Just picked up an 85 F150 with a 300 and 4 speed w/OD manual. I've been searching the the net and come across a few posts, but many seem to be centered more around performance. I'm looking for the best gas mileage out of the truck and wondering if I should work on getting the feedback system working as it should or simply replace it and the distributor and go with an after market intake.

It seems to be a common recommendation to swap the exhaust manifold with those from an EFI. I'm thinking that'll be on the list no mater which way I go.

The Feedback on it now seems to run a little rich. I haven't done anything to it yet, but did unplug the connector to the solenoid on the carb and there was a difference in idle. When the coolant temp senor was unplugged the RPM increased by about 1000 RPM. (guess without a tach..) So it seems parts of it are working. The smog pump is long gone...

Its a truck and I plan on using it for that. I have other cars to go fast if needed. I'm wanting the best torque and gas mileage out of the truck and wondering if leaving the feedback carb is the best option or replacing it. Parts don't seem too expensive, but I want to put my money in the right place from the start.

Emissions aren't an issue.

Thoughts?

Thanks!

 

I'm not familiar with the 300, but my '82 351W originally had a feedback carb. However, when I got it that carb had literally been thrown away and a simple Autolite 2150 put in its place. I'm guessing that was done because it ran poorly, but that step didn't help much since whomever did it didn't know what they were doing.

The problem was that the truck had EEC IV and DS III on it and any problem, even a little one, can throw the system into limp-home mode which guarantees it'll run, but not well nor fast. The guy that worked on it seems to have had no clue about that and tried to make small changes. That doesn't work as it is all or nothing. EEC IV/DS III or simple carb/DS II ignition.

I'm glad I made the change because it is a simple system that I can maintain. I didn't do it for performance, but I'm confident that the truck performs at least as well as it did new, and possibly better. As for gas mileage, I get ~12 running a true 60 on the highway, but I have 3.50 gears and the engine is really spinning at that speed.

 

If you are up to learning about the feedback system, it will give you better mileage than a conversion(if it's working correctly).

 

I think I am up to learning it. It can't be all that complicated and certainly not as much so as my other two cars. I'm just trying to determine if it is worth keeping and dumping money in it.

From a high level view I'd think any kind of feedback system would do a little better than a plain carb'd system. EPA rated this truck at 22 mpg highway from the factory and I'd be tickled to get any where near that!

This weekend I'll do the normal tune up maintenance on it and dig a little deeper in to it all. I need to replace the valve cover gasket so I'll be taking a lot of it off.

Would the EFI exhaust manifolds be a good idea even if I leave the rest stock? Seems from what I have read it would be and they can be had cheaply on the bay.

 

If its running a little rich, you could replace the O2 sensor. It screws into the exhaust system on the passenger side of the engine. It is hard to see and hard to reach, both from above and from below the truck! I was irritated with Ford engineers for putting it in such a difficult place.

It's easier to remove with the engine warm. I found that if you turn the steering wheel all the way to the left before turing off the engine, you can reach through the passenger front wheel well to get at the sensor with a wrench.

 

As was said by Galendor, Replace the o2 sensor. They have a service life of between 30,000 and 50,000 miles on these older trucks. Some are factory original. (Scary thought)

And replace your smog pump and components. If the smog pump isn't there, the computer will put the truck into, limp home mode, where all the settings are at predetermined levels.

If you want to keep the feedback carb system, and the fuel mileage, you have to get everything back into working order, to reap the rewards.

Doing a computer diagnostic would be a wise idea as well. It will tell you what systems are not functioning properly.

 

So far I found the PCV connection hose disconnected and a spark plug that wasn't seated all the way. I could spin it with my fingers.... Replaced all plugs and reconnected the hose and its running much better. I'm surprised at how well it was running before correcting those two issues....

I have an new O2 sensor that will probably go in today along with a new valve cover gasket. I added sea foam last night and will be doing an oil change.

 

Pulled codes. Non-running I got a system Pass. Running I got 41 and 77. I'm guessing I need to floor it before the test starts to get rid of the 77 code? Do I do that before I start the truck or after?

I replaced the O2 sensor. That was a real pain... I disconnected the battery and will reconnect to drive for a while to see if the 41 clears. I was surprised to find only those two codes.

 

If this is just like the EECIV fuel injection test, the computer will give you a signal when you are supposed to goose the throttle. You do this with the engine running tests on the fuel injected engines. It tests the knock sensor, makes sure the engine got above 2000 rpm, etc.

Is 41 a oxygen sensor code on your truck? If so, then you did the right thing and disconnected the battery to clear the old code after you changed the sensor.

 

The 41 is supposed to be "oxygen sensor circuit indicates system always lean" from what I found online. The 77 is "brief wide open throttle not sensed during test".

Getting ready to take it out for a spin to warm it up and then do an oil change.

 

Truck seems to be running better. It has been raining all day and after replacing the O2 sensor the back wheel(s) break free with without any change in driving habits. I got used to giving it a bit of gas before and now it is too much.

When I pulled the running codes though, I still get the 41 and also get 3 more codes I didn't get last time... 12, 13, 58. These all seem to be related with the carb so not happy to see them. I'm curious why I am seeing them after replacing the O2 though.

I did rev it a few times to get rid of the 77 error and now wondering if that is what caused them. I would have thought the O2 error would go away. Is it stored in a way that won't clear by disconnecting the battery? I can put a MM on it tomorrow to see what it is reading.

 

Today I filled the truck up and took it out for almost a 100 mile drive. The trip included about 60 - 70% highway driving at least 65 mph with an average probably a bit over 70 mph with a top speed of 82 according to GPS.

I averaged 18.4 mpg the entire trip which seems pretty darn good considering the speed I was running and town driving. I can only assume the system is working as it should given some of the feedback I have read here about feedback carbs failing and getting poor gas mileage.

So far I have changed the PCV valve, spark plugs, O2 sensor, and fixed a vacuum leak. I have ran sea foam through the crank case, carb, and brake booster hose. Also did an oil change. I have plug wires that I'll put on soon and will also replace the cap and rotor.

At this point I am really leaning towards keeping the feedback system in place.

 

It sounds like it is finding things that it couldn't find with the old worn out o2 sensor.

Although you must be pretty close to having it right if you managed to get over 18 mpg.

 

I found a small vacuum line leak that was tied in to the same port the MAP was connected to. It was a small line so not sure if it would cause much of an issue.

I also hooked the EGR back up the way it is supposed to run. It was completely disconnected. I connected it to match the diagram on the front of the truck. I assume it works because when I applied vacuum to the EGR port it would bog the engine to the point of stalling. I have no idea if the solenoid controlling vacuum to the EGR works or not...

I replaced plug wires and tightened the valve cover gasket. I'll need to take the carb off at some point and tighten it back down as there is a small bit movement with the bottom plate.

I guess I should have named this thread differently...

 

Arcaidy, there's still some of us here using the stock feedback carb system. My truck is similar to yours, I'll bet you have a 3.08 rear gear ratio like mine. Our trucks with 300-I6, 4spd manual with OD and "highway" gearing can get almost 20 miles per gallon with the feedback system working properly and good driving habits. I think they were advertised as getting 22 mpg.

Plus we probably pollute a bit less, better for mother earth.