northern pointer

Hello. I've a 2008 F150 HD 4x4 with the 5.4L engine and with about 158,000kms or 98,750 miles. For some time now I've been hearing what sounds a lot like a plastic bag blowing in the wind from under the truck somewhere, and it seems to be getting louder. It doesn't change pitch, and becomes quite audible from around 1100 rpm and up. This happens as the engine is under load, i.e. giving it gas and holding the gas pedal at normal speed down the road. It stops when going downhill or backing off on the gas pedal. It's also NOT there when idling or revving the engine in neutral. I've looked underneath and all heat shields are on solid, no play. I'm thinking maybe something to do with the exhaust system but nothing is obvious. Any ideas as to what is causing this noise like a plastic bag in the wind? Has anyone experienced this, and if so, what was it and how did you fix it? Hoping to hear soon as this is driving me nuts. Thanks.

ShirBlackspots

Could be the piece of metal that covers the second set of cats.

LariatDude

Your IWE's (four wheel drive you mentioned) are losing vacuum under load. Symptoms you mentioned make sense. When you hear it again, shift in to four wheel drive and if it goes away, that confirms it. Then you'll need to check to see if any of the check valves in the engine bay or the solenoid on the firewall behind the battery no longer hold vacuum. If those check out tho, you'll check to see if the IWE hubs on both wheels still hold a vacuum by attaching a hand pump with a gauge to them. If even still those check out, then your vacuum lines have a crack, are loose, or the vacuum reservoir by the battery is shot.

These all seem to be a possible root cause, easiest problems first to the more serious ones.

northern pointer

Thanks LariatDude. I took the truck out on the road and put it into four wheel drive, and sure enough the noise stopped. Where would I look under the hood for the check valve? Is it attached to the solenoid or located elsewhere? I know the solenoid is on the upper right corner of the engine bay, and it's the newer style with the plastic cover to redirect water. Unfortunately I don't have the tools or the knowledge to try all the things you suggested, so I guess it may be off to my favorite mechanic to do the job. It's probably something quite simple.

LariatDude

Everything circled in red are the cheapest and easily checked. You mentioned you have no rain cover over that solenoid like I do. I advise you upgrade that anyway to the new one, as it is the first step to 4x4 issues. You'll find the next three images circled are the cheap check valves you need to check to see if they hold vacuum. I've had new valves fail outside the packaging from Ford. Test new ones if ordered. There should be a total of 3 of them in the engine bay. The last image is one of the CV axles. Circled is the nipple on the IWE I mentioned (the silver part of the assembly). This is also where you test if they hold a vacuum. Your mechanic should have a handheld vacuum pump and he could test it himself. The airbox I also mentioned earlier is hidden right behind the battery and will only need to be looked at if everything else above check out!

LariatDude

Lemme know if you have anymore questions!! Been thru this system and its issues but it taught me well

BeaverLakeF150

Good call on the vacuum leak, mine never made that sound. I found a short piece of rubber vacuum tube you could reach through the right wheel well that had dry rotted. Replaced that and solved the issue with mine leaking down. All the rest were in good shape, just that one 3" piece that was a problem.


Also a hand vacuum pump for 25 bucks at harbour freight was just the thing for testing where it was leaking at. Ford has an old video of someone on youtube demonstrating the vacuum IWEs with a hand vacuum pump that is informative.

LariatDude

Love the idea of using vacuum for the 4x4 but it opens a whole new mess of problems that can arise. Definitely check the lines for cracks or breaks, Northern

BeaverLakeF150

Vacuum is so easy to work on, Much better than the old hubs that were such a pain. . . did I drive backwards far enough to disengage them yadda yadda. . . .

My son has a 1962 falcon with vacuum windshield wipers(144 inline 6) He has to let off the gas when the window gets too wet. . . . always good for a laugh. . .

kw

northern pointer

Thank you so much for the pics LariatDude. Makes it so much easier. You must have misread my post regarding the solenoid cover. My truck DOES have the plastic cover over the solenoid to redirect water. I replaced the check valve (Motorcraft YG360) nearest the solenoid today, but made no difference. I didn't realize there were two others as well but did notice the next one that you show in picture 3. If I pull them out and blow through them, one side should let air through and the other side not, correct? If blowing from either end lets air through, then the check valve is faulty as I understand it??? Are all 3 check valves the same ones? They look identical.

I found out too that the noise stops right after I put the truck into 4x4 mode for a moment, and then back to 2x4. Once in 2x4, there is no noise until I shut the truck off. However, after restarting it and begin driving, the noise is back until again I choose 4x4 momentarily and immediately back to 2x4.