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Power loss, 1996 F-150, 300 six

I have had a persistent problem with power loss, and some MPG loss as well. I find going up highway hills starting at 60 mph I am struggling at 45 mph at the top in third gear. I sometimes pull a 6500lb. boat up a steep, paved hill and end up creeping along in first gear with some engine overheating.
My friend said that the manifold sometimes warps with age. To check this I sprayed staring fluid near where the manifold mates with the head while idling, no increase in idle speed.
I have replaced plugs, wires, dist. cap, rotor, all filters (including in-line fuel filter) and no change. It has good compression, burns a little oil. It has 112K miles.

No Check Engine light?


can it be due to exhaust restriction?
not sure how to test, but seems to be correlating high load to the power loss?

Right, no check engine light.

I wondered about the cat. converter. I could eliminate it since we do not have emissions tests here in Alaska. Another friend suggested ramming rebar through it to open it up. I thought that might do other damage.

wow… ramming it with rebar seems to be a little bit drastic if you do not know if it is the root cause.

when I was replacing my failed cats, I could “see through” their combs visually, not sure you would be able to remove it and inspect visually like this… messy to remove as all nuts are rusted, I would looks for other things to check before getting this route

A vacuum gauge will tell you if your exhaust is restricted. Instructions for using one are easy to find online.

I’m shooting in the dark here, but does this engine have a carburetor? Is the cold start mechanism not releasing when the engine is warmed up?

It has EFI

Connect a vacuum gauge, you should see approximately 17" of vacuum at idle and ~22" @3000 RPMs in neutral/park. If the vacuum is lower @3000 RPMs than at idle there is likely an exhaust restriction.

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Thanks, this makes sense, I will do it.

Makes sense. I will try a vacuum test.

Fuel injected.

Okay, finally got around to the vacuum test. It had about 18 at idle and revved it (no tach) to high rpms with no change in vacuum, remained at 18. The gauge instruction said it should drop in vacuum if there is exhaust restriction. So likely it is not the cat converter.

If reving the engine didn’t result in the vacuum reading immediately dipping significantly I suspect that you connected to the heater control check valve.

I think you’re asking a bit much of this truck. At most this truck was rated to tow 5700 pounds ( regular cab, 2WD, 3.55 rear end, automatic. Towing capacity is less with 4WD, manual transmission, 3.08/3.31 gears or any combination there of) . 20+ years later, it’s probably not up to the task to towing a 6500 pound boat, uphill at highway speeds.

That’s a good possibility @FoDaddy. The tall geared light duty models were limited in their towing ability while the F250s and F350s with compound low four speeds and 4.00+ final drives could likely drag the boat easily.


Thanks for the feedback. I don’t pull the boat on the highway much. On a
flat highway my top speed is about 45 mph when pulling the boat. That
was an example of the truck struggling uphill on a side road pulling the
boat, I need to go to 1st gear. With a light load on the highway, or
even empty bed, it still struggles on hills at 60mph, I end up in third
gear at 45 mph by the time I reach the top of the highway hill.

I get your point, it’s showing it’s age. It has been a very dependable
vehicle and not worth selling it at the Blue Book value of less than
$4K. It does everything else admirably!


Presuming it had more power before, and less now, given what you’ve already done, I’d be looking at

  • Something partially blocking the airflow between the air intake and the throttle body
  • Fuel pressure dropping when engine is under a load
  • Clogged injectors
  • Too much valve clearances
  • Even tho the manifold vacuum seems ok, if nothing else is helping, I’d still be inclined temporarily remove the cat and do a test drive, see if that has any effect.

Timing chain might be worn.

Remove the distributor cap to expose the rotor.

Turn the engine over by hand in the normal direction and then stop.

Now turn the engine over by hand in the opposite direction while watching the rotor.

If the engine can be turned over in the opposite direction more than a few degrees before the rotor in the distributor begins to move, the timing chain is worn.