Something spraying above Rearend

[Always Learning]

I just picked up a 1972 F100 and there is a big black spot on the underside of the bed just above the rear end. I'm a bit of a newbe to major maintenance so my questions may seem a bit overly simple. Anyway here are a few questions:
1. If the rearend is leaking, will it spary on the underside of the bed?
2. How can I determine where the leak is?
3. How and where do I add fluid?
4. Is there a seal between the two housing portions and if so, what does it take to replace it?
5. If it does have a slow leak and I drive it after topping it off, what kind of risk am I taking?

That should be enough to get me started and thanks for all your advice in advance.

Frank

[BobbyFord]

It's the grease spun out of the rear u-joint.

[Always Learning]

That was quick, thanks. So, how can I tell if too much has spun out and needs to be serviced?

[BobbyFord]

That was quick, thanks. So, how can I tell if too much has spun out and needs to be serviced?

Check the u-joints for play. If they're loose, replace 'em. The throw-off is normal. Grease the u-joints at every oil change.

[Always Learning]

Cool, thanks

[Skifu]

He said it was "spraying" out,could it be a busted fuel line? the u-joint does sound more logical,I guess.

[fordman]

theres not fuel line that far back.

[lee_ford]

… a big black spot on the underside of the bed just above the rear end.

How big? If it is kind of elongated side to side then depending on EXACTLY where it is, it could be U-joint grease build up over the years or the front pinion seal on the rear end went out at one time. Remember this is a 37 year old truck. There may be nothing wrong now. I would check to see how wet it feels. If it is still even a little moist, I would check further.

To answer your questions…

1. If the rearend is leaking, will it spary on the underside of the bed?
Yes, a pinion seal leak will be elongated in a direct line with it on the underside of the bed.

2. How can I determine where the leak is?
Where is the oil wettest? Oil away from the leak will dry out faster than fresh oil comong directly from any leak.

3. How and where do I add fluid?
If it is a 9 inch, there is a plug on the back side of the housing. On others, just look for a plug just above the centerline of the pinion shaft. Pull that plug and stick your finger in. If you get oil on your finger, you’re good. If not then add until it is filled to the opening.

4. Is there a seal between the two housing portions and if so, what does it take to replace it?
That seal is a gasket and I have seen VERY few leak. If it is leaking there it will be obvious and not gat on the underside of the bed.

5. If it does have a slow leak and I drive it after topping it off, what kind of risk am I taking?
A slow leak could take years to get low enough to hurt. Still, fix it and friends will not mind you parking in their driveway.

Let us know what you find.

Lee

[lee_ford]

I forgot to answer a question.

4. Is there a seal between the two housing portions and if so, what does it take to replace it?

That is a whole nother project. Let us know if that is what you need before we get into tearing down a rear end.

Lee

[BobbyFord]

If the pinion was leaking there would be oil under the truck too, not just on the underside of the bed. Oil and grease on the underside of the bed look very different.

[lee_ford]

If the pinion was leaking there would be oil under the truck too, not just on the underside of the bed. Oil and grease on the underside of the bed look very different.

When a seal starts leaking it will only seep a little. Since he said he just got the truck, he may not have noticed the few drops on his driveway yet. The black spot on the underside could have been there for YEARS. We still have not heard from “Always Learning” as to whether the black spot feels wet. A sure sign it is a recent leak and not just u-joint grease buildup over the years.

But you are right about watching your driveway for an oil spot under the rear end. That is a sure sign of a problem.

As for oil and grease looking different, they both would have collected their share of road grime, if this is an old problem. In that case it would be hard to tell which caused it.

Lee

[fordman]

or it could be blinker fluid

[lee_ford]

or it could be blinker fluid

No way!!! The reservoir holds so little the blinkers would have quit long before a big spot showed up.

Wait!!! You may be right. Maybe the PO sold it because the price of blinker fluid was keeping him broke. It seems my shop bill was high all the time because I kept using my blinker too much. When I ask my semi mechanic about the bill he said I needed to start rolling my window down and using my arm to signal turns.

Lee

[fordman]

or it could be blinker fluid

No way!!! The reservoir holds so little the blinkers would have quit long before a big spot showed up.

Wait!!! You may be right. Maybe the PO sold it because the price of blinker fluid was keeping him broke. It seems my shop bill was high all the time because I kept using my blinker too much. When I ask my semi mechanic about the bill he said I needed to start rolling my window down and using my arm to signal turns.

Lee

how often does the disel mechancic recommend to replace a disel engine sprak plug? what kind of mileage do they get? every 6 months or 3000 miles like a gas engine should?

[BobbyFord]

Grease clumps, gear oil does not.