JayCarr97

I have a 1988 F150 XL Lariat, 4.9 5-Speed Manual
I don’t have much knowledge about vehicles. Just your basics (oil change.brakes,how to change a tire). Recently changed the spark with a buddy of mine. And now the truck is idling really high, the throttle is as low as it can go. I’m just not sure what wrong. I’m sure I didn’t mess up the firing order as I replace one spark plug and wire at a time. I asked autozone what the gap should be and they said .045 so that’s what I did. Any suggestions?

16IngotFX4

Vacuum line that wasn't reconnected? Double check the work.

If you have a crossed wire (plug wire on wrong plug), you'll notice the motor shaking from the misfire. If it's smooth and fast idle it wouldn't be cross wires.
Vacuum line is first thing that comes to my mind as that cause a high idle condition, or erratic.
Also double check there aren't any electrical connections disconnected in the area you guys worked on. There were two of you, maybe you each thought the other one reconnected everything.

Steve83

Your first mistake was axing the zone for advice. They hire people who can't cut it as burger-flippers to stand behind a desk and look up part numbers. They are NOT mechanics, and probably know less about how vehicles work than you do. I hope you didn't actually buy any parts there.

Click this, read the caption, and follow the links in it:

(phone app link)


Eventually, you should read it cover-to-cover at least once. Keep a hard copy in the truck with the irrelevant sections crossed out with a light-colored crayon or colored pencil (so you can still read them if necessary).

Lastly: it's very possible (since you didn't say why you were changing the plugs, or what the old ones looked like) that the truck was running THAT badly, and it now runs so much better that the idle is noticeably high. Since you've made a significant change/repair/improvement, it's probably worth clearing the KAM to erase the EEC's adaptions by disconnecting the battery for ~5 minutes. The engine may run poorly for ~10 miles as the EEC re-learns the adaptions.

But it's also cheap & easy to check for a vacuum leak using a mechanic's stethoscope, or ~3' of garden hose:

(phone app link)


This thread explains how to upload & embed pics in your posts:
https://www.supermotors.net/forums/t...-and-or-videos

This caption explains how to collect all your truck's details, and put them into your signature so we can review them with each reply:

(phone app link)

raski

I know on the 4.9 that the # 6 spark plug (the one closest to firewall) is very tight with the solenoids hovering over the spark plug. I have to disconnect everything to access# 6. I would look in that area to see if you broke any of the plastic lines which is easy to do. Also look where the large vacuum line attaches to the brake booster for a split grommet or the PCV valve which may have pulled loose, sucking air.
New plugs will not cause your issue and if the engine idled fine before, you must have done something in the process of changing the plugs.
You mentioned "idle is as low is it can go", I hope you didn't go messing with the hard stop adjusting screw on the throttle body,if you did, you need to put it back to where it was.It is not meant to control idle speed.

JayCarr97

We didn’t mess with the screw on the throttle body, my buddy just told me it’s all the way closed so we couldn’t if we wanted. When I bought the vehicle it was running well just a little rough and felt like there was more power to what I was getting. So decide to do a tune up. The old spark plugs were burnt to hell. I mean 5 out of 6 were white.

JayCarr97

will update profile once I get off work. But I changed the plugs because I wanted to do a tune up. Bought the truck off a farmer but before him it was a city vehicle. Just figure it needed some tlc and it could use it. Old plugs were burnt to ****, they were white when we pulled em.

Steve83

EFIs don't need tune-ups; they re-tune themselves ~200x per second. Only carburetors need to be tuned. EFIs only need maintenance & repairs, so follow the published schedule, and catch up anything that's overdue.

(phone app link)