88 Surging Upon Startup

Recently, my XJ has been surging up to approximately 2000 rpm upon startup for about 5 seconds before setting down to about 1000 rpm. The surging occurs only when starting the engine. It has not surged during operation at all. It does this whether the engine is hot or cold, or whether the heater is on or off. This is highly annoying. Anyone have a clue as to what may be causing the engine to surge during startup? My XJ has 186K, although I replaced the engine with a rebuilt longblock 12K ago. The surging has only been occuring the last 2K or so.

Mine stopped doing that for the longest time and just started up again, after I´d messed up my TPS adjustment, by the tiniest amount.
But I´ve had it happen for other reasons.
Next week I´m gonna recheck my TPS settings and maybe swap in a spare TPS and see what happens.

I've noticed mine jumping up to about 1500 or so. Seems that the cold weather today seems to be doing it. [-16C abouts outside today] Typically mine jumps to about 800rpm on startup and drops to about 500 or so.
Is it cold where you are? maybe that might be causing it to an extent like I see with mine?

It´s cold where I´m at. I turned the throttle stop screw on mine, to up the cold idle just a bit, which also affects the TPS adjustment and mine started idling up to around 2000 on occasion at start-up. Doesn´t do it every time, just occasionally.
Could just be the cold, like you mentioned.
I had a vacuum leak, cut line, the vacuum resivoir would loose the stored vacuum, had a high idle start most everytime. Went away after fixing the vacuum leak, though I was messing with the TPS, at about the same time. My daughter discovered, that starting the XJ with the heater controls off, did away with most of the high idle starts, that´s what tipped me off to the vacuum leak.
I suspect there is more than one thing, that causes this problem.
Recently for me at least, it seems to happen on a warm, but not hot (full operating temp.) motor or full cold motor. Mostly around 1/4 scale on the heat gauge, mostly but not always.
One of those intermittant things that will drive you nuts.

I can imagine! Going to try the heater idea and see what happens when I go to work. This is my first XJ so learning all it's little idiocicracies [sp?] but have learned alot as we had to rebuild the front end, and did some work on the engine along with a few other things. Replaced the doors as I eventually want to have it painted, so it's rather 'colorful' but driving it across from Seattle, the parts there tend to have no rust on them.
I had all recalls finished on it as soon as I got it back, and the mech. dd a thorough check of the entire vehicle as well for me. The next things I have to do is replace the gas tank as it's seeping a bit near the top somewhere, and my front u-joints are worn, so that's next on the list. Other than looking like it's in need of a paint job, is running great. Made it from Seattle to Sudbury in 2 1/2 days non-stop, so has been reliable with being garaged for 3 yrs.
I've heard that one of the issues they can have is that many things work on vacumes in it. So far I've not had any problems at all that way on it, and much of the hoses have been changed out to newer ones before it was garaged, and still in great shape.

I don't think its the temperature, as the temp has varied quite a bit over the last couple of months - anywhere from the 30's to 60's. The surge upon startup has occurred in both the cold and warmer temps. The heater controls being on or off also does not seem to have much of an effect. The only other thing I have noticed is that my mileage has dropped 1-2 mpg, which could just be the effect of the surging.

Maybe I better give my vacuum system a good once over.