Any ideas why my 1998 F150 4.2 liter automatic sometimes “rumbles” or vibrates sometimes? Sometimes when I’m stopped at a light the truck has an unusual vibration as soon as I step on the accelerator the noise subsides. Seems to run fine otherwise

 

Could be a loose or fouled plug or plug wire. I had an 83 f150 that did that. Turned out to be a bad plug wire.

 

How many miles does it have on it? If it's got 105,000+ miles and has never been tuned up, it's about time to do it.

 

Actually, now that mine did that to me today, it jogged my memory about this post. Mine has that same thing going on sometimes. I've got almost 67K on mine. It never does it when I'm actually going any kind of speed at all, but it will do it occasionally at red lights.

I have found out that, at least for me, it's just some kind of synergistic thing going on with the A/C. Whenever I turn off my A/C, the rumble goes IMMEDIATELY away. However, it'll go away all by itself as soon as the fan kicks into higher gear, so I don't worry much about it. Just one of those things.

So, see if this rumble is connected to your A/C unit.

I see you're in CT, so I guess I should say "See if this rumble is connected in some way to your heater." At least you'll know.

 

Did y'all ever find a solution to this problem... i currently have the same symptoms... and it's frustrating because i can't narrow it down.....

 

vacuum leak.

Check the following list of suspects (don't ask):

The elbow on the bottom side of the intake plenum just aft of the throttle body

The elbow at the PCV valve (passenger side valve cover, aft)

The plastic tube that sneaks between the 2 items above

The 3/8" hose/elbow on the back side of the intake plenum (I didn't even know it was there until I took the plenum off). It's actuator vacuum for a valve and tees off of the PCV line.

Good Luck

Jim

 

mine used to do that alot. i just changed the plugs, plug wires and fuel filter. now there no shake or rummble. i would try these first.

 

The slayer may very well be correct - those are good things to do anyway.

The distinctive symptom of a vacuum leak in this engine (in sadly vast experience) is that it runs OK under load, but idles badly - just like every other engine with a small vacuum leak. When stopping at a light, if it idles OK for 10 to 20 seconds and THEN starts to get crappy, it is probably vacuum. My guess is that the computer starts messing with IAC and EGR positions at 'extended' idle. The computer starts chasing it's own tail and the idle oscillates all over the place. Whatever it does, vacuum leaks really show up at that point.

Jim

p.s. if it gets worse when the outside air temp is low, that also points to vacuum.

 

same thing happens to me. mines an 01 and i used to get in all the time when it was still under 50k. trucks up to 106 now and it rarely does it anymore. havent tried anything with the plugs but maybe i will

 

Well, I just learned something today. Whenever you hear a low level hum that sounds like a mixer hitting a metal bowl, it's time to have the tranny dropped and looked at. Now my truck might as well be a Chevy because it's sitting 'like a rock' out in front of my house.