hey all so the jeeps been feeling real sluggish lately, slowly gotten worse noticeable mostly on the highway. and since I'm currently making 140mi round trip weekend journeys from college to Denver. Hitting I-25 trying to keep up with 70-75mph traffic is a little difficult and I'm not convinced that with the 4.0 it should be such a slouch. I've replaced the muffler with a glass pack, (only performance updgrade). Overall it just feels gutless, and i dont think its a gearing issue. I hooked up a Vacuum gauge attempting to find a possible cause (head issues, sloppy valve guides, etc) and read approx 13-14inHG, which according to the guage is below normal, and chiltons says should be 15-22inHG. Im thinking this is my prob. or is it low because of the elevation? (Colorado being the mile high state). I'm thinking possible vacuum leak, gasket etc, or just worn out head, piston rings. any ideas? thanks in advance,(and the long post!)

I'd start with a compression test as that'll give you a much better idea and starting point. And if your numbers are good, you've eliminated a number of variables. And while the plugs are out, take a close look at them as they can also give you a good idea of what's going on...

That vacuum is not really abnormal at idle. In fact, for a high mileage engine (I'm guessing your '88 is fairly high mileage) it's pretty normal as vacuum does tend to decrease over the engine's life span. I completely agree with EdW in that you should pull the plugs and check for compression.

Here's a great article (about a Jeep no less!) on vacuum if you're interested.

http://tc.wagoneer.net/sblog/static...._vac_emissions

Check for vacuum leaks by squirting some carb cleaner or even WD-40 around the intake gaskets, the vacuum lines etc. With the engine idleing. When you hear a distinct change in engine RPM you found a leak !!...Just in Case ya need to...

My 4.0 has 190K on it and it is no Slouch, that motor is very strong you must have something going on....

Another thought: Check the Air filter !! If it's clogged you will se a drastic drop in power as well..