Hi - I'm new to this excellent forum. I just bought an '86 300 powered F150 XLT Lariat from the original owner in New Jersey. It's super rusty underneath and has over 250,000 miles but I just drove it down to Florida without a hitch. The PO wasn't a car/truck guy so he paid a mechanic to do everything - I have a stack of receipts dating back to when he took delivery all those years ago.

Long story short. The Feedback carb system is about 98% intact but every hose is a crumbling, dried out mess. I don't hear any vacuum leaks but I'm sure it'll happen soon. Also the 'emissions' light doesn't come on at start-up. I believe it's been removed. I haven't yet pulled any codes but it runs pretty well - issue is I only got 15 MPG on the trip from Jersey to FL. That was with 2 200lb people in the car and about 400 pounds of stuff in the bed. Also running snow tires in the back. We were babying it a little and staying around 70-75mph. 15 MPG seems a little low - just wondering what you guys think?

Also - I've spent a lot of time searching this site but I wasn't able to find an answer to this question: The parts of the feedback system that are really in bad shape are the Air-pump, vapor recovery and EGR... If that stuff isn't working correctly will it affect how the ECM functions? Would an issue with it put the computer into 'limp home mode'? Can I just get the various engine management sensors into shape and leave the other emissions stuff alone and still expect the computer to function properly? Need to get it running as well as I can as cheaply as possible.

Thanks!
rustfreemike

 

15 mpg isn't too bad, considering the variables you listed.

The computer system inputs (sensors) are:
1) crankshaft position (PIP)
2) throttle plate position (TPS)
3) engine coolant temperature (ECT)
4) exhaust oxygen level (EGO)
5) manifold absolute pressure (MAP)

The above need to be functioning because the computer will look for input signals from these sensors and will become unhappy (limp home mode) if one or more are missing or malfunctioning.

In contrast, the vapor recovery (canister purge valve), EGR valve, feedback solenoid, and thermactor are on the "output" end of the system. So the computer cannot actually "see" these components, but can control them in most cases using vacuum solenoids to turn them ON or OFF.

 

Your truck was designed to operate when the national maximum speed limit was 55 MPH, 70-75 is really pushin' it.

Yes, pull codes, too.

Here is all about the EMISSIONS light:

https://www.ford-trucks.com/forums/1...ons-light.html

 

"...We were babying it a little and staying around 70-75mph. 15 MPG seems a little low..."

60 mph might be considered babying it...70-75 mph not. 15 mpg in an aerodynamic-as-a-brick truck at 70-75 mph isn't bad.

"... every hose is a crumbling, dried out mess..." There is a good starting place. What Galendor said X2. All systems in a computer controlled vehicle have to be functioning properly

 

Galendor - thanks for confirming that I can get the computer inputs in order first. I wasn't aware that there was a crank position sensor input in this system until you mentioned it.

ctubutis - The 55 mph speed limit when it was new crossed my mind - glad to hear 15mpg isn't too terribly off at 70-75. Also thanks for that link.

Harte3 - definitely going to replace all the old hoses but the truck has one more big trip in it before I do. Gotta drive back up to North GA with a final load of stuff. After that the truck's only duties will be getting me up and down the mountain to the home depot and back.

-Mike
86 F150 XLT Lariat

 

X3 what Harte said. I wonder what transmission you have, to be "babying it" at 70-75 mph!

Pull the codes. Or, a quick check if you have a timing light is to just check the base timing (with PIP disconnected), then check the timing advance (PIP reconnected) when an assistant gives it gas to raise the engine speed up to say 2000 rpm (in neutral, obviously, and after engine is fully warmed up). If timing advances with engine speed, then the computer IS NOT in limp home mode.

 

It has the 4 speed overdrive manual transmission -which is most of the reason I bought such a rusty truck - I was having too much fun driving it to think about rust.

Had no problem keeping at 70-75 in overdrive.... But actually today I had a GPS hooked up and noticed that the speedo was reading 5 mph low at highway speed. So I'm guessing the tires on the truck are too small (235-75R15. What did it come with originally?). - I should amend my '70-75 mph' to 65-70 mph.

Going to try the timing-light test you outlined tomorrow - really curious to know if it's in limp-home mode.

Thanks,
~Mike
86 F-150 XLT Lariat

 

Oh, ok - same transmission as mine. Yes it will happily do 75 mph and be under 2000 rpm in OD, especially if the rear differential gear is 3.08 or higher.

The "stock" tire size is given on the information sticker inside the drivers side door jam. 235 75 R15 sounds about right, actually.

Once the engine is warmed up, if your truck is advancing timing with increasing engine speed, then the computer is not in limp home mode (I think it is always in limp home mode until the engine warms up).