I have a 1997 XLT 4.0 AWD w/Rear A/C that has 175,000 miles. Approximately 500 miles (give or take a thousand) I replaced the cylinder heads and everything in them (valves, lifters, etc...) described in other threads.

The van is running great, but I recently removed the air intake duct to look inside the throttle, which always had some brown or black deposits buildups in the past, which I cleaned once a year. I was shocked to see that it was at least 3 X the usual quantity. I can only assume that the mechanic cleaned the throttle before putting it back onto the engine, so it is a very short time to collect a tangible black layer completely coating everything inside. I sprayed and wiped as best as I could through the throttle door, but I was not up to actually opening up or removing the whole throttle body.

Any ideas as to WHY, and WHAT TO DO about it?

Unfortunately after putting the last $2000 into that rebuild, I am resigned to not spending too much more on it. Though I am praying and hoping to get as many more years as I can out if it, but only without major costs. I love the van and would like to keep it forever, but I finally reached the point where the next major problem of any kind will be the final blow.

So this could be indicative of a terminal problem, or it could be an early warning for the stitch in time that saves a 1997...

 

Was the PCV valve replaced? are all the hoses attached correctly?

Also, I doubt the TB was cleaned when the heads were replaced. I even doubt that it was removed from the intake when the heads were replaced.

I would clean the TB and see if the issue continues.

CRC Industries 5078 - Throttle Body & Air-Intake Cleaner | O'Reilly Auto Parts

 

I didn't think it was possible to remove and remount the intake manifold head with the TB still attached, so I assumed it was taken apart. Then, I assumed no idiot would leave it so dirty if it was open and easy to clean....

As for the PCV hoses, the mechanic who did the work maybe be a slob but he is a real old-time pro. I wouldn't know better than him to verify.

I changed the PCV valve for a complete tune up a year or so ago. It looked fine then, did not change again recently. Problem then was that the plugs kept fouling - which I now understand to have been from the bad heads. Also, the IAC kept needing cleaning or changing, which is why I cleaned the TB every year. I was hoping the head job would fix that too.

 

Is the buildup gritty or fine?

Most pavement dust is black from tire rubber and asphalt worn off. tires in front of your rig throw it up into the air and into the air intake

Got an air leak somewhere between air filter and throttle body. Check at 1500>2000 rpms to develop a high vacuum. Can also be the air filter seal to the box, boxes sometimes warp distorting the sealing surface.
Smoke generator works well for the tube connections. These are getting old, look for splits in the ends.
Hope you are using the stock filter and a brand name one such as WIX?

Check to see how much blowby is coming out of the engine PCV hole at 2000 rpm. Should be next to none but with 175k rings may be shot.

What's the oil usage?

If there's lots of blowby or high oil consumption, can put an oil collection unit on the PCV line and collect the oil before it hits the throttle body intake. Any oil into the TB and the slightest dust leak in makes for crud buildup in TB and IAC.

Any of the Ford 4.0L OHV V6s with more than 125k miles and bad heads, the whole engine should be rebuilt or long blocked

 

I think it was fine, black soot, but a surprisingly thick layer. Not oily. No oil consumption.

Could be coming through air cleaner. Haven't changed the filter in a long time. I live in New York - much of my mileage is through heavy pollution areas!

My mechanic suggested changing the block as well, but I couldn't afford or justify yet another $2,000 and more onto an already insane expenditure for an old rattilly van. Not sorry.

How easy is opening the TB to clean it? Is a gasket or sealant needed to put it back?

 

There is a gasket, and if it has not been replaced, it probably needs to be.

Fel-Pro 61055 - Throttle Body Gasket | O'Reilly Auto Parts

CRC TB cleaner and some shop rags work well on cleaning the TB.