I got this as a PM from Ralph and decided to start this thread for him.

'00 XJ. Stock. 150K. Has a 0331 head but thinking I do not have the cracked head issue. No coolant loss to speak of, 45lbs at start up, and 23lbs of oil pressure at a very hot idle. Never have done a compression test or put a mechanical gauge on her to check oil pressure. Never have done a coolant pressure check either. Probably should do all that at some point though. I upgraded to 4 hole injectors and swapped out all 4 O2 sensors. Did the O2 sensors, CA Emissions, justifying it by thinking they had 150K on them, were original, and needed to go. Did a valve cover gasket while I was there and everything in there looked clean. Flashed the computer so it could relearn fuel trims and whatnot. Then I get a P0432 code. Fuel trims seem to be within reason. O2 sensor voltage also. Best I can tell is that there is no exhaust leak, even though I did find that one of my rear exhaust manifold bolts was backed out about a 1/4". So I tightened them all. Replace the rubber elbows on the front 2 vacuum lines off the manifold. Also the little elbow under the MAP Sensor. No luck. Thinking about swapping out the rear downstream O2 sensor. Would not be the first time a new part was defective out of the box. Did use NTK's. From poking around I realize that it is usually the Cat but not always. Because of the timing I not convinced either. If I need a new Cat I need a new Cat. But would be pissed if this did not solve the problem. Thus trying to rule out other suspects and the reason for the PM.

Sorry for the long winded PM but I figured some background was in order.

Thinking I would buy the Mighty Vac one. Questions as follows:
Would those reading that you should get apply to a '00 4.0?
I have 3 lines coming off my intake. Thinking the front 2 are vacuum and the one nearest the firewall with a check valve in the line is not. So which one should I hook up to?
When you say: Start the engine and note the vacuum reading. It’s usually 17 to 21 inches of vacuum. Is this when the engine is stone cold?

Cold or warm Ralph, vacuum should be the same.