I just brought home my new to me 93 F150 5.0. On the way home (50 miles) it started running rough or shaking. It would only happen when I would let off the throttle and then give it a little throttle, like when a I would come up on a slow car. Giving it more throttle would make it clear up until the next time.

Before the trip home i filled it with 85 octane gas, I don't know if that might have anything to do with it.

 

Welcome idahofordguy, I've never heard of that problem before but if anything it might be the fuel. Take it to one of the parts stores that will run the codes for you and that will take the guess work out of it.

 

Might be a low idle caused by a dirty IAC (idle air control). Pretty easy to pull its right on the throttle body. Take it off and clean it out. It's held on by two screws. While you are at it you may wanna get a new gasket for it, what was causing rough idle and stalling on mine was a combination of bad gasket and dirty IAC.

 

It could be such things as O2 sensor, and Throttle Position Sensor, airflow sensor. Need to check for codes. Make sure airfilter is clean, all vac hoses good.

 

sounds like a confused throttle position sensor, maybe a dead spot in it. if it runs perfectly fine at any other position including idle. it might not be pickin up that slight increase.

 

I second brisk_truck's suggestion.

When you release the throttle, the throttle body valve closes and the IAC opens. The engine, at that time, gets it air thru the IAC. They get dirty all the time.

Pull the codes. There are many threads here that describe how to do it without a code reader. You could also have an auto parts store read the codes or buy a code reader for less than $50.

 

on my 95 f150 5.0 while changing dist. cap, rotor, and wires after changing ran awful. According to my haynes manual my plug wires were on wrong, put them back on, ran worse.Book says 95 up to install one way and 94 down another. Had to put back as was man. in 94, confusing? Also showed engine light had bad reading on tps. Found wires pulled apart when changing plug wires, now runs good, was easy to change.Note: check door for date of man.

 

Never hurts to change the fuel filter. Check fuel pressure at the fuel rail valve, etc. I got a bad batch of gas on the interstate, and the engine would miss every 5 minutes or so, but otherwise the engine ran fine. Freaked me out, but wouldn't hurt to put some Seafoam or whatever in the gas tank once. Maybe some gas drier? Maybe you have some water in your tank.

 

Not to get off on a drift here - but 85 octane? The lowest I can get in the northeast is 87. I don't think I've ever even seen 85.... When I first read this post, I thought the writer meant that they filled up with E85, and that was the problem....

WOW! 85 huh? Live and learn... Now back to your regularly scheduled thoughts....

 

I know they carry 85 in the higher elevations. When I was in Colorado, I saw it there.

I third the vote for the IAC, clean it or at least check it. Or it may need a good tune up. Use Motorcraft Copper plugs, Motorcraft 8mm or better wires, and Motorcraft cap and rotor. These engines really do better to keep with the OEM stuff. Oh yeah, Motorcraft oil filters as well, stay away from Fram oil fiters.

 

One more thing on these 5.0s, make sure to keep the plug wires seperated at leeast an inch or cross at 90 degree angles. They have a crossfire problem on 7&8 and 5&3, I believe.

 

Thanks for all the replies guys, I will take a look at the IAC in a couple days when the snow and wind is gone. I always like to do a tune up after buying any vehicle, I will get all motorcraft stuff. I will also try to pull any codes if there are any. The check engine light does not come on so I am not sure if there will be anything.

I have been running Mobile 1 in all my cars for a few years now and like it, would you guys switch a 5.0 to Mobile 1 with 103,000 miles on it? It has one small leak at the rear between the tranny and the engine but that is it.


As far as 85 octane gas all over the intermountain west we have it. The oil companies say carburated vehicles run fine with since the elevation is high. Only problem is how many people still run carburated vehicles? I have been running 85 in all my vehicles like most people around here without any problems.

For 2 years I lived in western Oregon and was shocked to find that 87 was what they call regular. I grew up in Colorado so I am used to 85.

 

Mobil 1 should be fine. That leak at the rear is probably the oil pan gasket. Or maybe the valve cover gasket leaking down the back of the block. These oil pans are notorious for rusting out, so keep an eye on that leak.

 

After taking her for a long trip today I discovered that she does not hardly warm up and she is only getting 11MPG. Maybe that is normal but I think 11MPG is too low.

It is supposed to be dry and warm tomorrow so I will see if there are any codes.

I will look at the IAC, coolant temp sensor, and thermostat. I think it is running rich trying to warm up.

 

Good advice above and you should check your cooling, but I thought I'd add another thought here.

If you have access to a pressure gauge for fuel injection, it's an easy check to rule out the fuel pressure regulator. There's a schrader valve on the fuel rail, preassure when running should be around 35 pounds. Also pull the vacuum line on the FPR, if it's wet it's definitely bad. (Dry does not mean automatically good).