My 91 F150 4x4, 351, can hardly maintain highway speed in overdrive, when it has to climb a small incline on the freeway. I'll get my foot into it until I feel it is about to shift down, but it seems there isn't much there. WHen it shifts down, it goes OK. It has 106k on it, runs, idles fine, gets about 14 mpg. IT SEEMS TO ME that an engine this size, pulling an empty truck, should easily have the power to maintain 70 mph in overdrive, even over mild hills on an interstate freeway. SO - is it possible my catalytic converter is plugged? I keep air filters clean on regular basis. And why couldn't I just take the catalytic converter off and put a straight pipe there. It is bound to reduce backpressure. THis is a truck that gets driven only 2k a year, at most, never had any work done on it except regular maintenance. Changed plugs 10k ago (5 years) ANY IDEAS? Thanks.

 

Well if the cats are origninal to the truck then you might consider replacement or deleting them, very well could be the problem. IF you are real concerned you might do a simple compression check just to see where the cylinder pressure is at. I would also recommend installing new wires plugs and cap and rotor. May try pulling some codes just to see if something out of the ordinary is showing up. Later

 

Fuel filter...

 

What axle ratio do you have? 3.08?

 

Sounds like my '93 5.0. it's always been very weak. I think it must be geared fairly high. I have over double your miles and my converters aren't clogged. i don't think that's your problem. Might think about a new O2 sensor. They supposedly have a life of around 60k miles. you won't get any codes saying it's bad because it's not out of range but it's probably not accurate. I'm going to change mine as soon as I can. KS

 

Compression test?

Nobody on here ever suggests a compression test.

 

Testing of any kind seems to be a last resort, trial and error parts swapping is the preferred method.

 

testing should be first proirity so u dont change out good parts and waste $$. only the good techs know how a part is designed, works and how to test it. sure changing a fuel filter and such isnt much and i would start with that but know the symptoms and what they are before changing out parts in a trial n error method.

 

It should be... but it takes me 15 min to change a fuel filter...closer to an hour for a compression test. May as well swap plugs/wires at the same time as comp test. An O2 sensor will take me 5 min at most also.

Cap and rotor take me 10min tops also.

If the vehicle has never been maintained properly it is amazing how much better it runs after a tune up. General maintainence that needs to be done regardless is easy to do...that may solve the problems you are incurring anyway.

 

Thanks for all the suggestions. I don't see how a compression test is warranted - as it idles fine, runs out fine, uses no oil, etc. What else would indicate bad compression?
REgarding O2 sensor, how do I know if it is bad? I am all for testing and not just replacing parts. Thanks.

 

They make a tester for O2 sensors. I wonder if you can take the sensor to autozone and have it checked? trouble is, they're rather fragile and if you take it out, you risk ruining it. If you've got over 100,000 miles on the truck and you plan on keeping it, you should replace the o2 sensor. It is considered to be a tune up item although kind of pricey. KS

 

Because engines can run fine with bad compression.
If you put strain in the engine, they do not perform as well.

 

When's the last time you did a full tune up on your truck?

O2 sensors last 50-80K mi tops. When I got my truck a few years back it still had the original O2 on it from '88!

I change plugs more often than most people trying to get more power. I'd change them at 25K mi.

Fuel filters I swap at 10K mi.

 

Getting back to the catalytic converter - Can a muffler shop take that off for me, or is it illegal for them to do that? Suppose I would have to go to a private welding shop -

 

It's illegal. New cats that are high flow sell for $40+shipping on ebay...