While driving home from work a couple of weeks ago, my 1998 XJ8L went in to fail safe mode and the idle was really high.the list of what can be the problem
I connected the code reader to it and got a couple of codes, one was because I had disconnected the battery and the testing cycle was not finished. That one should go away once I get about a 100 miles or so on the car.

The second code was a P0507, if I remember correctly.
Here's what the possibilities are:
Faulty or clogged IAC valve
Carbon buildup on the throttle body
Faulty power steering pressure switch
Alternator that has failed
A vacuum leak
Leaking air intake after the throttle body
EGR valve leaking vacuum
A faulty positive crankcase ventilation (PCV) valve
Damaged/failed/dirty throttle body
Failed EVAP system
Failed IAC (idle air controller) or faulty IAC circuit


I started with vacuum leaks.
I was also replacing the thermostat and housing because it was overheating while driving home. It would overheat and then the needle would suddenly drop to the normal range. I was surprised to find that I needed to remove the intake to replace it. It turned out to be fortuitous.

Here's what I found.
Brake booster hose that goes from the bottom of the throttle body to the booster. Broken at the throttle body. RTV and a standard vacuum fitting inside the hose was used to fix it. I don't think it was doing the job, but the brakes did work, or maybe it was.
Someone had put RTV on the hose nipple on the exact opposite side of the throttle body, to stop that one from leaking and the hose was just laying open in the engine bay.
The air intake turn down at the top of the throttle body is cracked and I believe leaking there also.

I'll check the EGR next.
Not sure how to check the EVAP or the IAC.











I've attached pictures.