Hi all, I am on my 2nd 86 F150 4x4. This one is a 351HO with the C6 transmission. The axle code is 16 which I believe means the gears are 3.50. I'm running into several issues especially when dtiving on the highway which I do a lot. Any recommendations y'all could offer would be great.

1) at 70mph the truck is doing about 4100 rpm. This seems pretty high to me. Is this indicitave of a problem? The truck has 28" tires. I know the C6 doesn't have overdrive, but this seems high. What should I check?

2) After cruising on the highway for a while if I stop for gas or something, when I try to start it back up it cranks really really slow. Sounds like the battery is almost dead or something. If I wait a bit it cranks normally and starts right up. It hasn't stranded me yet, but it makes me nervous.

3) lately I've been having mild issues with cold starts. When I first try, it cranks real fast but doesn't turn over or sometimes it'll barely turnover and immediately die. The second crank it typically starts up rarely easily with a couple pumps of the pedal. Not a major issue, but annoying.

Any insight you could provide on any of these issues would be appreciated.

 

I have a C-6 with 3.55 gears and @ 65 mph I am turning near 3000 RPMs if I remember correctly. Sounds like something is amiss.

 

Any recommendations on what to check first? Got a long road trip pulling a trailer coming up.

 

Check your axle tag. It's a small metal plate bolted to your rear diff with a bunch of numbers and letters embossed on it. It should have a numeric axle ratio somewhere on it, like 3 55 or 3 73 or 3 08 or 4 10 for example. This should get you a pretty positive indicator of what gears you have, in case the diff was swapped out.

Other than that I don't know why it would be running so high. Your truck is a 2wd, which rules out having it in 4-low or something silly like that... The other thing is if your 3-speed C6 transmission is stuck in second gear. I guess you should check and make sure it shifts twice when you go from a stop to highway speed.

As for your starting troubles, check to make sure your battery has good connections. Mine had a bad positive cable and eventually it started making a harsh grinding sound instead of cranking the starter when I would try to start it. A $3 cable from O'Reilly's fixed that.

EDIT: I re-read your post, and your truck is indeed a 4wd. Make sure your transfer case lever is in the full forward position in 2-hi. I've heard of some t-cases allowing you to find a mysterious 2-low mode if the shifter gets caught in between 2-hi and 4-hi.

 

If you replace the starter cable and the starter is still dragging, you could just be having heat soak issues with the original direct drive starter

On cold starts, always give it one full pump to the floor, then release the pedal, and do not pump it while starting. The single pedal-pump sets the choke for cranking and primes the carb with a shot from the accel pump

 

Thanks for the tip on cold starts. I'll try that tomorrow morning. With the possible heat synch issues, what do I do to check for and potentially fix that?

 

Most likely, the battery cables - to block & frame, solenoid etc etc, are original to your truck. These should be replaced on general principles, due to their age and corrosion going on these 32 years. They tend to corrode under the insulation where it is not visible, at the crimp bond, and inevitably start to drop significant voltage under heavy starting loads. Most people know about cold weather starting problems though high temperatures also cause real trouble.

There are other possibilities without being there in person to check - over advanced ignition timing will act the same way aka "starter kickback". The next time you're on a highway run or the problem presents itself disable the ignition and see if normal cranking returns.

And a starter itself on the way out will often begin to drag and draw way more battery current than spec'd. Heat soak can often be minimized with adequately sized cables and clean tight connections, makes for happy electrons. It's kind of a package deal, as the resistance increases the alternator too struggles to plow through it and the battery doesn't get plussed up as well as it should. Let us know what you find.

 

I'm probably showing my ignorance here, but what does "The next time you're on a highway run or the problem presents itself disable the ignition and see if normal cranking returns" mean? How do I disable my ignition?

 

Well let's say you stop for gas and it turns over real slow when you try to restart. Ground the coil wire and see if crank speed returns to normal, only for test purposes.

 

Headers will heat the starter and cause the slow crank. And like was said, advanced timing will cause it also. A trick I used to do on my chevy when the advance weights would rust and get sticky, on a hot restart I would pump the gas pedal a couple of times before cranking. This would flood the engine and it would not fire, but it let it crank over faster till it cleared up, and by that time it had built momentum and it would start right up.