I just bought a 1991 F150 Lariat, ext cab, 5 litre 2WD. Not sure if it has 103,000 or 203,000 miles on it but it is in pretty good shape for its age and is all I can afford these days. It starts and runs smooth and clean (no smoke) but does not have as much power as it should. I was thinking it feels like a fuel-mixture problem, but that's just a guess. Also, there seems to be a huge amount of accelerator-pedal travel before the engine starts reacting. I have already replaced the fuel filter, air filter, plugs and wires. The CHECK ENGINE light goes on after about 20 minutes of driving. I checked the codes and found only one; "126 MAP voltage above spec". Does that mean that that sensor is bad or does it mean whatever is feeding that sensor is bad? Would it cause the lack of power? In addition to changing that sensor, since there are so many miles on this truck and most sensors have a life of 100,000 miles or less, I'm also planning to replace the Oxygen Sensor(s) and Throttle Position Sensor hoping that will fix the low-power problem. Also, when I depress the accelerator pedal, I can feel a slight binding instead of a smooth movement, so I was going to replace that cable as well. Is it reasonable to expect one or all of these to fix my power problem or is there something else I should be looking at? Are these sensors located and accessable with one hand (I have a broken arm)? Are there more than one O2 sensors? O'Reilly's has several choices to choose from for each of the parts so I'm a bit confused as to which to buy.

 

I'd go w/ a MAP & O2 sensor, disconnect the battery for 5 min. after replacement. Then drive it for a while. You should see a very noticeable improvement. Sensors will code if their pooched. Although an o2 sensor will slow in switching speed & loose accuracy long before it codes out. However, removal & cleaning your EGR & IAC is a good idea. These are valves and won't display a code directly themselves. A clogged EGR can result in a Insufficient EGR flow code by the EVP sensor though. Otherwise, Id wait till they actually break before fixin em! Should have only 1 o2 ahead of the Cat. Converter.

 

I'll give those things a try and update with the results in about a week or two. Thanks for the post.

 

Followed your suggestions and I did notice an improvement. Not as much as I was hoping for but I suppose that's my fault for having a 302ci, not a strong engine. Afterwards, I got another noticeable increase in performance by sealing a vacuum hose that was open (1" hose going to the blower motor). It wasn't connected because it was broken-off and the remained length was too short to connect both ends. So only one end was connected and the other end was open and continuously sucking air. Several self-proclaimed mechanics told me that the open line wasn't hurting anything but that didn't seem quite right to me. For example, if you have a pin-hole in your brake-line, some energy is wasted as it escapes through that hole as your apply the brakes. In my mind, the same logic applies to an open vacuum line. Other components that require a vacuum are going to suffer if there is an open line or leak in the system. Anyway, thanks again for the helpful input.

 

You may want to go to a muffler shop and get your cats checked - they may be partially clogged.

 

I'll do that, thanks.

 

Bump your base timing to 14 BTDC (if you get ping under load, retard 2 degrees) if all your codes are gone & after treating the deposits in your combustion chambers w/ Seafoam. I'd skip adding it to oil unless your planning on changeing oil soon. Just 1/3 through the power brake booster Vac hose at warm idle, shut off for 15 min. then restart & rev out the crap! Put the other 2/3's in your gas.http://www.f150forum.com/f33/how-sea...-1991-a-31505/ & http://www.seafoamsales.com/ These two things will make a noticeable difference also! Including better MPG!