I have a '99 XJ 4.0. My problem began when I stepped on my brake and the engine stalling/stuttering. I replaced pretty much all the "usual suspects" on the intake manifold to no avail. I suspected a vacuum leak or an issue with the brake booster. I verified no vacuum leaks then verified the booster was okay. Now the strange part. I removed the brake switch from it's position but had it still plugged in. I depressed the switch plunger and started the engine, no problem. I released the plunger and the engine stalled and stuttered. I bought a new brake switch and repeated the same test getting the same results. Suffice it to say, I am baffled! Sensors replaced; TPS, MAP, Idle Air Control. Again, the XJ idles fine but release that brake switch plunger and the idle goes awry. I suspect the switch is communicating to another sensor that is creating the havoc, but which?

 

Still trying to find the cause of my issue today. Next step; remove the throttle body and clean the daylights out of it, Add a dose of "Oops". Yesterday I was looking at the brake switch and starting the engine with the plunger in the up and down positions. I forgot that I left the ignition on for a few hours. I forgot that with the plunger laying on the floorboard in the default up position, the brake lights were on. With the bright sun out all day, I never noticed the brake lights on until dusk. Had to get my battery charger out last night for a re-charge as I ran it down to zilch.

 

Problem solved! I didn't have to clean the throttle body. My thoughts were to see if I could pick up an ECM from a bone yard (who also services vehicles) as I figured I might need to replace mine. When I got to the boneyard, I talked to a guy there who said he could get one for me, but I was looking at a whopping $400! As he looed, I told him about the issue I was having and how all the sensors were good, to include a new brake switch. The boneyard keeps records online of every vehicle they worked on and he said my problem sounded familiar. He looked up an XJ they worked on that had identical symptoms, They worked on it and their records say they found a rear brake light ground wire located on the right rear driver's side) of the XJ that created a short. He said those wires to the brake light are ties to the fuel pump. So, every time the customer stepped on the brake, the brake wire short cut the electric to the fuel pump stalling the engine, just like mine.. I just looked up into the air and shook my head. I told him that two weeks ago, I installed a new trailer wiring harness to those same rear driver's side wires. As soon as I got home, I just pulled the new trailer harness out using wire caps on the exposed XJ's wires. As soon as I did, I fired up my XJ and it's running running great again. I had no idea those brake wires were also tied to the fuel pump and a short could cause such a headache!