My "03 3 liter has a new quirk. When I first start it the idle is very rough. When I back down my long steep drive way and apply the brakes it is difficult to stop and the brake peddle feels like there is no vacuum assist. Once I drive about a block the brakes feel normal and the idle is normal after the engine warms up. In addition, twice the "restricted performance" warning came on when I started it first thing in the AM and was restricted (back fired under hard load) but went off and the car performed normal after I shut it off and started it later. The last time the restricted warning came on so did the check engine light. I took it to a parts shop and when they hooked up their fault reader it showed no fault codes and the light finally went off. Sounds like a vacuum issue some way tided into the power brakes but why does it correct itself after the engine warms up? Thanks for any help,. Don R

 

Imagine it's an air (aka vacuum) leak. The brakes will be as you describe and the car idle will struggle.

To confirm, get an OBD tool with live data such as elm327 and look at fuel trims. Hot engine, parked, at idle and again at about 2500rpm.

You can find leaks either by spraying tiny amounts of a burnable gas near suspect parts or by a smoke test.

The leak may partially correct by something expanding and reducing a gap. Don't sweat the detail - just hunt the leak.

 

Have you noticed a hissing sound from under the dash? It's most likely when you take your foot off the brake pedal. If so, your brake booster is most likely at fault.

I'm not 100% about this version, but on all other vehicles I've owned with power brakes, the booster also acts as a vacuum reservoir via a check valve. The idea is to maintain enough boost force for applying the brakes at least twice even when the engine is off. This is part of the design so you could still safely stop with normal pedal effort after the engine stalls, for example.

No access to my Jag manuals at the moment, but part of the typical booster checkout is to run the engine to charge the booster ("fill" it with vacuum, so to speak), shut off the engine, and then make sure you get at least two full brake applications before the pedal force increases.

The next part of the checkout is a leakdown test, to make sure the booster can hold a vacuum. This is the same as above, but let the car sit for hours after shutdown before trying the brakes.

Just curious, do you apply the foot brake before starting the engine? I was taught to always do this, and not just to make sure the car doesn't accidentally move. By making sure the pedal effort is normal BEFORE starting the engine, this completes a leakdown test of the booster. High pedal effort before starting indicates a failed booster, either a bad check valve or a leaky diaphragm.

It sounds like your car has already failed the leakdown test. I'd be real curious to hear how the booster behaves before engine start.

This is also most likely the source of your vacuum leak. There may be a restrictor in the vacuum line, to prevent a booster leak from causing a vacuum leak bad enough to kill the engine, too.

Please note if the booster is leaking, it may not be your only vacuum leak. There could be another leak(s), but the booster leak adds enough volume to put the total leakage over the threshold that causes rough running and restricted performance.

 

You have a vacum leak, probably from IMT valves, vacum lines or upper or lower manifolds.
Site has many threads on how to find?
Joycesjag has a great set of pictures showing all the possible places.

Dont think its your brake booster, so wait until after checking for vac leaks.

 

First I would check the vacuum line from the manifold (back) to the brake booster.

 

Here it is:

 

Yeah he does:
https://www.jaguarforums.com/forum/s...ics-faq-52720/

 

Another easy test would be to disconnect and cap the vacuum line feeding the brake booster. Break the connection as close to the manifold as possible, in case the line is leaking, not the booster.

Then start the car (don't drive it yet, as you'd have no power assist to the brakes) and see if it runs okay now. If so, you have a major vacuum leak either in the booster or the line.