Hey some of you maybe don't remember me.

I finally found 1990 F350 with 300/C6 with 4.10 in rear axle IT WAS RARE it came from Metro Detroit Airport. It was nice condition not to bad for $600 not rusty.

Okay ran in some lot trouble.

Gas mileage is HORRIBLE I filled complete full drove 20 miles in 2 day it already on half tank.

I replaced dist cap and rotor they were SHOT full of white crap.

Now I need replace thermostat, oxygen sensor, and cap some vacuum leak.


What I noticed they remove emission. They cap the 6 fittings on head for air injection. EGR pipe is welded closed. Egr sensor is still there but vac cap.

It running rough but lot better after dist cap. Hard to START it get few seconds of crank before it catch up. The colder the worse it is.

 

I was so confused when I first read the title of this thread. I thought you were claiming 300.5 mpg lol.

Hard starts when cold would lead me to believe a vacuum leak. They can be some of the hardest things to track down. I've found the propane trick to work well: Run over the engine with a propane torch (no flame obviously) and if it finds a vacuum leak the idle should rise slightly.

 

At 5 mpg, I would expect the exhaust to be belching black smoke. Is it?

Make sure the brakes are not dragging and that the tires are fully inflated.

If the thermostat is not functioning correctly, the engine will not warm up and the fuel mixture will be rich.

If the Oxygen sensor is bad [you said it needed replacing], then the computer is "blind" and cannot properly adjust the air/fuel mixture.

Is the check engine light on? Have you had the codes, stored in the computer] pulled?

 

There's a fsuperduty in my juckyard with a service bed and a removed zf5 with a efi 300...man I drooled for a good twenty minutes

 

I didn't have enough of sleep last night so opp.

Yes it really badly. No black smoke out exhaust. But It possible fuel transfer issue to front tank. Front tank not work.


Right my plan is do thermostat and oxygen sensor then plug my code reader.

Engine light is on.


Transmission C6 won't shift itself. And vacuum system just mess.

 

Sort out the major vac. issues to start . Codes second .

 

The C6 transmission will not shift properly without a strong vacuum signal.