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I thought this was initially a problem - or a need to grease the slip yoke in my 2005 F150 FX4, (130,000 miles), but have done so using basic red grease twice with no change in performance.
The issue is that when accelerating - but only when going in an upward/uphill direction, there is a clunk or thud when the accelerator is first pressed. It doesn't do it a second time if a longer incline/uphill direction - just at the initial acceleration. This is also regardless of speed, just cause by the sudden acceleration. The UVs look fine with no play and transmission shifting is fine in all other cases...though I don't believe this is an issue with a transmission or transfer care, but thought I'd throw that out there.
My next plan was to use the Ford XG-8 grease - which I should've used to begin with, to regrease the slip yoke but thought I'd pose the question here first to see if this does still indeed sound more like an issue with the slip yoke and not something else.
I had the same problem on a 2009 F150. Greased the slip yolk with some wheel bearing grease and cured the problem. I drove another 40,000 miles with no problems.
Question...
Now that I've done some reading, it seems the "clunk" should be happening when coming to a stop or starting from a stop. However, that is not when mine is doing it. The issue, I guess as I stated in my original message, is when I'm quickly accelerating when going up an incline - generally only at higher speeds. Could this still be the slip yoke or something else?
Have you looked at the carrier bearing (if yours has one). It could be the bearing there is worn out too. Another place to check is the transmission mount. My Firebird goes through these regularly and I'm currently in the process of changing to a poly mount. You could also check your U-joints. These will clunk too when there are needle bearings missing.
OP- did you ever figure out this "clunk at speed going up hills" issue? I have it on a 01 f350 and this is the only thread I've found with the same symptoms.
When im accelerating up a long hill at around 60mph i will get this clunk with a slipping sensation, often combined w something that feels/ sounds like gear teeth slipping. None of the normal "differential noises " seem to cover this situation.
truck is a 01 f350 4x4 scab longbed, psd, manual, 260k, stock.