1998 f150 xlt 4.6 248k on the dash, so i have been having a misfire problem for over 2 months now on cylinder 1. At first it was 1,2, & 3 but i did a compression test (only on the 3 cylinders(110 psi)) i found that spark plug 1 & 3 were pretty loose but 2 was tight. Anyway i clean all three of em and the misfires went away fir 2 days. Now I'm stuck with cylinder 1 misfiring again. I have replaced the coil packs about 300 miles ago (motorcraft), the plugs only have 21k on em (motorcraft), the wires have 21k on em as well (some cheap brand), I've checked for bubbles on the oil dip stick, milky oil, oil in the coolant, but I can't seem to figure out what it's from. It happens mainly at idle and when i take off. If i hold the rpms above 1500 it smooths out.

 

If you haven't done so already, put two bottles of Gumout for high mileage engines in a tank of gas and run it through. The injectors could be a little dirty. It will also help clean the intake ports and backsides of the intake valves.

 

I'd suspect those cheap wires. I'd also do a compression test.

A bit vague - you checked for these but what were the results?

 

I'll get some gumout on the home tonight & the results were nothing. No oil in coolant, no milky bubbly oil. Yeah I'll probably do a full compression test, does anyone know if advanced rents the compression tool?

 

Can that wire be swapped to another cylinder to see if it travels? I too would suspect the wires.

 

Most parts stores have loaner compression gauges. Make sure it has a straight shank so it will go down the well and comes with the right screw in adapter for Ford Triton.

Make sure the Gumout lists polyetheramine (PEA) as an ingredient. Not all Gumout does. Otherwise use Techron, it definitely does and is the only additive approved by Ford.

Take this opportunity to use a torque wrench on the plugs - 28 ft/lb. There's only 3 threads in the head and the plugs like to loosen and blow out, taking the threads with them.

 

he said he checked the compression in the cylinders that are missing. 110 psig sounds a little low to me but it is a high mileage engine.

 

Need to check all 8 and check for consistency. All cylinders should be in a 15% window.

 

I put gumout in it and I've driven 200 miles already, the misfire is still there but it's not as bad. Probably gunna try the compression test tomorrow and check the injectors too.
I might be able to swap out the wire with a different but it's pretty short so I'll have to see.
The last compression test that was done all three were checked three times and every time they were 110 psi.
Any way the timing could be the cause? It's still got the original chains, guides, tensioners, and cam gears.
Thank y'all for the help too

 

Alright y'all, i know it's been a few days but after some testing and test runs i finally got it fixed. It ended up being the spark plug wires, so i replaced em with motorcraft ones. Thank you to y'all for the help.

 

Congrats!