I need some help. I bought my 1995 F150 5.0 2wd back in December. The beginning of January had an issue with a misfire. Narrowed that down to o2 sensor. New sensor fixed that. I have noticed since that was fixed that it will idle rough and have hesitation upon acceleration. At cruising speeds the truck seems to run perfectly fine. The only thing is that it seems to happen off and on. Runs fine (somewhat) for a while then the rough running/hesitation comes back. I have cleaned the IAC (it also looks new). New cap and rotor. I was told by my mechanic that the spark plugs look fine. The wires look new. The TPS is new. The MAF looks older. I disconnected the intermediate intake tube to somewhat bypass the MAF. Truck seems to have more power. The other thing that I noticed is when I have my foot on the brake pedal it will hiss. The master cylinder just started leaking at the brake booster. I know I need to replace both of those. I had a carbureted 302 before this truck so I am not afraid to work on it. I just don't want to be shooting in the dark since I'm not as familiar with all the sensors and emissions stuff. (The air system has been bypassed since we don't have emissions testing.) I just took it on a drive about 130 miles going 70 mph and I got about 12-13 mpg. The truck has 232k mi. Please give me some ideas on what I should look at now?

 

That leaking brake booster is not helping you any, it's a vacuum leak. Start with what you know is wrong before tossing parts at the unknown.

 

That's what I was thinking. I will keep you all posted.

 

Never trust unknown age/quality spark plug wires. They'll bite you every time. Also make sure the plug wire routing matches the revised layout Ford issued years ago.


courtesy of Steve83


Might as well change the spark plugs too even though someone stated they were good. Plain Ford copper core are highly suggested.

 

Replaced the brake booster and master cylinder. No more hissing/leaking. Seems to accelerate alot better. But I'll have to wait for the weather to be back up in the 70s to 80s. Today was in the 60s. (weird Florida weather)

 

Sounds like you might have it covered. But just in case, see if you can get a fuel pressure gauge and check the pressure at the rail. A bad fuel pressure regulator can throw O2 codes, have a rough idle, and crappy mileage. There was (is?) a recall and free dealer replacement of the original FPR's . Unfortunately I didn't find out until after replacing mine.

 

Today was warmer for the truck. I tried accelerating from 35 mph to 45 mph and it had no power. It does idle a little rough. Other than acceleration and idle, truck seems to run fine. I don't get it. Runs fine when it's cold out (below 70° ish) and then doest run well when it warm out (above 75° ish).

 

I think it's been replaced at one point. The truck has 232,000 miles on it.

 

It only idles rough when it is warm outside. Any suggestions?
Also, I replaced all 8 spark plugs.

 

Did you also change the plug wires as previously suggested? Did you verify the plug wire routing matches the TSB I previously posted? Have you verified the fuel pressure?

Another suggestion is to remove the Ignition Control Module (ICM) from the heat sink mounted near the driver side hood hinge then inspect the thermal compound that should be on the back side of it. After years of use the compound may have dried up. As a preventative action I would clean off what is there then apply new thermal compound. BTW..the ICM should be black. If you have a gray version, it's the wrong one.

Also highly suggest to run the Key On Engine Off (KOEO) tests and Stored Code display.