump shuts off I may run it until it clicks off a couple of times. I then zero the trip mileage meter. The next time I fill it I do the same thing with the same shut off clicks etc.
Our 97 Blazer 4x4 gets anywhere from 16.4-22 MPG over a 2 year & 50,000 miles of driving. This avg 19.2 MPG. Winter gas almost always cost me a couple of MPGs in all our vehicles because of smog gas & cold temps. Our Blazer gets worse mileage in town & around town than on the highway. Living in Nevada we deal with winds & anything but flat running.
Our gas gauge is like the rest of the gauges AWFUL. I run 290 miles & the gauge shows E & the most gas I have ever put in is 14.8 gallons when it is on EMPTY.
As a rule I never run the tank down past 1/2 tank. It is cheaper to fill the top half than the bottom half. Also I have never lost an Electric in tank fuel pump (except this Blazer right after we bought it) which who know what or how it was driven before us. One reason I believe that is true is because I never run a tank low or out. Also because I use a lube like Marvel Mystery Oil or Lucas Injector cleaner on a regular basis. I have owned several vehicles with in tank pumps & have driven them many miles.
One thing I can tell you for sure is GM products get the best mileage over any of the rest. We own a Jeep GC 5.9, I have owned Explorer's, Dakota's, Ranger's & several other vehicles. Believe me GMs get the better mileage.

 

If you reset the trip meter, you can use the following formula to check your milage wether you use 5 gallons or 25 gallons.
Get a receipt, or check the numbers on the pump.

 

Technically, all 4.3L's have the vortec cylinder head design although it did evolve over the life cycle of the motor, but that's been covered in other threads. As has this; 92 thru 95 = CPI (although there is an overlap with the TBI in this range as well), 96-mid-02 = SCFI, & late-02-05 = MFI.

 

I treat the 1/4 mileage mark as a empty gas tank. my friend told to never let the tank go below that level as the fuel pump will go out and it was expensive to fix. I know it to be true as every last one of my friend who have blazers and suburbans has their fuel pump go out while mine never have.

I set my trip to zero at full up and when it got to the quarter tank and I would get 300 miles. that was the average it road and freeway put together every time. I knew that I got more miles on the freeway. I just didn't pay attention to it then because just on average driving this truck was by far the best mileage vehicle I had, by far, which was a surprise to me

I used to have an 8,000 watt system in the blazer with 2 batteries and a 100 farad cap. I put an gm 150 alternator in that you can get if you go to the store and order an alternator from a later model blazer and change the fan belt.

anyway when I took my 100 amp alternator and I lost 50 miles a tank with the big alternator (250 miles per tank). Now I got 20 mile a gallon on it while freeway driving with this alternator.

I accepted that tradeoff for my music. my wife got a job working out of town. so I gave her the blazer and I took her Cadillac as that made sense to do. I took the system and alternator out. I was back at 300 mpt

Now I'm starting to drive the blazer because the Cadillac now has better miles per gallon. freeway driving gets me approx 250 mile mpt.

I would just be happy to get 50 more mile per tank as that was what I was use to when I had the big alt in

 

I don't think that running the tank to below 1/4 will have any effect on the pump. The pump in my blazer sits in a "bucket" thats always filled with gas inside the fuel tank. So it's constantly submerged in fuel. The 1 gen s-series had the pump just hanging in the tank with no "bucket", so I could see where keeping fuel in the tank would help in those applications.

Just my .02