Hey guys my truck currently has a 302 in it that gets 12mpg. I am looking for more low end torque(and hopfully mpg) and have found a parts truck with a 300 and a 5 speed in it. The parts truck is an 89 so I am pretty sure it is fuel injected. From the research I did I will need to switch the motor mounts but the trans mounts should be the same. Is there anything else I should know about this swap? Will I need the driveshafts changed at all? Is there any performance mods I should do to the motor while it is out? What does it take to get 200hp out of the 300(what the 302 makes stock)?

 

What year is your truck? If it's a '96 as your handle indicates this is a complicated swap with the truck being OBD2, You really need to source a '96 300 for all the 1 year specific parts it has.

If your 5.0 only gets 12mpg there is something wrong with it, it should do much better than that unless you only make shirt trips and it never gets up to operating temp. If that is the case substituting a different 300 cu in motor in won't help the situation.

 

Why would I need specific ob2 parts, wouldn't I use the computer from the truck with the 300 which would be ob1? My trips get the truck up to operating temp, I know that somthing is definatly not right with the engine hence why I am looking for another one.

 

What year is your truck? If it's a '96 then an OBD1 PCM won't even plug in.

 

Its is a 96 and I didn't think you could just plug the new computer in even if they were the same year because the would have a different wiring harness anyway?

 

That means the truck is OBD2 and the PCM connector is 108 pins, the OBD1 PCM only has 60 pins.

The engine portion of the harness is different for each engine but the PCM connector is the same except when they switched over to OBD2, then everything is different. So yes if you had an OBD2 PCM for a 4.9 it would plug in but you don't so this is a complete no go.

 

So basically to do the swap I would have to pull all of the wiring out of the donor truck and rewire my whole truck?

 

Yes, and then forget passing emissions if that is a concern.

The better thing to do here is to just fix what you got and make some changes to improve the trucks performance, that is the big upside with the windsor V8 it's dumb easy and cheap to get lots more out of them.

 

​​​​​​​So there is a reason its getting 12mpg. What size tires are you running? Lift? What gears? You'd be better off sourcing a 351

 

Its has 265/75/r15 tires(about 31s). It has no lift and 3.55 gears. The check engine light is on somthing to do with the egr, is this making that much of a difference?

 

Anything that prevents the motor from operating at maximum efficiency will affect fuel consumption. Post up the exact code numbers you are getting for suggestions as to what the problem might be, 20 heads are better than 1 when it comes to figuring out problems.

 

I can get the exact codes later, I believe I was getting both egr flow insufficient and excessive. I was also planning to do a compression check somtime this week.

 

Was the speedo recalibrated for the "31"s if not its off by about 6.5% there could be some of your mileage. Also, with 31s and auto and 3.55s your revs are way low in overdrive, try cruising with OD off and see how it does.

 

I am getting three codes, 401(Egr flow insufficient) 411(air injection flow incorrect) 136 (o2 sensor bank 1 sensor 2)

 

So you have at least 2 problems, the O2 sensor problem itself could be enough to keep the PCM in open loop and that would definitely impact fuel milage. Bank1 sensor 2 would be the downstream sensor though, do you have an aftermarket exhaust on the truck or have the cats removed?
Code 401 is telling you the EGR system isn't working correctly, the DPFE system on this truck measures gas flow through the EGR tube so you could have a bad control valve or a broken vacuum line between it and the control solenoid, or a faulty DPFE sensor.
Code 411 confuses me, I didn't think these OBD2 trucks had an air injection system. Does your truck have an air pump?