99 Expy EB
5.4L AWD<O</O
252,000 mi

Have to make a 600+ mi trip this weekend and noticed the following:

<O</OUnder a load, there seems to be a drivetrain related “clunk” coming from middle-front end of underside of vehicle. Only happens when accelerating hard from a stop or jumping on throttle as if passing, does not occur at any other time (not while turning, stopping, etc.). Sound can be avoided altogether if acceleration is easy (as I usually do). From reading posts here it sounds like u-joints (?), but welcome any other ideas or T/S suggestions. Truck is in great shape otherwise.

Question: is this a “critical” fix before upcoming trip – or if by accelerating slowly and avoiding the "clunk" can I put off further diagnosis/repair until I return? Definitely don’t want to wind up stranded on side of the road, but also really don’t have a lot of time to address it this week so I'd have to have a pro look at it ASAP (big $$$!). If you were me – what would YOU do?

Appreciate any/all input - thanks.

 

I have the same thing but it does it less when the truck is not in overdrive...so if you don't get any advice before you leave, try taking overdrive off---it appears to put less strain on "it".

 

Thanks for the quick response, my2000xp. Will keep that suggestion in mind.

 

That would've been my first suggestion of what to check out.

 

alloro - thanks for the confirmation.

Have not worked with them before - based on the brief description offered, any suggestions re: what to look for/check? Specifically, are there any go/no-go points to consider, given to my upcoming trip?

 

I've also seen posts on here about the a torque converter being the cause...i even had one mechanic tell me that it was my transmission...well that was a year ago and I think if that were the case, it would have went by now

 

To be a bit more specific. The front u-joint of the rear drive shaft is actually a double cardan u-joint. This cardan joint is really a glorified double u-joint assembly. In between the two u-joints is a spring mechanism to keep a constant pressure on the 2 u-joints to effectively keep them centered to each other. Sometimes this spring loses grease and will cause a clunking on acceleration. If your upcoming road trip is a long one, or if you're pulling a trailer, or going into a mountainous area, or basically if you're going to be loading down the drive shaft more than normal, I would have it inspected before you go. Maybe it is something as simple as sticking a grease gun on the u-joints and giving them a bit of grease to tie you over until you're back.

Here's a pretty decent write-up with pictures of a double cardan joint rebuild.
http://www.mallcrawlin.com/forum/showthread.php?t=7478

 

Thanks very much for the description and link - that sheds a lot of light on what to look for and expect. Trip won't be mountainous or involve pulling anything - but will definitely have a look-see before leaving.

One last question - I didn't notice them on the photos of the rebuild (maybe missed them?) - but is there normally a grease fitting on the double cardan joint - or would just shooting some grease into the spring mechanism area work as a short-term precaution?

 

Whoops - went back again and clearly see at least one fitting in a photo - just not sure if that's accessible without disassembling the joint. Can I get to that while everything's still in place?

 

Sometimes you can, but it depends on the size of your grease gun head. They do make slim heads for grease guns just for the purpose of greasing u-joints. But the way I've always done it, was to detach the drive shaft from the rear flange and bend it over enough to gain access to the grease fittings.

Unfortunately there is no way to grease the centering spring assembly without disassembly.