I've got a 2000 Expedition with a 4.6 and just over 100,000 miles. When driving at highway cruising speeds I get a vibration and a rumble sound as soon as I hit a slight incline. It just seems to happen when the throttle opens a little above the cruising level when encountering an increase in load. Other than that it runs pretty smooth. I have changed all 8 coils twice (don't trust the cheap ebay coils) and plugs with no change in the rumble. The engine seems to run great. It accelerates smoothly and free-revs like it should. I can't get it to vibrate while powerbraking, but I can get it to do it while driving in 3rd or 4th gear. I have also had the trans flushed. I had suspected the egr, but I'm getting no codes whatsoever. I looked in the throttle body and the egr ports look good from there. I have run seafoam in the tank and in the brake vacuum line. I have also checked fuel pressure and found it to be good. I changed the fuel filter anyway. I haven't checked fuel pressure when it happens at highway speeds, but maybe I'll try to see if I can do that. I'm thinking maybe IAC or TPS. Any thoughts? Thanks.

 

Does it smooth out if you give it more gas?

 

Yes, it will downshift and smooth out.

 

Oh yeah, it also smooths out immediately if you let off the gas. But it will pick it right back up again as soon as you have the same circumstance: Give a little gas at a hill to maintain speed without downshifting. It feels down on power a when it happens, also. It seems to need to downshift on small hills that it didn't used to downshift on. Does it with cruise control on or off. I have also recently replaced tires, front brakes, front hub/bearings, ball joints, and tie rod ends as routine maintenance. The problem was there before and after those were replaced. U-joints all seem to be good. Don't feel play in pinion bearings on the front or rear diff.

 

From what you have said, I think the problem is in the engine and most likely something very minor. I suggest that you check all of the vacuum hoses and especially the inside of the elbows, they are known to break there. I think there is a real PIA at the very back of the intake manifold that rarely gets checked because of it's location, if there is any vacuum hoses going back there you have to check it. You may also have a bad PCV.

 

Thanks. I have done a visual-feel around-kind of check of the vacuum lines including the PCV at the back of the intake. The whole line is covered with a foam insulator which I haven't taken off yet. I know that there's a plastic elbow back there that has been troublesome to some others on this site! When I looked inside the throttle body I did see some oil inside at the PCV port. I'll try a new PCV valve tomorrow. It's weird that it's not throwing a lean code. It feels lean when it does it.

 

Mine has been doing something similar. I got a CEL code for a miss on #7 a couple weeks ago and I have had a miss since then. I am doing plugs this weekend and will be replacing the coils. We'll see if it resolves my problem.

Have you had a CEL code for a miss recently?

Chris

 

Take that 90* elbow off the intake and look at it. They often collapse in the middle and then start leaking there. Replace the PCV first and take a test ride, that could be your whole problem, but check the elbow also.

 

Thanks guys. No codes whatsoever right now. A couple months ago I was getting those cyl # codes with my first set of new coils and plugs within a few hundred miles. That coincided with a definite miss under load. Second set of new coils and plugs and no more codes.
I'm on my way out the door to go get a new PCV valve. It makes sense that it would be a problem with a manifold vacuum (below the throttle plate) accessory because it happens at a transition when opening the throttle from cruising. Thats why I suspected the egr. That, and it seems to happen when the egr should be open. Also, I have opened and closed the egr valve with a vacuum source and found the valve to be functional. Since I can't do that at highway speeds I have no idea if it's actually working when it's supposed to. Anyone have an egr issue without throwing a code?
Thanks again.

 

Sounds like the symptoms of a bad U-joint, might want to check the drive shaft U joints.

 

Okay, I changed the PCV valve. No change. I also pulled the whole PCV line off and inspected it. This truck doesn't have a plastic elbow in the back, just the line. I picked up a vacuum pump, and so far everything seems to be okay. I went back and double checked the u-joints, and the one on the front driveshaft at the transfer case has the slightest play in it. I'll replace them in order to rule it out. I've had a bad u-joint before, but I've never seen it cause this problem. Hopefully that's it because I'm running out of ideas. Maybe time to trade it in!! Thanks.

 

Did you run a compression check? Sounds like whats been going on with mine over the past 4 months. Replaced everything. Right now it boils down to pressure in the cylinders.

 

Nope. I haven't done a compression check yet. It doesn't feel like a compression issue, but at this point who knows?! I know that it runs smooth under all other conditions, even wide open throttle and idle. The muffler needs to be replaced, but I don't think that's causing it either. Vacuum stays good as rpms increase. Did yours have a compression issue with the same symptoms? Thanks.

 

Mine runs smooth under all conditions except when going up a hill, when OD needs to kick in, or when it's very cold. I torn apart my whole TB and cleaned it top to bottom, turns out I had roughly 97psi in 2 cylinders on my pass side.

Currently my motor has a skip that needs to be looked into, but for the hell of it, check your compression. It cant hurt.

 

I would agree that 97 psi is too low! Thanks, I'll look into it. Seems like you'd get a code for it being rich on bank 1 (or 2, whichever is the pass. side) with two lower-compression cylinders on that bank. The injectors would still be operating under the assumption that the cylinders are good, right? And they'd be drawing less air with significantly less compression. Anyway, It's one more thing to look into. A mechanical issue like low-compression could be exactly the reason there are no codes. It just seems like it'd be a little rich. Thanks again.