My 98 Eddie Bauer Explorer is having a "low power" problem with the 4.0 SOHC engine. Any suggestions would be greatly appreciated.

Symtoms: 1. I have a lot less power off the start from a dead stop
2. Idle is rough. Similar to an ignition miss but is not as consistant (hunting)
3. Engine surges/bucks while holding accelerator steady at speeds under 55 MPH. Runs decent at highway speeds.
4. My vacuum gauge (mounted on my A-pillar) shows only 10 inches of vacuum at idle in park (used to be 17-21 inches)
5. Check engine light is not on and I have no trouble codes (DTC's) stored.
I have replaced the fuel filter (which was pretty clogged), spark plugs, fuel pressure regulator, PCV valve. I cleaned my K and N air filter and ran a fuel injector cleaner through the system and had no changes in the problem. I have done a compression check (105-115psi between all 6 cylinders cold) and I have spark at every cylinder. I pulled off the upper and lower intake manifolds and checked all the O-rings, and checked all vacuum lines coming off of the upper intake and found no leaks. Fuel pressure is @38 psi at idle which I thought seemed a little low but it is EXACTLY the same as my friend's 98 Explorer. I pulled the EGR valve off, cleaned the carbon off the pintle and checked it's operation, looks fine.
My Haynes manual states there is no way to check base timing so I haven't done anything with timing. She' got 126,000 miles on her and I have NEVER had any engine problems with her. It's a great truck and I really need to get her back in shape because I'm driving my 78 F-350 meantime and getting about 6 MPG out of the 460!
ANY help or information anyone has would be greatly appreciated!

 

I know there has to be a few other mechanics out there!! My back is against the wall on this problem as I can't find another route to approach this problem from. Come on guys, lets here your ideas!!

 

A few thoughts in no particular order:

*EGR not operating properly.
*EVAP emission system problem. How is the seal on the gas cap and is the cap on tight?
*Vacuum leak somewhere. Visually inspect and use carb cleaner on faying surfaces. Although rare but possible, a bad power brake booster causing a vacuum leak.

 

With that low vacuum reading, I would be looking into the possibility of a pratially plugged up cat / muffler. Looks like the engine cannot breath properly.
Dialtone

 

While I agree that plugged cats could do it, I would put them more akin to constipation than breathing problems . First place I would suspect is vacuum leak. However, one way to help determine if it's plugged cats / muffler is to have a helper press the accelerator while you feel the force of the exhaust coming out (CAUTION: Exhaust gases are hot). IF the exhaust is very gentle when the engine is revved, suspect pluggage. Had this problem on my sister-in-laws old V6 Mustang once. They thought the engine was completely shot, but it was constipated. Pull the cats loose and went for a test drive, and it ran like a scalded dog (A loud one). Plugged cats can also tend to make an engine run hot.

 

Thanks first of all to Cowboy, Dialtone and Jerry for your responses, the first I have gotton on this problem.
When I previously pulled the EGR to clean it, I tested it with a vacuum pump and it worked fine. The pintle seats all the way and seems OK. Gas cap seal is good as when I replaced the fuel pressure regulator I blew compressed air though the fuel return line while my brother listened at the fuel cap for leaks. I used an ENTIRE can of carb cleaner around the upper and lower intake sealing surfaces, as well as all visible vacuum lines with no change in idle speed noted. The brake booster would be an interesting idea, however if a leak is present it would have to be internal. Either way I had removed the vacuum line going to it and plugged the line with no change in idle speed or the problem.
The plugged exhaust therory would explain everything including the rough idle, power loss and vacuum problem, but I haven't noticed any increase in engine temerature. I guess the exhaust could be only partially plugged to where it wouldn't increase engine temp but cause the problem. I'll break the exhaust open upstream of the cats and see if that does it. I'll check that one out this weekend.
Thanks a bunch guys for your help, I hope that ends up being the problem as I just can't seem to find anything else!