I have 91 f-150, 4.9 L. It has seen better days ! It starts great, but idles rough. As you accelerate, it idles up roughly. About 2,000 rpms it seems to even out. Went to do a compression test today, as I was taking out the back plug I heard a crunching sound. Pulling the plug out I found the porcelain was all cracked, AHAAA!!!! I found the problem ! Put in a new plug and found out it still ran rough, crap !!!!! I did notice that the tip of the cracked spark pug was very blackish as though it had not been firing, certainly not clean tip like the other 5 plugs I had pulled. Compression ran 170 on all cylinders. Clogged fuel injector ??? Thanks for any inputs dennis

 

EXACTLY the same pressure on all 6 cylinders??? Even the one that had been misfiring? I find that VERY suspicious. Sounds more like a bad compression tester - you weren't using the press-in kind with the rubber cone tip, were you?

 

Steve, I said 170, but we ran between 165 to 170, new compression tester, the screw in kind. Even though the truck is a 91 it only has about 92k miles, sat around a lot up in Alabama. After reading some more I am leaning towards a fuel problem, could it be possible that one injector could be dumping too much fuel in that one cylinder ? thanks for your input, dennis.

 

check the idle air control motor, usully the case in 5.0s worth a shot for your sixer

 

A bad IAC will cause a low or high idle, but not rough.

The blackness on the plug is from rich burning, so it WAS sparking.

Yes, an injector could be dumping too much fuel, but it's VERY rare. Check the red line on the FPR for liquid gas, or even the smell of it. If so, replace the FPR. Yours is inside the intake cage near the front, just behind the alternator, but it looks like this.

 

Thanks Steve for the input and picture, I wiill ck today ! dennis

 

stang mann, thanks for the input !!!! dennis

 

My 2006 with a 5.4 and only 5000 miles has a rough idle until it warms up. Also a longer crank to start vs my trade-in Dodge. But I assume this is normal????

 

No, a 5Kmi engine shouldn't have a rough idle OR a long start-up. Take it in & complain.

 

I had a rough idle problem on my old 96, I found out working for a ford dealer
that using other than motorcraft plug wires can cause rough idle and missing.
I changed from "high dollar" plug wires to motorcraft and problem was solved.

 

paintboy, wires look good ! the more I look at my problem it really appears to be a fuel issue. today while running, i pulled my # 6 wire and nothing changed, so #6 is not firing, hence the roughness. however the plug itself is firing good. the plug electrode is covered with wet blackish carbon lookin sh-t. compression is good- 170lbs. the only thing aI can figure is the fuel injector is stuck open/leaking ? I give up ! thanks for the input dennis

 

Brand new wires that arent motorcraft will do this. My neibor wasted $200 chasing the same problem before he would try them. $45 later problem gone

 

paintboy, it is certainly worth a try ! i have ck'd most everything else, thanks dennis