The most common cause of this problem is the passageways for the exhaust get plugged with oily carbon.
It is relatively easy to do.
Things which can be plugged include:
The EGR valve;
The passageways inside the plenum it is bolted to;
The tube leading from the exhaust pipe or manifold to the intake system;
The rubber hoses connected to the DPFE and the ECU.
I have found that cleaning the EGR valve and the plenum passageways usually fixes the problem. It really is easier than working on a carburator!
The EGR is easy enough to clean with a wire brush, if it is clogged. The plenum passageways are the hardest to clean, but only because you need to remove the throttle body and the plenum to get to everything. Once everything is apart, you can use a screwdriver to get the worst out. I used a rifle bore brush to finish the job inside the long deep passageway.
I would recommend you look closely at the EGR valve. If it will open with vacuum, and will allow air to get through, then I highly recommend you go through the trouble to remove the throttle body to get to the upper plenum. That is where your trouble most likely lies.
Easy enough to do, too. I took all apart, cleaned everything, and had it back together in less than 2 hours.
By the way, I am referring to my '98 with a 4.6, and am assuming yours is similar.
Check the port in the intake behind the throttlebody. I have seen the hole clogged. You will need to hold the throttle plate open and peek into the intake with a light. You should be able to see it, if not, you may have to remove the TB. to access it.
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