Head replacement issues

I had a couple bad exhaust valves on my 98 xj (4.0L), so I dropped in a reman. head yesterday. The stumbling went away, and the ECU isn't throwing any codes. However... *sigh*....

During my first test drive, I did some hard acceleration, and then came to a stop and idled for a few seconds, and heard some fairly serious tapping. Sounded like valves to me, but I'm no expert. The tapping went away after about 2 seconds and hasn't returned since.
Next, on real hard acceleration I heard what sounded like scraping for a second or two. I've been fooled before into thinking this was a cylinder wall being destroyed when in fact it was just a vacuum leak, but I can't find any leaks.

Next, I'm not positive but I think my oil pressure is lower at idle than it used to be. I did change to a non-synthetic oil for the break in and cleaning, but I'm not sure that'd make that big a difference.

The fuel economy has increased substantially from 12/16mpg to 17/21mpg, but I don't feel any increase in power. I've got 4.10's on 31". I should be screamin' off the line with a new head. My 92 with 500,000Km on it accelerated slightly faster.

Did I just trash my block?

I'm supposed to be heading out to BC on holidays (1400KM drive) tomorrow. Would be nice to know if I should bring a spare engine and my garage with me

1. The 4.0 is a hydrolic lifter engine. You probably have some week lifters unless you replaced those too. If not, they are probably tired.
2. If you not positive.... the only oil pressure related items to the head are the rocker tubes. Did you replace those too?
3. You might not see an increas in power but the fact that you see a big MPG increase is a good thing! Your computer hasn't finished "learning" yet. It takes about 25 or more starts with running before it has all the data needed.
4. Not likely with such good MPG.

All of the above is just my opinion.

Hrm.. Didn't replace pushrods or lifters, but they looked to be in pretty good condition.
I did find the source of the mystery noise. When I put the fan shroud back in, I forgot to slot it into the bottom of the frame, so its just flapping around and the fan was rubbing
I'm hoping you're right about the ECU, and that I'll start seeing a bit more power/economy after a few runs. 1400km should be enough for it to sort itself out

Its not Km as much as frequency of starts with running. Your noise could have been the fan shroud/fan interference.

You can't tell by looking at those parts. There is a special test for lifters that determines if they are worn out. They are inexpensive enough that replacement should be part of the head removal/replacement if over 100K. They need oil to operate and if the oil doesn't flow correctly, they are "colapsed" and you get the noise you heard. Lack of oil will do the same thing. There might have been air in the line which cause the lifter to not function correctly when the pressure was reduced from acceleration to idle. The flow from the lifter goes up through he rods and lubricates the rockers. If you look at a lifter, there is a small hole in the middle where the rod is placed.

If the head has experienced a lot of heat or marginal oil is used, there can be carbon build up in the rods. Also they should be removed in order and replaced in the same location when reassembling. They wear to both the lifter and the rocker. They should be kept as a set, even though you have new rockers they should be matched to the lifters. This may be another contributor to the noise if you didn't match them in assembly.

Anyway, none of this stuff is always critical and it is amazing how tolerant parts can be.

Fixing the leaking valves has made the engine run better, you are not wasting fuel.

Unless you had some custom port/polish work done you aren't going to see any more "performance" gains from the engine that what you are already getting--smooth running and better mileage.