I Just Bought A 86 F250 4x4, 4 Speed Std Transmission With 302 Ci Engine That Runs Poorly. It Has About 110,000 Miles On It. I've Changed The Plugs, Wires, Rotor Button, And Cap. Have Not Checked Timing Or Done A Compression Check Yet. It Ran Much Better After I Changed The Electrical Items But Still Doesn't Have The Power It Should. I Pulled A Trailer With About 2000# Load And It Wouldn't Pull Up A Slight Grade Without Shifting To 3rd!!

When I Replaced The Plugs The Two Back Cylinders #4 And 8 Were Fouled. The Others Looks Ok--typical Wear Decent Shape With Gray Color. No 4 Was Black With Some Carbon, No 8 Was Fouled With Gray Carbon Under The Electrode.

Any Ideas Where To Look Next.

 

Number 8 tends to foul because the PCV hose dumps into the upper intake right next to #8. Plug that input line on the upper intake and re-route your PCV to the main vacum tree up on top on the upper intake. Or into the brake booster line going into your upper intake. But this will not help in the power dept. much. Check your rear end gear and see what it says on the tag.
I've got an 86 F150 302 4x2 with AOD trans and 3.55 rear gear. And 31 inch tires. How big are your tires?
My engine runs strong (rebuilt with 30,000 on it) but on the slightest grades it goes out of overdrive into drive. Without towing anything.

 

The Tires Are 245 65r 16. Will Check Rear Axle Tag But I Suspect It Is 3.55. I Tried To Determine Ratio While I Had Both Rear Wheels Jacked Up And Had Problems. I Was Turning The Drive Shaft And Counting The Axle Shaft. Can't Be Correct Because When I Turned The Drive Shaft One Revolution The Axle Shaft Turned About 1 3/4 Turns! Will Check The Tag And Try To Determine The Ratio.

There's A Problem Because Of The Two Fouled Spark Plugs And I Can Feel The Hesitation In The System While Driving Without A Load. Somethings Not Right.

Thanks For The Reply

 

Forgot To Say The Tag On Door Jam Says The Axle Is #293. Don't Know If That Is A Code Or What?

 

Ya, it also says it on the door jam. But there should be a tag on one of the bolts of the rear end cover. I'm not sure what the 4x4s came with because I don't have any experience with them. Maybe you are geared on the high side.

Check you ignition wires and make sure #7 and #8 aren't close to each other.
Maybe you are getting some cross fire. 7 and 8 fire one right after the other.

 

The Rear End Is A Ford 10.25 And Has A 3.55 Ratio. Found The Tag. Also Checked The Plug Wires And All Looks Ok. None Touching Or Too Close To Each Other.

The Vacuum Hose(s)-2 Hoses Y Into One- Coming Off The Intake Manifold Just By The Throttle Body Is Not Hooked To Anything. I Have Stuck A Plug Into It But Can't Find Where It Goes. Don't Hear Any Vacuum Leaks On The Engine.

Can't Figure It Out. The Engine Runs Really Smooth Until You Put A Load On It. I Can Shift Into A Higher Gear A Little Early And Feel The Hesitation And Maybe A Miss. Just Seems To Be No Power. Still Need To Do Timing And A Compression Check.

 

well, i finally got around to doing a compression test and guess i know now why it doesn't have much power. the first 7 of the cylinders tested good at 172 to 180#. number 8 tested 50#. after squirting some oil into the cylinder it read 125#.

does this sound like i have bad valves in this cylinder? since i'm planning on making a swamp buggy and will only run in low side and the 1st two gears in the transmission can i get by with only pulling the head and doing a valve job on that side--until something else goes wrong. it doesn't smoke and idles smooth.

 

Hi Sawdust,

I have an 1985 F150 302 AOD 2x4 and I have the same problems, but i'm tracking mine down to a major intake/vacuum leak.

The vacuum lines that connect right behind the butterflys on the throttle body connect to a ?carbon? element box on the right hand inner fender below the battery. Mine was disconnected as well but it is for the gas tank, thankfully I have an almost complete parts truck to look at.

I too have the problem with the PCV valve pouring oil into the intake, but the prev owner of my truck apparently tried to replace the gasket between the upper and lower intake with out the proper tools or patience as the torx bolt head was stripped and cross threaded into the intake. It was about a quarter inch out of the intake, so I have to fix that before I can find out if my truck is ok.

Mine has the same problem with no power under load, but I could hear a major leak somewhere on mine

Good luck with your project

Mike

 

Hi Guys

My '86 did the same thing too. I've been chasing this for over 2 years!

A coulpe of things, The vaccum hose with the "Y" in it, That goes to the upper and lower nipples on the right side of the throttle body.

If you have a lot of oily gunk getting into the upper intake, (plenium) you may have the same problem mine did. Valve Guides are shot.

Have the truck scoped out, Do a cylinder shorten test. In my case Cylinders 1, 2,& 3, when shortened out, the engine actually reved up, (not down as it is supposed to) Leading to the valve guides are worn.... since they are worn, all the gasses are backing up out of the cylinder and into the intake and down to "good" cylinders. A compression test will not show anything, all cylinders had @160#

I had no codes, checked TPS voltage, New Cap, Rotor, Wires, TFI, EGR, even a new dist body from Ford. 7 & 8 were not crossed.

Another hint, take off the plenium, if it is full of oily goo, at some point the valves may not be seating right while running. Rest the plenium on the floor with the port holes down, if oil runs out after some time you may have an expensive fix on your hands!

That's why a compression test won't help in this case. Having the engine rebuilt as we type.

Let us know what you find

Reamer