Ok, the story is that I bought a '91 F-150 a few months back. It ran great, but used a quart of coolant a day and had a very, very occasional miss under full throttle. Happened maybe twice a month. I drove it everyday at least 60 miles.

I figured the miss was just the truck telling me it needed a tuneup. I ended up replacing the motor because of the massive coolant leak (head gasket). Of course, at the same time I also replaced the cap, wires, cap, & rotor.

The truck still runs great, but the miss was still there at about the same frequency. So now it's probably the coil.

I read the thread where the guy claimed 22 mpg in his '96 F-150, and I wanted to try that upgrade to improve the mileage AND get rid of the miss.

I did the ignition last weekend - BOY, what a difference. This truck runs absolutely fantastic. No mileage reports (YET), but it's got considerably more power. I can now accelerate up hills in overdrive that slow trucks down to 20 mph and other cars down to 50 (although that may be driver-related)... It starts easier, and really seems to want to run. I'm very pleased. Seat of my pants, it probably feels like an extra 10 to 15 lb-ft of torque.

But I STILL HAVE THE MISS!!!!

New motor, accel cap & rotor, autolite plugs, MSD wires, MSD coil, MSD multi-spark box, and it still has a very occasional miss under full throttle. What am I missing? Do I need a new dizzy? That's the only ignition component that hasn't been replaced!

 

I am guessing you checked for stored codes and found none.

The computer can store codes and the check engine light never come on.

Have you changed your fuel filter, have you ruled out the fuel delivery?

Is it specific to an rpm, maybe fualty egr function?

Vacuum leak, say around the throttle body?

It could be darn near anything.

Good luck

 

\

x2

 

Thanks guys... no codes, new fuel filter, good fuel pressure.

I didn't think about a vacuum leak. I'll check for that next. Considering I didn't use a torque wrench once while putting this motor in, that's a good possibility.

 

Hey andym,

Check your vacuum caps on the vacuum tree.
I replaced all of mine last week (one was cracked and leaking pretty good) and my truck smoothed out a bit more.
Also, you might try and put a hose clamp on each end of the PCV hose, mine was leaking and resulted in a "serious" vac leak.

BTW.....
Where abouts' are you in Nor Cal.
I'm in Chico....you near there ???

Just curious.


Bob

 

A vacuum leak won't usally cause a miss under full throttle because at WOT there is no vacuum to leak. However, anything is possible with EFI because the WOT maps are guestimated based on the part throttle maps. Misses under high load are usually caused by the ignition system, but the intermittent nature of the problem kinda sounds like a fuel injector might be going south.

 

Bob, I'm not that close to Chico - I live in the bay area.

I think I found the problem. I was putting in some autolite double platinum plugs gapped at .55 to go with the upgraded ignition stuff, and one of the old plugs had a very short gap. I must have bumped it when I put them in.

Remember the thread where some guy had 96 F-150 and claimed he got 22 mpg? I now have everything he has in his truck - I'll post the mileage when I burn a couple tanks with this setup.

I also bumped the timing to 12* - it has a little bit more power and runs smoother with the plugs all gapped correctly.

 

Andym,

I hope you meant gapped @ .055 rather than .55

Anyway, was there that much of a "seat of the pants" difference in "smoothness" when you upped the gap with the msd stuff ???
I've got the "Blaster" ignition box---single hot spark---and my truck hasn't smoothed out like my previous 81' did at idle. I'm running .050 gap on motorcraft plugs with MSD superconductor wires and a MSD Blaster coil and timed at 10* btdc.

I may be a little "undergapped" but my truck will still run pretty decent at that gap with the MSD box disconnected (just in case the box ever decides to quit working).

And PLEASE keep us updated on your gas mileage.
With the price of gas where it is now, it wouldn't take long for a 1+ mpg increase to pay for an MSD upgrade.

It's pretty danged bad when you have to have a fist full of cash to fill up at the pumps !!!

Bob

 

Haha, yeah .055.

It definitely smoothed it out, even my girlfriend noticed it. But I really think that was because of the mis-gapped plug. That plug was pretty burned up, too. It did not run that well. Now that I have six properly gapped plugs, it's much smoother.

Still waiting to put enough miles on it to empty a tank. I'll keep you guys informed.

 

Interested in your mpg gain as well, wondering if the other post was a fluke, as I would imagine but you just never know...good luck

 

Just filled it up the other day.

Before the motor swap, I was getting about 15 mpg.

After the motor swap, I was getting about 15 mpg.

After spending $300 to upgrade the ignition to make it identical to the guy who claims his 300 powered F-150 gets 22 mpg - I'm getting about 15 mpg (15.4, to be exact.)

I do notice a little spark knock going up hills at WOT now - I'll have to bump it back down to 10 BTDC.

 

Thats no good.

On the plus side, you may have just saved someone reading this $300.

 

I doubt it. It has been pretty well documented that ignition upgrades are a poor upgrade for the money, yet people continue to buy them.

 

yup, only time i would upgrade would be if my coil went because i seen replacement coils for a lot and an msd or crane is cheaper...

for made a good iginton system on these trucks.

 

Thanks for the update Andym,

Disappointing................yes
Valuable information.......yes
One for the archives.......yes

Bob