My 99 xjr occaisionally, and without warning, of course, will fall into some sort of restricted performance mode. I am not talking about the failsafe mode, as I am waiting on a brake switch to solve that problem. Independently, without any lights or messages, occaisionally the car acts like the supercharger is disconnected. I put my foot in it, and it gives me about half the acceleration it normally does. I can still get to fifty or sixty, I have not tried to go faster, as it never happens on the freeway. It happened today when I was going to get tires. The tires were replaced and I drove home and found that the car healed on the way. It is the darndest thing. I thought that maybe it had to do with some sort of cool valet mode designed to keep garage jockeys from smoking the hides, but as far as I know valet mode just deals with truck locks....any help would be appreciated.

 

when it's going at half, is it smooth throughout? or are there brief periods where the power cuts in or out?... kind of like if you were at half throttle while accelerating, then punch it a little more, then back off again to half... does that make any sense? I remember this happening to me a couple of times, no lights or warnings... it ended up the intake tube connection (right where it meets the MAF) loosened up. To be honest, it was probably like that for month or so but didn't give me any check engine or restricted performance lights even with a big gaping hole at that connection (the hose clamp literally was so loose it was just dangling there).

 

It is smooth, the problem, that is...I had the problem you described on my sl600...but it threw a dozen fault codes. I will check and advise. It makes sense, as the problem seems alot like the back two barrels of a four barrel carburetor not opening, if anybody is nostalgic.

 

The problem could be ethanol gas related try finding and running straight gas.blog around to find these rare non ethanol gas locations.It worked well for me and I currently have a thread asking help to locate 100% non ethanol gas. Good Luck.

 

technetx, you are my hero. the clamp was loose, allowing the hose to come loose when in a turn. Works great now.

 

John, that's awesome you found it I love it when these problems are fixed without some crazy dealer cost!... if you had taken it in to the dealer, you would have walked out with a new MAF sensor and housing (and $400 less in your pocket).

Now that you know it came loose, you may want to check up on it every once in a while to look for cracks. That specific junction gets all heated up with oil inside the tube and the proximity to the engine, and will puff out, expand, and crack on the underside right where the clamp is suppose to tighten up on it. If you have any further problems, check the underside of the tube (on the outside of the tube) for any oily sludge.