I just purchased a 1988 XJS V12 that has been with the same owner for 15 years. It surges at low rpms and sounds like it is misfiring from about 2000 to 3500 rpm. During this it bucks but never dies and then once rpms rise it stabilizes. At idle it is quiet and smooth but after it warms up the oil pressure begins to drop quickly. Oil pressure is not restored until it smooths out at higher rpm. Engine temp stays stable.

I will update this after I do all the general maintenance (Plugs, plug wires, clean and lube distributor, fuel filter, oil change, and then AAV "thumb" test as stated by another user)
Manual Timestamp: 12:14pm CST 3/29/2022

If anyone has any suggestions it would be a huge help.

 

Oil pressure reading low at hot idle is normal. No cause for concern. Its the gauge, not the pressure.

Air leaks can cause surging at idle, but there's a fuel cutoff at 1500 rpm, so that's probably not the problem.

 

Welcome!

I had similar idling issues when I first purchased my xj-s and it turned out to be bad oxygen sensors. To troubleshoot, disconnect both of them and see how she runs. With the sensors unplugged the ECU sticks to its base fuel map. In my case with the bad sensors it was adding way too much fuel at idle. If you unplug them and the idle is immediately improved, it's time to replace.

You're on the right track with the general maintenance items you're already tackling. Make sure the mechanical and vacuum advance mechanisms are working properly while you're in the distributor.

Check for vacuum leaks.

The xj-s exhaust system has no crossover or x pipe, so you can put a hand behind each tailpipe and narrow down your misfire to one cylinder bank or the other based on the exhaust pulse.

Also check all the fuel lines under the hood. If they're cracking or hard as a rock put them at the top of your list; this is the #1 cause of engine fires. If your injector hoses are original as they very well may be, your injector seals are probably also toast which can cause a vacuum leak/misfire.

Hope this helps

 

Welcome to the Forums.
Please do an Intro in the New Members Area when time permits.

Clean the throttle body discs and internal bores.
Run a socket over the 24 Inlet manifold nuts, NOT Gorilla tight, "nice and firm" is all taht is needed.
Set the throttle rods CORRECTLY. ALWAYS look at this engine as Double Six, which is what it really is, so if the throttles are NOT balanced properly, the rev range you have issues with will never go away.
TPS may be flaky in that rev range spot.
CTS Coolant Temp Sensor, B Bank thermostat housing rear casting. Well known PITA, and cheap as chips to replace, and has sorted many a V12 in less than 60 seconds.
Spark plugs gapped wrongly, 0.025" is required.

GOOD idea on the basics you mention, that is called "catch up" and that list is generally quite long.

These be SIMPLE beasts, so methodically working through items is the ONLY way.

Good luck.